feat(ri-050): RI-5-001 typed freshness states and stale-safe Mission Control surfaces (#1275)

Reviewed-on: #1270

apps/web/src/components/freshness/freshness-notices.tsx

@@ -0,0 +1,110 @@
+'use client';
+
+import type { ReactElement } from 'react';
+import { formatAge, type FreshnessLabel } from '@/lib/freshness/model';
+
+/**
+ * Rendering rules for non-current freshness states (RI-5-001).
+ *
+ * - `unavailable` renders an explicit failure panel — never an empty
+ *   healthy collection.
+ * - `stale` may render last-known data, but only under a visible label
+ *   carrying source identity, snapshot version, and age.
+ * - `partial` renders the verified parts plus an explicit list of what is
+ *   missing.
+ */
+
+interface RetryableNoticeProps {
+  readonly onRetry?: () => void;
+  readonly retryLabel?: string;
+}
+
+function RetryButton({ onRetry, retryLabel }: RetryableNoticeProps): ReactElement | null {
+  if (!onRetry) return null;
+  return (
+    <button
+      type="button"
+      onClick={onRetry}
+      className="mt-2 rounded-lg border border-surface-border px-3 py-1.5 text-xs transition-colors hover:border-gray-500"
+    >
+      {retryLabel ?? 'Retry'}
+    </button>
+  );
+}
+
+export interface UnavailableDataNoticeProps extends RetryableNoticeProps {
+  /** What is unavailable, e.g. "Tasks". */
+  readonly title: string;
+  /** Optional underlying failure detail (network message, invalidation reason). */
+  readonly detail?: string | null;
+}
+
+/** Explicit `unavailable` state. Never renders as an empty healthy collection. */
+export function UnavailableDataNotice({
+  title,
+  detail,
+  onRetry,
+  retryLabel,
+}: UnavailableDataNoticeProps): ReactElement {
+  return (
+    <div role="alert" className="rounded-lg border border-error/40 px-4 py-3 text-sm">
+      <p className="font-medium text-text-primary">{title} are unavailable</p>
+      <p className="mt-1 text-text-muted">
+        This is not an empty result — the data could not be verified from the gateway.
+        {detail ? ` ${detail}` : ''}
+      </p>
+      <RetryButton onRetry={onRetry} retryLabel={retryLabel} />
+    </div>
+  );
+}
+
+export interface StaleDataNoticeProps extends RetryableNoticeProps {
+  /** Provenance of the last-known snapshot being displayed. */
+  readonly label: FreshnessLabel;
+}
+
+/**
+ * Situational-awareness banner for `stale` data: last-known data may render,
+ * but visibly labeled with source identity, snapshot version, and age.
+ */
+export function StaleDataNotice({
+  label,
+  onRetry,
+  retryLabel,
+}: StaleDataNoticeProps): ReactElement {
+  return (
+    <div role="status" className="rounded-lg border border-warning/40 px-4 py-3 text-sm">
+      <p className="font-medium text-warning">Showing last-known data — it may be out of date</p>
+      <p className="mt-1 text-xs text-text-muted">
+        Source {label.source} · snapshot v{label.version} · fetched{' '}
+        {formatAge(label.fetchedAt, Date.now())}. Verdicts derived from this data are unknown and
+        changes are disabled until it is revalidated.
+      </p>
+      <RetryButton onRetry={onRetry} retryLabel={retryLabel ?? 'Revalidate'} />
+    </div>
+  );
+}
+
+export interface PartialDataNoticeProps extends RetryableNoticeProps {
+  /** Display names of the sections whose collections are unavailable. */
+  readonly missing: readonly string[];
+}
+
+/** `partial` surface banner: verified parts render, missing parts are explicit. */
+export function PartialDataNotice({
+  missing,
+  onRetry,
+  retryLabel,
+}: PartialDataNoticeProps): ReactElement {
+  return (
+    <div role="status" className="rounded-lg border border-warning/40 px-4 py-3 text-sm">
+      <p className="font-medium text-warning">Some data could not be loaded</p>
+      <p className="mt-1 text-xs text-text-muted">
+        {missing.join(', ')} {missing.length === 1 ? 'is' : 'are'} unavailable — sections below show
+        an explicit unavailable state instead of an empty list. Derived verdicts remain unknown
+        until every collection is revalidated.
+      </p>
+      <RetryButton onRetry={onRetry} retryLabel={retryLabel ?? 'Revalidate'} />
+    </div>
+  );
+}

apps/web/src/lib/freshness/model.spec.ts

@@ -0,0 +1,324 @@
+import { describe, expect, it } from 'vitest';
+import type { Task } from '@/lib/types';
+import {
+  acceptSnapshot,
+  assertMutable,
+  canMutate,
+  combineFreshness,
+  computeDigest,
+  computeFreshness,
+  DEFAULT_FRESHNESS_POLICY,
+  formatAge,
+  type FreshSnapshot,
+  invalidationReasonLabels,
+  StaleMutationError,
+  UNKNOWN_VERDICT,
+  verdictValue,
+} from './model';
+import { validateProjectCollection, validateTaskCollection } from './validators';
+
+const NOW = 1_800_000_000_000;
+
+const policy = { ...DEFAULT_FRESHNESS_POLICY, staleAfterMs: 60_000 };
+
+const taskPayload: Task[] = [
+  {
+    id: 'task-1',
+    title: 'T1',
+    description: null,
+    status: 'not-started',
+    priority: 'high',
+    projectId: 'project-1',
+    missionId: null,
+    assignee: null,
+    tags: null,
+    dueDate: null,
+    metadata: null,
+    createdAt: '2026-08-01T00:00:00.000Z',
+    updatedAt: '2026-08-01T00:00:00.000Z',
+  },
+];
+
+function acceptedTaskSnapshot(
+  overrides: Partial<FreshSnapshot<typeof taskPayload>> = {},
+): FreshSnapshot<typeof taskPayload> {
+  const result = acceptSnapshot({
+    value: taskPayload,
+    validate: validateTaskCollection,
+    previous: null,
+    policy,
+    source: 'gateway:/api/tasks',
+    now: NOW,
+  });
+  if (result.outcome !== 'accepted') {
+    throw new Error(`fixture setup failed: ${result.reason}`);
+  }
+  return { ...result.snapshot, ...overrides };
+}
+
+describe('computeFreshness', () => {
+  it('treats a missing snapshot as unavailable, never as an empty healthy collection', () => {
+    expect(computeFreshness({ snapshot: null, policy, now: NOW })).toBe('unavailable');
+  });
+
+  it('returns current for a fresh verified snapshot regardless of data emptiness', () => {
+    const empty = acceptSnapshot({
+      value: [],
+      validate: validateTaskCollection,
+      previous: null,
+      policy,
+      source: 'gateway:/api/tasks',
+      now: NOW,
+    });
+    if (empty.outcome !== 'accepted') throw new Error('expected acceptance');
+    expect(computeFreshness({ snapshot: empty.snapshot, policy, now: NOW })).toBe('current');
+  });
+
+  it('degrades to stale once the snapshot ages past staleAfterMs', () => {
+    const snapshot = acceptedTaskSnapshot();
+    expect(computeFreshness({ snapshot, policy, now: NOW + 60_001 })).toBe('stale');
+    expect(computeFreshness({ snapshot, policy, now: NOW + 59_999 })).toBe('current');
+  });
+
+  it('degrades to stale when the latest revalidation failed', () => {
+    const snapshot = acceptedTaskSnapshot();
+    expect(computeFreshness({ snapshot, policy, now: NOW, degraded: true })).toBe('stale');
+  });
+});
+
+describe('mutation guard', () => {
+  it('permits mutations only on current data', () => {
+    expect(canMutate('current')).toBe(true);
+    for (const state of ['stale', 'partial', 'unknown', 'unavailable'] as const) {
+      expect(canMutate(state)).toBe(false);
+    }
+  });
+
+  it('refuses mutations on non-current data via assertMutable', () => {
+    expect(() => assertMutable('current')).not.toThrow();
+    for (const state of ['stale', 'partial', 'unknown', 'unavailable'] as const) {
+      let thrown: unknown;
+      try {
+        assertMutable(state);
+      } catch (caught) {
+        thrown = caught;
+      }
+      expect(thrown).toBeInstanceOf(StaleMutationError);
+      expect(thrown).toBeInstanceOf(Error);
+      if (thrown instanceof StaleMutationError) {
+        expect(thrown.name).toBe('StaleMutationError');
+        expect(thrown.freshness).toBe(state);
+        expect(thrown.message).toContain(state);
+        expect(thrown.message).toContain('revalidat');
+      }
+    }
+  });
+});
+
+describe('acceptSnapshot', () => {
+  it('accepts a valid payload with provenance', () => {
+    const result = acceptSnapshot({
+      value: taskPayload,
+      validate: validateTaskCollection,
+      previous: null,
+      policy,
+      source: 'gateway:/api/tasks',
+      now: NOW,
+    });
+    expect(result.outcome).toBe('accepted');
+    if (result.outcome !== 'accepted') return;
+    expect(result.snapshot.source).toBe('gateway:/api/tasks');
+    expect(result.snapshot.version).toBe(1);
+    expect(result.snapshot.fetchedAt).toBe(NOW);
+    expect(result.snapshot.data).toEqual(taskPayload);
+  });
+
+  it('invalidates a schema-mismatched payload instead of rendering it', () => {
+    const result = acceptSnapshot({
+      value: { not: 'an array' },
+      validate: validateTaskCollection,
+      previous: acceptedTaskSnapshot(),
+      policy,
+      source: 'gateway:/api/tasks',
+      now: NOW,
+    });
+    expect(result).toEqual({ outcome: 'invalidated', reason: 'schema-mismatch' });
+    expect(invalidationReasonLabels['schema-mismatch']).toContain('schema');
+  });
+
+  it('invalidates cross-workspace payloads', () => {
+    const userOne = acceptSnapshot({
+      value: [
+        {
+          id: 'p1',
+          name: 'P1',
+          description: null,
+          status: 'active',
+          userId: 'user-1',
+          metadata: null,
+          createdAt: '2026-08-01T00:00:00.000Z',
+          updatedAt: '2026-08-01T00:00:00.000Z',
+        },
+      ],
+      validate: validateProjectCollection,
+      previous: null,
+      policy,
+      source: 'gateway:/api/projects',
+      now: NOW,
+    });
+    if (userOne.outcome !== 'accepted') throw new Error('expected acceptance');
+
+    const switched = acceptSnapshot({
+      value: [
+        {
+          id: 'p9',
+          name: 'P9',
+          description: null,
+          status: 'active',
+          userId: 'user-2',
+          metadata: null,
+          createdAt: '2026-08-01T00:00:00.000Z',
+          updatedAt: '2026-08-01T00:00:00.000Z',
+        },
+      ],
+      validate: validateProjectCollection,
+      previous: userOne.snapshot,
+      policy,
+      source: 'gateway:/api/projects',
+      now: NOW,
+    });
+    expect(switched).toEqual({ outcome: 'invalidated', reason: 'cross-workspace' });
+  });
+
+  it('keeps the previous workspace for collections with no intrinsic identity', () => {
+    const userOne = acceptSnapshot({
+      value: [
+        {
+          id: 'p1',
+          name: 'P1',
+          description: null,
+          status: 'active',
+          userId: 'user-1',
+          metadata: null,
+          createdAt: '2026-08-01T00:00:00.000Z',
+          updatedAt: '2026-08-01T00:00:00.000Z',
+        },
+      ],
+      validate: validateProjectCollection,
+      previous: null,
+      policy,
+      source: 'gateway:/api/projects',
+      now: NOW,
+    });
+    if (userOne.outcome !== 'accepted') throw new Error('expected acceptance');
+
+    // Empty list after the user deleted every project: no identity to check,
+    // so the verified scope is retained and the empty state stays healthy.
+    const emptied = acceptSnapshot({
+      value: [],
+      validate: validateProjectCollection,
+      previous: userOne.snapshot,
+      policy,
+      source: 'gateway:/api/projects',
+      now: NOW,
+    });
+    expect(emptied.outcome).toBe('accepted');
+    if (emptied.outcome === 'accepted') {
+      expect(emptied.snapshot.data).toEqual([]);
+      expect(emptied.snapshot.workspace).toBe('user-1');
+    }
+  });
+
+  it('invalidates version regressions', () => {
+    const previous = acceptedTaskSnapshot({ version: 7 });
+    const regressed = acceptSnapshot({
+      value: taskPayload,
+      validate: validateTaskCollection,
+      previous,
+      policy,
+      source: 'gateway:/api/tasks',
+      now: NOW,
+      incomingVersion: 3,
+    });
+    expect(regressed).toEqual({ outcome: 'invalidated', reason: 'version-regression' });
+
+    const newerSchema = acceptedTaskSnapshot({ schemaVersion: 4 });
+    const downgradedClient = acceptSnapshot({
+      value: taskPayload,
+      validate: validateTaskCollection,
+      previous: newerSchema,
+      policy: { ...policy, schemaVersion: 2 },
+      source: 'gateway:/api/tasks',
+      now: NOW,
+    });
+    expect(downgradedClient).toEqual({ outcome: 'invalidated', reason: 'version-regression' });
+  });
+
+  it('increments the version monotonically across accepted snapshots', () => {
+    const first = acceptedTaskSnapshot();
+    const second = acceptSnapshot({
+      value: taskPayload,
+      validate: validateTaskCollection,
+      previous: first,
+      policy,
+      source: 'gateway:/api/tasks',
+      now: NOW,
+    });
+    expect(second.outcome).toBe('accepted');
+    if (second.outcome === 'accepted') {
+      expect(second.snapshot.version).toBe(first.version + 1);
+    }
+  });
+});
+
+describe('combineFreshness', () => {
+  it('gates the surface on the primary collection', () => {
+    expect(combineFreshness('unavailable', ['current'])).toBe('unavailable');
+    expect(combineFreshness('unknown', ['current'])).toBe('unknown');
+    expect(combineFreshness('current', [])).toBe('current');
+  });
+
+  it('degrades to partial when a secondary is unavailable', () => {
+    expect(combineFreshness('current', ['current', 'unavailable'])).toBe('partial');
+  });
+
+  it('degrades to unknown while a secondary is still loading', () => {
+    expect(combineFreshness('current', ['unknown'])).toBe('unknown');
+  });
+
+  it('degrades to stale when any collection is stale', () => {
+    expect(combineFreshness('current', ['stale'])).toBe('stale');
+    expect(combineFreshness('stale', ['current'])).toBe('stale');
+  });
+
+  it('propagates partial secondaries', () => {
+    expect(combineFreshness('current', ['partial'])).toBe('partial');
+  });
+});
+
+describe('computeDigest', () => {
+  it('is stable across key order and changes with data', () => {
+    const a = computeDigest({ x: 1, y: [1, 2] });
+    const b = computeDigest({ y: [1, 2], x: 1 });
+    expect(a).toBe(b);
+    expect(computeDigest({ x: 1, y: [1, 3] })).not.toBe(a);
+  });
+});
+
+describe('verdictValue', () => {
+  it('returns the value only for verified inputs', () => {
+    expect(verdictValue(true, '5')).toBe('5');
+    expect(verdictValue(false, '5')).toBe(UNKNOWN_VERDICT);
+    expect(verdictValue(false, '5')).not.toBe('5');
+  });
+});
+
+describe('formatAge', () => {
+  it('labels age in human terms', () => {
+    expect(formatAge(NOW, NOW)).toBe('just now');
+    expect(formatAge(NOW, NOW + 15_000)).toBe('under a minute ago');
+    expect(formatAge(NOW, NOW + 120_000)).toBe('2m ago');
+    expect(formatAge(NOW, NOW + 3 * 3_600_000)).toBe('3h ago');
+    expect(formatAge(NOW, NOW + 2 * 86_400_000)).toBe('2d ago');
+  });
+});

apps/web/src/lib/freshness/model.ts

@@ -0,0 +1,261 @@
+/**
+ * Typed freshness model for gateway-fetched collections (RI-5-001).
+ *
+ * A failed or stale fetch must never be indistinguishable from an empty
+ * healthy collection. Every fetched surface carries an explicit freshness
+ * state, a verified snapshot identity (source, workspace, version, age), and
+ * a mutation guard that refuses state-changing operations unless the data is
+ * verified current.
+ */
+
+/** Freshness states for fetched data. Never inferred from emptiness. */
+export type FreshnessState = 'current' | 'stale' | 'partial' | 'unknown' | 'unavailable';
+
+/**
+ * Reasons a snapshot is invalidated. An invalidated snapshot is treated as
+ * unavailable and is never rendered as current.
+ */
+export type InvalidationReason =
+  | 'cache-corruption'
+  | 'cross-workspace'
+  | 'schema-mismatch'
+  | 'version-regression';
+
+/** Human-readable labels for invalidation reasons (UI + error messages). */
+export const invalidationReasonLabels: Record<InvalidationReason, string> = {
+  'cache-corruption': 'cached snapshot failed integrity checks',
+  'cross-workspace': 'data belongs to a different workspace',
+  'schema-mismatch': 'response did not match the expected schema',
+  'version-regression': 'snapshot version regressed below the accepted version',
+};
+
+/** A verified snapshot of fetched data with full provenance. */
+export interface FreshSnapshot<T> {
+  readonly data: T;
+  /** Source identity of the fetch, e.g. `gateway:/api/tasks`. */
+  readonly source: string;
+  /** Workspace scope the data belongs to. */
+  readonly workspace: string;
+  /** Monotonic snapshot sequence number for this surface. */
+  readonly version: number;
+  /** Schema version of the validator that accepted this snapshot. */
+  readonly schemaVersion: number;
+  /** Epoch ms at which the data was verified. */
+  readonly fetchedAt: number;
+  /** Integrity digest of `data`, used to detect cache corruption. */
+  readonly digest: string;
+}
+
+/** Provenance label rendered next to last-known data. */
+export interface FreshnessLabel {
+  readonly source: string;
+  readonly version: number;
+  readonly fetchedAt: number;
+}
+
+/** Policy governing freshness for a surface. */
+export interface FreshnessPolicy {
+  /** Active workspace scope. Snapshots from other scopes are invalidated. */
+  readonly workspace: string;
+  /** Schema version of the current validator. */
+  readonly schemaVersion: number;
+  /** Age after which a verified snapshot degrades from current to stale. */
+  readonly staleAfterMs: number;
+}
+
+export const DEFAULT_FRESHNESS_POLICY: FreshnessPolicy = {
+  workspace: 'default',
+  schemaVersion: 1,
+  staleAfterMs: 60_000,
+};
+
+/** Payload returned by a successful schema validation. */
+export interface FreshPayload<T> {
+  readonly data: T;
+  /**
+   * Workspace identity extracted from the payload itself when the collection
+   * carries one (e.g. a uniform `userId` on projects). `null` when the
+   * collection has no intrinsic workspace identity.
+   */
+  readonly workspace: string | null;
+}
+
+/** Error thrown when a mutation is attempted on non-current data. */
+export class StaleMutationError extends Error {
+  readonly freshness: FreshnessState;
+
+  constructor(freshness: FreshnessState) {
+    super(`Refused mutation on ${freshness} data: revalidation is required before mutating.`);
+    this.name = 'StaleMutationError';
+    this.freshness = freshness;
+  }
+}
+
+/** Stable JSON digest used for snapshot integrity checks. */
+export function computeDigest(value: unknown): string {
+  // FNV-1a 32-bit over the stable JSON serialization. This is an integrity
+  // check against corruption, not a cryptographic guarantee.
+  let hash = 0x811c9dc5;
+  for (const byte of stableStringify(value)) {
+    hash ^= byte.charCodeAt(0);
+    hash = Math.imul(hash, 0x01000193) >>> 0;
+  }
+  return hash.toString(16).padStart(8, '0');
+}
+
+function stableStringify(value: unknown): string {
+  return serialize(value);
+}
+
+function serialize(value: unknown): string {
+  if (value === null || typeof value !== 'object') return JSON.stringify(value) ?? 'null';
+  if (Array.isArray(value)) return `[${value.map(serialize).join(',')}]`;
+  const entries = Object.entries(value as Record<string, unknown>)
+    .filter(([, item]) => item !== undefined)
+    .sort(([left], [right]) => (left < right ? -1 : left > right ? 1 : 0))
+    .map(([key, item]) => `${JSON.stringify(key)}:${serialize(item)}`);
+  return `{${entries.join(',')}}`;
+}
+
+export type AcceptSnapshotResult<T> =
+  | { readonly outcome: 'accepted'; readonly snapshot: FreshSnapshot<T> }
+  | { readonly outcome: 'invalidated'; readonly reason: InvalidationReason };
+
+export interface AcceptSnapshotOptions<T> {
+  /** Raw fetched value (untrusted JSON). */
+  readonly value: unknown;
+  /** Schema validator; returns `null` when the value does not match. */
+  readonly validate: (value: unknown) => FreshPayload<T> | null;
+  /** Previously accepted snapshot for this surface, if any. */
+  readonly previous: FreshSnapshot<T> | null;
+  readonly policy: FreshnessPolicy;
+  readonly source: string;
+  /**
+   * Version carried by the incoming payload when the transport exposes one.
+   * Must not regress below the accepted snapshot's version.
+   */
+  readonly incomingVersion?: number;
+  readonly now: number;
+}
+
+/**
+ * Validate and accept a fetched value as a snapshot, or invalidate it.
+ *
+ * Invalidation rules (each treated as unavailable, never rendered current):
+ * - schema mismatch: the payload fails validation
+ * - cross-workspace: the payload's workspace differs from the verified one
+ * - version regression: payload/schema version is below the accepted one
+ */
+export function acceptSnapshot<T>(options: AcceptSnapshotOptions<T>): AcceptSnapshotResult<T> {
+  const payload = options.validate(options.value);
+  if (payload === null) {
+    return { outcome: 'invalidated', reason: 'schema-mismatch' };
+  }
+
+  // Workspace identity: the payload's own scope wins; a collection with no
+  // intrinsic identity (e.g. an empty list after every project was deleted)
+  // keeps the previously verified scope rather than resetting to the policy
+  // default, so a legitimately empty response is not mistaken for a scope
+  // change.
+  const workspace = payload.workspace ?? options.previous?.workspace ?? options.policy.workspace;
+  if (options.previous !== null && options.previous.workspace !== workspace) {
+    return { outcome: 'invalidated', reason: 'cross-workspace' };
+  }
+  if (options.previous !== null && options.policy.schemaVersion < options.previous.schemaVersion) {
+    return { outcome: 'invalidated', reason: 'version-regression' };
+  }
+  if (
+    options.incomingVersion !== undefined &&
+    options.previous !== null &&
+    options.incomingVersion < options.previous.version
+  ) {
+    return { outcome: 'invalidated', reason: 'version-regression' };
+  }
+
+  const snapshot: FreshSnapshot<T> = {
+    data: payload.data,
+    source: options.source,
+    workspace,
+    version: options.incomingVersion ?? (options.previous?.version ?? 0) + 1,
+    schemaVersion: options.policy.schemaVersion,
+    fetchedAt: options.now,
+    digest: computeDigest(payload.data),
+  };
+  return { outcome: 'accepted', snapshot };
+}
+
+export interface ComputeFreshnessOptions {
+  readonly snapshot: FreshSnapshot<unknown> | null;
+  readonly policy: FreshnessPolicy;
+  readonly now: number;
+  /**
+   * True when the snapshot cannot be trusted as current regardless of age:
+   * the latest revalidation failed, or the snapshot was restored from cache
+   * and has not been verified by a fetch in this session.
+   */
+  readonly degraded?: boolean;
+}
+
+/**
+ * Compute the freshness state of a snapshot. A missing snapshot is
+ * `unavailable` (never "empty and healthy"); a degraded or aged snapshot is
+ * `stale` (situational awareness only).
+ */
+export function computeFreshness(options: ComputeFreshnessOptions): FreshnessState {
+  const { snapshot, policy, now, degraded = false } = options;
+  if (snapshot === null) return 'unavailable';
+  if (degraded) return 'stale';
+  if (now - snapshot.fetchedAt > policy.staleAfterMs) return 'stale';
+  return 'current';
+}
+
+/** Only verified-current data may back a state-changing action. */
+export function canMutate(state: FreshnessState): boolean {
+  return state === 'current';
+}
+
+/** Defense in depth: reject the mutation call itself on non-current data. */
+export function assertMutable(state: FreshnessState): void {
+  if (!canMutate(state)) {
+    throw new StaleMutationError(state);
+  }
+}
+
+/**
+ * Combine freshness across a multi-collection surface (primary + secondaries).
+ * The primary collection gates the surface: unknown while it loads,
+ * unavailable when it fails. Missing secondaries degrade the surface to
+ * `partial`; aged collections degrade it to `stale`.
+ */
+export function combineFreshness(
+  primary: FreshnessState,
+  secondaries: readonly FreshnessState[],
+): FreshnessState {
+  if (primary === 'unavailable') return 'unavailable';
+  if (primary === 'unknown') return 'unknown';
+  if (secondaries.includes('unavailable')) return 'partial';
+  if (secondaries.includes('unknown')) return 'unknown';
+  if (secondaries.includes('stale') || primary === 'stale') return 'stale';
+  if (secondaries.includes('partial')) return 'partial';
+  return 'current';
+}
+
+/** Render-safe age label for snapshot provenance. */
+export function formatAge(fetchedAt: number, now: number): string {
+  const ageMs = Math.max(0, now - fetchedAt);
+  if (ageMs < 10_000) return 'just now';
+  const minutes = Math.floor(ageMs / 60_000);
+  if (minutes < 1) return 'under a minute ago';
+  if (minutes < 60) return `${minutes}m ago`;
+  const hours = Math.floor(minutes / 60);
+  if (hours < 24) return `${hours}h ago`;
+  const days = Math.floor(hours / 24);
+  return `${days}d ago`;
+}
+
+/** Derived verdict placeholder for non-current inputs — never a green value. */
+export const UNKNOWN_VERDICT = '?';
+
+export function verdictValue(verified: boolean, value: string): string {
+  return verified ? value : UNKNOWN_VERDICT;
+}

apps/web/src/lib/freshness/snapshot-cache.spec.ts

@@ -0,0 +1,197 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { acceptSnapshot, DEFAULT_FRESHNESS_POLICY } from './model';
+import { clearSnapshotCache, readSnapshotCache, writeSnapshotCache } from './snapshot-cache';
+import { validateProjectCollection, validateTaskCollection } from './validators';
+import { projectFixtures, taskFixtures } from '@/spa/pages/page-fixtures';
+import type { Project, Task } from '@/lib/types';
+
+const KEY = 'test:tasks';
+const NOW = 1_800_000_000_000;
+const policy = { ...DEFAULT_FRESHNESS_POLICY, staleAfterMs: 60_000 };
+
+function storedTaskSnapshot() {
+  const result = acceptSnapshot({
+    value: taskFixtures,
+    validate: validateTaskCollection,
+    previous: null,
+    policy,
+    source: 'gateway:/api/tasks',
+    now: NOW,
+  });
+  if (result.outcome !== 'accepted') throw new Error('fixture setup failed');
+  return result.snapshot;
+}
+
+function storedProjectSnapshot() {
+  const result = acceptSnapshot({
+    value: projectFixtures,
+    validate: validateProjectCollection,
+    previous: null,
+    policy,
+    source: 'gateway:/api/projects',
+    now: NOW,
+  });
+  if (result.outcome !== 'accepted') throw new Error('fixture setup failed');
+  return result.snapshot;
+}
+
+function readTasks() {
+  return readSnapshotCache({
+    key: KEY,
+    workspace: policy.workspace,
+    policy,
+    validate: validateTaskCollection,
+  });
+}
+
+/** Write an arbitrary value directly at the raw cache slot. */
+function writeRaw(key: string, value: unknown): void {
+  sessionStorage.setItem(`mosaic:freshness:v1:${key}`, JSON.stringify(value));
+}
+
+/** Parse and re-write the stored entry (for tampering with internals). */
+function tamperStored<T>(key: string, mutate: (stored: T) => void): void {
+  const parsed = JSON.parse(sessionStorage.getItem(`mosaic:freshness:v1:${key}`) ?? '{}') as T;
+  mutate(parsed);
+  writeRaw(key, parsed);
+}
+
+beforeEach(() => {
+  sessionStorage.clear();
+});
+
+afterEach(() => {
+  sessionStorage.clear();
+});
+
+describe('readSnapshotCache', () => {
+  it('misses when nothing is stored', () => {
+    expect(readTasks()).toEqual({ outcome: 'miss' });
+  });
+
+  it('hits for a well-formed entry and preserves provenance', () => {
+    const snapshot = storedTaskSnapshot();
+    writeSnapshotCache(KEY, snapshot);
+
+    const result = readTasks();
+    expect(result.outcome).toBe('hit');
+    if (result.outcome === 'hit') {
+      expect(result.snapshot.data).toEqual(taskFixtures);
+      expect(result.snapshot.source).toBe('gateway:/api/tasks');
+      expect(result.snapshot.version).toBe(snapshot.version);
+      expect(result.snapshot.fetchedAt).toBe(snapshot.fetchedAt);
+      expect(result.snapshot.workspace).toBe(snapshot.workspace);
+    }
+  });
+
+  it('invalidates unparsable entries as cache corruption', () => {
+    sessionStorage.setItem(`mosaic:freshness:v1:${KEY}`, '{not json');
+    expect(readTasks()).toEqual({ outcome: 'invalidated', reason: 'cache-corruption' });
+  });
+
+  it('invalidates structurally wrong entries as cache corruption', () => {
+    const malformed: unknown[] = [
+      'nested but not a snapshot',
+      { data: taskFixtures }, // missing provenance fields
+      {
+        data: taskFixtures,
+        source: 1,
+        workspace: 'w',
+        version: 1,
+        schemaVersion: 1,
+        fetchedAt: 1,
+        digest: 'x',
+      },
+      null,
+      17,
+    ];
+    for (const entry of malformed) {
+      writeRaw(KEY, entry);
+      expect(readTasks()).toEqual({ outcome: 'invalidated', reason: 'cache-corruption' });
+    }
+  });
+
+  it('invalidates digest mismatches as cache corruption (tampered data)', () => {
+    writeSnapshotCache(KEY, storedTaskSnapshot());
+    tamperStored<{ data: Task[] }>(KEY, (stored) => {
+      stored.data = [...stored.data, { ...stored.data[0]!, id: 'injected-task' }];
+    });
+    expect(readTasks()).toEqual({ outcome: 'invalidated', reason: 'cache-corruption' });
+  });
+
+  it('invalidates entries scoped to another workspace', () => {
+    const snapshot = storedTaskSnapshot();
+    writeSnapshotCache(KEY, { ...snapshot, workspace: 'someone-else' });
+    expect(readTasks()).toEqual({ outcome: 'invalidated', reason: 'cross-workspace' });
+  });
+
+  it('invalidates entries written by a newer schema as a version regression', () => {
+    const snapshot = storedTaskSnapshot();
+    writeSnapshotCache(KEY, { ...snapshot, schemaVersion: policy.schemaVersion + 1 });
+    expect(readTasks()).toEqual({ outcome: 'invalidated', reason: 'version-regression' });
+  });
+
+  it('invalidates entries whose data no longer validates (schema mismatch)', () => {
+    writeSnapshotCache(KEY, storedTaskSnapshot());
+    tamperStored<{ data: unknown }>(KEY, (stored) => {
+      stored.data = { malformed: true };
+    });
+    expect(readTasks()).toEqual({ outcome: 'invalidated', reason: 'schema-mismatch' });
+  });
+
+  it('never reports a corrupted raw entry as a hit (negative control)', () => {
+    for (const raw of ['{oops', 'null', '"string"', '[]', '12']) {
+      sessionStorage.setItem(`mosaic:freshness:v1:${KEY}`, raw);
+      const result = readTasks();
+      expect(result.outcome).not.toBe('hit');
+      expect(result.outcome).toBe('invalidated');
+    }
+  });
+
+  it('scopes project collections by their workspace identity', () => {
+    const snapshot = storedProjectSnapshot();
+    writeSnapshotCache('test:projects', snapshot);
+
+    const sameScope = readSnapshotCache({
+      key: 'test:projects',
+      workspace: 'user-1',
+      policy,
+      validate: validateProjectCollection,
+    });
+    expect(sameScope.outcome).toBe('hit');
+
+    const foreignScope = readSnapshotCache({
+      key: 'test:projects',
+      workspace: 'user-2',
+      policy,
+      validate: validateProjectCollection,
+    });
+    expect(foreignScope).toEqual({ outcome: 'invalidated', reason: 'cross-workspace' });
+  });
+});
+
+describe('writeSnapshotCache round-trip', () => {
+  it('round-trips an accepted project snapshot', () => {
+    const snapshot = storedProjectSnapshot();
+    writeSnapshotCache('test:projects', snapshot);
+    const result = readSnapshotCache({
+      key: 'test:projects',
+      workspace: snapshot.workspace,
+      policy,
+      validate: validateProjectCollection,
+    });
+    expect(result.outcome).toBe('hit');
+    if (result.outcome === 'hit') {
+      expect(result.snapshot.data).toEqual(projectFixtures as Project[]);
+    }
+  });
+});
+
+describe('clearSnapshotCache', () => {
+  it('drops the entry so the next read misses', () => {
+    writeSnapshotCache(KEY, storedTaskSnapshot());
+    expect(readTasks().outcome).toBe('hit');
+    clearSnapshotCache(KEY);
+    expect(readTasks()).toEqual({ outcome: 'miss' });
+  });
+});

apps/web/src/lib/freshness/snapshot-cache.ts

@@ -0,0 +1,154 @@
+import {
+  computeDigest,
+  type FreshPayload,
+  type FreshSnapshot,
+  type FreshnessPolicy,
+  type InvalidationReason,
+} from './model';
+
+/**
+ * Session-scoped last-known snapshot cache (RI-5-001).
+ *
+ * Restored snapshots are situational awareness only: they surface as `stale`
+ * until a fetch re-verifies them. A cache entry that is corrupted, belongs to
+ * another workspace, was written by a newer schema, or no longer validates is
+ * invalidated (treated as unavailable, never rendered as current).
+ */
+
+const CACHE_PREFIX = 'mosaic:freshness:v1';
+
+interface StoredSnapshot {
+  data: unknown;
+  source: string;
+  workspace: string;
+  version: number;
+  schemaVersion: number;
+  fetchedAt: number;
+  digest: string;
+}
+
+export type SnapshotCacheRead<T> =
+  | { readonly outcome: 'hit'; readonly snapshot: FreshSnapshot<T> }
+  | { readonly outcome: 'miss' }
+  | { readonly outcome: 'invalidated'; readonly reason: InvalidationReason };
+
+export interface ReadSnapshotCacheOptions<T> {
+  readonly key: string;
+  readonly workspace: string;
+  readonly policy: FreshnessPolicy;
+  readonly validate: (value: unknown) => FreshPayload<T> | null;
+}
+
+function cacheKey(key: string): string {
+  return `${CACHE_PREFIX}:${key}`;
+}
+
+function isStoredSnapshot(value: unknown): value is StoredSnapshot {
+  if (typeof value !== 'object' || value === null) return false;
+  const candidate = value as Record<string, unknown>;
+  return (
+    typeof candidate['data'] === 'object' &&
+    candidate['data'] !== null &&
+    typeof candidate['source'] === 'string' &&
+    typeof candidate['workspace'] === 'string' &&
+    typeof candidate['version'] === 'number' &&
+    typeof candidate['schemaVersion'] === 'number' &&
+    typeof candidate['fetchedAt'] === 'number' &&
+    typeof candidate['digest'] === 'string'
+  );
+}
+
+function getStorage(): Storage | null {
+  try {
+    return globalThis.sessionStorage ?? null;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Restore a cached snapshot under the active workspace scope. Every failure
+ * mode maps to an explicit invalidation reason or a miss — never to data
+ * that renders as current.
+ */
+export function readSnapshotCache<T>(options: ReadSnapshotCacheOptions<T>): SnapshotCacheRead<T> {
+  const storage = getStorage();
+  if (storage === null) return { outcome: 'miss' };
+
+  let raw: string | null;
+  try {
+    raw = storage.getItem(cacheKey(options.key));
+  } catch {
+    return { outcome: 'miss' };
+  }
+  if (raw === null) return { outcome: 'miss' };
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw);
+  } catch {
+    return { outcome: 'invalidated', reason: 'cache-corruption' };
+  }
+  if (!isStoredSnapshot(parsed)) {
+    return { outcome: 'invalidated', reason: 'cache-corruption' };
+  }
+  if (parsed.workspace !== options.workspace) {
+    return { outcome: 'invalidated', reason: 'cross-workspace' };
+  }
+  if (parsed.schemaVersion > options.policy.schemaVersion) {
+    // Written by a newer build than the running client: version regression.
+    return { outcome: 'invalidated', reason: 'version-regression' };
+  }
+
+  const payload = options.validate(parsed.data);
+  if (payload === null) {
+    return { outcome: 'invalidated', reason: 'schema-mismatch' };
+  }
+  if (computeDigest(payload.data) !== parsed.digest) {
+    return { outcome: 'invalidated', reason: 'cache-corruption' };
+  }
+
+  return {
+    outcome: 'hit',
+    snapshot: {
+      data: payload.data,
+      source: parsed.source,
+      workspace: parsed.workspace,
+      version: parsed.version,
+      schemaVersion: parsed.schemaVersion,
+      fetchedAt: parsed.fetchedAt,
+      digest: parsed.digest,
+    },
+  };
+}
+
+/** Persist a verified snapshot. Failures are non-fatal (cache is best-effort). */
+export function writeSnapshotCache<T>(key: string, snapshot: FreshSnapshot<T>): void {
+  const storage = getStorage();
+  if (storage === null) return;
+  const stored: StoredSnapshot = {
+    data: snapshot.data,
+    source: snapshot.source,
+    workspace: snapshot.workspace,
+    version: snapshot.version,
+    schemaVersion: snapshot.schemaVersion,
+    fetchedAt: snapshot.fetchedAt,
+    digest: snapshot.digest,
+  };
+  try {
+    storage.setItem(cacheKey(key), JSON.stringify(stored));
+  } catch {
+    // Quota or serialization failures simply skip caching.
+  }
+}
+
+/** Drop a cached snapshot (used when a surface invalidates its cache entry). */
+export function clearSnapshotCache(key: string): void {
+  const storage = getStorage();
+  if (storage === null) return;
+  try {
+    storage.removeItem(cacheKey(key));
+  } catch {
+    // Ignorable: a wedged storage entry is detected as corruption on read.
+  }
+}

apps/web/src/lib/freshness/use-fresh-collection.spec.tsx

@@ -0,0 +1,372 @@
+import { act } from 'react';
+import { createRoot, type Root } from 'react-dom/client';
+import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from 'vitest';
+import type { Task } from '@/lib/types';
+import { acceptSnapshot, StaleMutationError, DEFAULT_FRESHNESS_POLICY } from './model';
+import type { FreshnessFailure } from './use-fresh-collection';
+import {
+  describeFailure,
+  useFreshCollection,
+  type FreshCollection,
+  type UseFreshCollectionOptions,
+} from './use-fresh-collection';
+import { validateProjectCollection, validateTaskCollection } from './validators';
+import { projectFixtures, taskFixtures } from '@/spa/pages/page-fixtures';
+
+/**
+ * Failure-matrix coverage for the freshness seam (RI-5-001): network failure,
+ * auth failure, malformed response, cache corruption, stale age, schema
+ * mismatch, cross-workspace, recovery, and stale-action rejection — with
+ * negative controls proving no case yields current data or an enabled
+ * mutation.
+ */
+
+const NOW = 1_800_000_000_000;
+
+interface Deferred<T> {
+  promise: Promise<T>;
+  resolve: (value: T) => void;
+  reject: (reason?: unknown) => void;
+}
+
+function createDeferred<T>(): Deferred<T> {
+  let resolve!: (value: T) => void;
+  let reject!: (reason?: unknown) => void;
+  const promise = new Promise<T>((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+  return { promise, resolve, reject };
+}
+
+let root: Root | null = null;
+let container: HTMLDivElement;
+let latest: FreshCollection<Task[]> | null = null;
+
+function Probe({
+  options,
+}: {
+  options: UseFreshCollectionOptions<Task[]>;
+}): React.ReactElement | null {
+  latest = useFreshCollection<Task[]>(options);
+  return null;
+}
+
+beforeAll(() => {
+  Object.defineProperty(globalThis, 'IS_REACT_ACT_ENVIRONMENT', {
+    configurable: true,
+    value: true,
+  });
+});
+
+beforeEach(() => {
+  sessionStorage.clear();
+});
+
+afterEach(async () => {
+  await act(async () => {
+    root?.unmount();
+  });
+  document.body.replaceChildren();
+  root = null;
+  latest = null;
+  sessionStorage.clear();
+  vi.restoreAllMocks();
+});
+
+async function renderCollection(
+  options: UseFreshCollectionOptions<Task[]>,
+): Promise<FreshCollection<Task[]>> {
+  container = document.createElement('div');
+  document.body.append(container);
+  root = createRoot(container);
+  await act(async () => {
+    root?.render(<Probe options={options} />);
+  });
+  if (latest === null) throw new Error('hook did not run');
+  return latest;
+}
+
+function taskOptions(
+  overrides: Partial<UseFreshCollectionOptions<Task[]>> = {},
+): UseFreshCollectionOptions<Task[]> {
+  return {
+    source: 'gateway:/api/tasks',
+    fetcher: () => Promise.resolve(taskFixtures),
+    validate: validateTaskCollection,
+    cacheKey: 'tasks',
+    clock: () => NOW,
+    ...overrides,
+  };
+}
+
+function authError(statusCode: number): Error & { statusCode: number } {
+  return Object.assign(new Error(`Request failed with ${statusCode}`), { statusCode });
+}
+
+function seedCache(key: string): number {
+  const result = acceptSnapshot({
+    value: taskFixtures,
+    validate: validateTaskCollection,
+    previous: null,
+    policy: DEFAULT_FRESHNESS_POLICY,
+    source: 'gateway:/api/tasks',
+    now: NOW,
+  });
+  if (result.outcome !== 'accepted') throw new Error('fixture setup failed');
+  sessionStorage.setItem(`mosaic:freshness:v1:${key}`, JSON.stringify({ ...result.snapshot }));
+  return result.snapshot.version;
+}
+
+describe('useFreshCollection failure matrix', () => {
+  it('is unknown (not empty) while the first validation is in flight', async () => {
+    const deferred = createDeferred<Task[]>();
+    const collection = await renderCollection(taskOptions({ fetcher: () => deferred.promise }));
+
+    expect(collection.freshness).toBe('unknown');
+    expect(collection.validating).toBe(true);
+    expect(collection.data).toBeNull();
+    expect(collection.canMutate).toBe(false);
+
+    await act(async () => {
+      deferred.resolve(taskFixtures);
+      await deferred.promise;
+    });
+  });
+
+  it('becomes current with provenance after a verified fetch', async () => {
+    const collection = await renderCollection(taskOptions());
+
+    expect(collection.freshness).toBe('current');
+    expect(collection.data).toEqual(taskFixtures);
+    expect(collection.snapshot?.source).toBe('gateway:/api/tasks');
+    expect(collection.snapshot?.version).toBe(1);
+    expect(collection.failure).toBeNull();
+    expect(collection.canMutate).toBe(true);
+    // Verified snapshot is persisted for last-known restore.
+    expect(sessionStorage.getItem('mosaic:freshness:v1:tasks')).toBeTruthy();
+  });
+
+  it('treats a network failure as unavailable — never an empty healthy collection', async () => {
+    const collection = await renderCollection(
+      taskOptions({ fetcher: () => Promise.reject(new Error('network down')) }),
+    );
+
+    expect(collection.freshness).toBe('unavailable');
+    expect(collection.data).toBeNull();
+    expect(collection.failure).toEqual({ kind: 'fetch', message: 'network down' });
+    expect(collection.canMutate).toBe(false);
+    expect(describeFailure(collection.failure)).toBe('network down');
+  });
+
+  it('treats an auth failure as unavailable and drops the last-known snapshot', async () => {
+    let call = 0;
+    const collection = await renderCollection(
+      taskOptions({
+        fetcher: () => {
+          call += 1;
+          return call === 1 ? Promise.resolve(taskFixtures) : Promise.reject(authError(401));
+        },
+      }),
+    );
+    expect(collection.freshness).toBe('current');
+
+    await act(async () => {
+      await collection.revalidate();
+    });
+
+    expect(latest?.freshness).toBe('unavailable');
+    expect(latest?.data).toBeNull();
+    expect(latest?.failure?.kind).toBe('fetch');
+    // The previous user's data must not linger in the session cache.
+    expect(sessionStorage.getItem('mosaic:freshness:v1:tasks')).toBeNull();
+  });
+
+  it('invalidates a malformed response as a schema mismatch', async () => {
+    const collection = await renderCollection(
+      taskOptions({ fetcher: () => Promise.resolve({ malformed: true }) }),
+    );
+
+    expect(collection.freshness).toBe('unavailable');
+    expect(collection.data).toBeNull();
+    expect(collection.failure).toEqual({ kind: 'invalidated', reason: 'schema-mismatch' });
+    expect(collection.canMutate).toBe(false);
+  });
+
+  it('keeps the previous snapshot as labeled stale when a later payload mismatches', async () => {
+    let call = 0;
+    const collection = await renderCollection(
+      taskOptions({
+        fetcher: () => {
+          call += 1;
+          return call === 1 ? Promise.resolve(taskFixtures) : Promise.resolve('garbage');
+        },
+      }),
+    );
+    expect(collection.freshness).toBe('current');
+
+    await act(async () => {
+      await collection.revalidate();
+    });
+
+    expect(latest?.freshness).toBe('stale');
+    expect(latest?.data).toEqual(taskFixtures);
+    expect(latest?.failure).toEqual({ kind: 'invalidated', reason: 'schema-mismatch' });
+    expect(latest?.canMutate).toBe(false);
+  });
+
+  it('drops the snapshot when the workspace changes under it (cross-workspace)', async () => {
+    let call = 0;
+    const collection = await renderCollection(
+      taskOptions({
+        fetcher: () => {
+          call += 1;
+          return Promise.resolve(
+            call === 1 ? projectFixtures : [{ ...projectFixtures[0], userId: 'user-2' }],
+          );
+        },
+        validate: validateProjectCollection as unknown as (value: unknown) => {
+          data: Task[];
+          workspace: string | null;
+        },
+        source: 'gateway:/api/projects',
+      }),
+    );
+    expect(collection.freshness).toBe('current');
+
+    await act(async () => {
+      await collection.revalidate();
+    });
+
+    expect(latest?.freshness).toBe('unavailable');
+    expect(latest?.data).toBeNull();
+    expect(latest?.failure).toEqual({ kind: 'invalidated', reason: 'cross-workspace' });
+  });
+
+  it('ages from current to stale and refuses mutations on stale data', async () => {
+    let fakeNow = NOW;
+    const collection = await renderCollection(
+      taskOptions({
+        clock: () => fakeNow,
+        policy: { staleAfterMs: 40 },
+        tickMs: 10,
+      }),
+    );
+    expect(collection.freshness).toBe('current');
+
+    // Age the snapshot past the policy and let the tick recompute.
+    fakeNow = NOW + 60;
+    await act(async () => {
+      await new Promise((resolve) => setTimeout(resolve, 25));
+    });
+
+    expect(latest?.freshness).toBe('stale');
+    expect(latest?.data).toEqual(taskFixtures);
+    expect(latest?.canMutate).toBe(false);
+
+    const operation = vi.fn(async () => 'result');
+    await expect(latest?.mutate(operation)).rejects.toBeInstanceOf(StaleMutationError);
+    expect(operation).not.toHaveBeenCalled();
+  });
+
+  it('recovers to current after a successful revalidation', async () => {
+    let call = 0;
+    const collection = await renderCollection(
+      taskOptions({
+        fetcher: () => {
+          call += 1;
+          return call === 1
+            ? Promise.reject(new Error('first attempt failed'))
+            : Promise.resolve(taskFixtures);
+        },
+      }),
+    );
+    expect(collection.freshness).toBe('unavailable');
+
+    await act(async () => {
+      await collection.revalidate();
+    });
+
+    expect(latest?.freshness).toBe('current');
+    expect(latest?.failure).toBeNull();
+
+    const operation = vi.fn(async (data: Task[]) => data.length);
+    await expect(latest?.mutate(operation)).resolves.toBe(taskFixtures.length);
+    expect(operation).toHaveBeenCalledOnce();
+  });
+
+  it('restores a cached snapshot as unverified stale data, then verifies it', async () => {
+    const seededVersion = seedCache('tasks');
+    const deferred = createDeferred<Task[]>();
+    const collection = await renderCollection(taskOptions({ fetcher: () => deferred.promise }));
+
+    // Restored data is situational awareness only: labeled stale, never
+    // current, and mutations are refused before verification.
+    expect(collection.freshness).toBe('stale');
+    expect(collection.data).toEqual(taskFixtures);
+    expect(collection.canMutate).toBe(false);
+    await expect(collection.mutate(vi.fn())).rejects.toBeInstanceOf(StaleMutationError);
+
+    await act(async () => {
+      deferred.resolve(taskFixtures);
+      await deferred.promise;
+    });
+
+    expect(latest?.freshness).toBe('current');
+    expect(latest?.snapshot?.version).toBe(seededVersion + 1);
+  });
+
+  it('never promotes corrupted cache data to current (cache corruption)', async () => {
+    sessionStorage.setItem('mosaic:freshness:v1:tasks', '{"data":');
+    const collection = await renderCollection(
+      taskOptions({ fetcher: () => Promise.reject(new Error('still down')) }),
+    );
+
+    expect(collection.freshness).toBe('unavailable');
+    expect(collection.data).toBeNull();
+    expect(collection.canMutate).toBe(false);
+    // The corrupted entry is dropped so it cannot come back.
+    expect(sessionStorage.getItem('mosaic:freshness:v1:tasks')).toBeNull();
+  });
+
+  it('refuses mutations while unknown or unavailable — the call itself, not just the button', async () => {
+    const deferred = createDeferred<Task[]>();
+    const unknown = await renderCollection(taskOptions({ fetcher: () => deferred.promise }));
+    const operation = vi.fn(async () => 'result');
+    await expect(unknown.mutate(operation)).rejects.toBeInstanceOf(StaleMutationError);
+    expect(operation).not.toHaveBeenCalled();
+    await act(async () => {
+      deferred.reject(new Error('failed'));
+      await deferred.promise.catch(() => undefined);
+    });
+
+    const unavailable = latest!;
+    await expect(unavailable.mutate(operation)).rejects.toBeInstanceOf(StaleMutationError);
+    expect(operation).not.toHaveBeenCalled();
+    expect(unavailable.canMutate).toBe(false);
+  });
+
+  it('degrades to stale with last-known data when a revalidation fails after success', async () => {
+    let call = 0;
+    const collection = await renderCollection(
+      taskOptions({
+        fetcher: () => {
+          call += 1;
+          return call === 1
+            ? Promise.resolve(taskFixtures)
+            : Promise.reject(new Error('connection lost'));
+        },
+      }),
+    );
+    expect(collection.freshness).toBe('current');
+
+    await act(async () => {
+      await collection.revalidate();
+    });
+
+    expect(latest?.freshness).toBe('stale');
+    expect(latest?.data).toEqual(taskFixtures);
+    const failure: FreshnessFailure | null = latest?.failure ?? null;
+    expect(failure).toEqual({ kind: 'fetch', message: 'connection lost' });
+  });
+});

apps/web/src/lib/freshness/use-fresh-collection.ts

@@ -0,0 +1,281 @@
+import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
+import {
+  acceptSnapshot,
+  assertMutable,
+  computeFreshness,
+  DEFAULT_FRESHNESS_POLICY,
+  invalidationReasonLabels,
+  type FreshPayload,
+  type FreshSnapshot,
+  type FreshnessPolicy,
+  type FreshnessState,
+  type InvalidationReason,
+  StaleMutationError,
+} from './model';
+import { clearSnapshotCache, readSnapshotCache, writeSnapshotCache } from './snapshot-cache';
+
+/**
+ * Freshness-aware collection fetch hook (RI-5-001).
+ *
+ * One hook owns one gateway collection end to end: fetch, schema validation,
+ * snapshot acceptance with provenance, session-scoped last-known caching,
+ * aging, and the mutation guard. Pages consume `freshness` and never infer
+ * health from emptiness.
+ */
+
+/** Why the latest validation did not produce a current snapshot. */
+export type FreshnessFailure =
+  | { readonly kind: 'fetch'; readonly message: string }
+  | { readonly kind: 'invalidated'; readonly reason: InvalidationReason };
+
+export interface UseFreshCollectionOptions<T> {
+  /** Source identity for provenance labels, e.g. `gateway:/api/tasks`. */
+  readonly source: string;
+  /** Performs the unvalidated fetch. The hook owns abort and verification. */
+  readonly fetcher: (signal: AbortSignal) => Promise<unknown>;
+  /**
+   * Runtime schema validator. Returning `null` invalidates the payload
+   * (`schema-mismatch`) instead of letting malformed JSON flow into render.
+   */
+  readonly validate: (value: unknown) => FreshPayload<T> | null;
+  /** Overrides of the default freshness policy. */
+  readonly policy?: Partial<FreshnessPolicy>;
+  /**
+   * Session cache key for last-known snapshots. `null`/omitted disables
+   * restore. Restored snapshots are unverified: they render only as
+   * labeled `stale` data until a fetch re-verifies them.
+   */
+  readonly cacheKey?: string | null;
+  /** Injectable clock for deterministic age transitions in tests. */
+  readonly clock?: () => number;
+  /** Aging tick interval override (default derived from `staleAfterMs`). */
+  readonly tickMs?: number;
+  /** When false, no fetch runs (surfaces stay `unavailable`/`unknown`). */
+  readonly enabled?: boolean;
+}
+
+export interface FreshCollection<T> {
+  /** Last verified (or restored-unverified) snapshot, or `null`. */
+  readonly snapshot: FreshSnapshot<T> | null;
+  /** Snapshot data or `null` — never a fabricated empty collection. */
+  readonly data: T | null;
+  readonly freshness: FreshnessState;
+  /** True while a validation request is in flight. */
+  readonly validating: boolean;
+  /** Outcome of the latest failed validation, `null` when healthy. */
+  readonly failure: FreshnessFailure | null;
+  /** False unless freshness is `current`; drives disabled UI affordances. */
+  readonly canMutate: boolean;
+  /** Re-run the fetch and re-verify. Always allowed (it is a read). */
+  readonly revalidate: () => Promise<void>;
+  /**
+   * Run a state-changing operation against verified-current data only.
+   * Rejects with `StaleMutationError` on any other state — the guard fires
+   * even if a disabled button was bypassed (defense in depth).
+   */
+  readonly mutate: <R>(operation: (data: T) => Promise<R>) => Promise<R>;
+}
+
+const defaultClock = (): number => Date.now();
+
+function resolveTickMs(policy: FreshnessPolicy, override?: number): number {
+  if (override !== undefined && override > 0) return override;
+  return Math.min(5_000, Math.max(250, Math.floor(policy.staleAfterMs / 4)));
+}
+
+function isAuthFailure(caught: unknown): boolean {
+  return (
+    typeof caught === 'object' &&
+    caught !== null &&
+    'statusCode' in caught &&
+    ((caught as { statusCode?: unknown }).statusCode === 401 ||
+      (caught as { statusCode?: unknown }).statusCode === 403)
+  );
+}
+
+function fetchFailureMessage(caught: unknown): string {
+  if (caught instanceof Error && caught.message.trim().length > 0) return caught.message;
+  return 'The request failed.';
+}
+
+/** Human-readable summary of a failure for unavailable/stale notices. */
+export function describeFailure(failure: FreshnessFailure | null): string | null {
+  if (failure === null) return null;
+  if (failure.kind === 'fetch') return failure.message;
+  return `The snapshot was invalidated: ${invalidationReasonLabels[failure.reason]}.`;
+}
+
+export function useFreshCollection<T>(options: UseFreshCollectionOptions<T>): FreshCollection<T> {
+  const optionsRef = useRef(options);
+  optionsRef.current = options;
+
+  const policy = useMemo<FreshnessPolicy>(
+    () => ({ ...DEFAULT_FRESHNESS_POLICY, ...options.policy }),
+    [options.policy],
+  );
+  const policyRef = useRef(policy);
+  policyRef.current = policy;
+
+  const clockRef = useRef(options.clock ?? defaultClock);
+  clockRef.current = options.clock ?? defaultClock;
+
+  const [snapshot, setSnapshot] = useState<FreshSnapshot<T> | null>(null);
+  const [failure, setFailure] = useState<FreshnessFailure | null>(null);
+  const [unverified, setUnverified] = useState(false);
+  const [validating, setValidating] = useState(options.enabled !== false);
+  const [now, setNow] = useState(() => (options.clock ?? defaultClock)());
+
+  const snapshotRef = useRef(snapshot);
+  snapshotRef.current = snapshot;
+  const failureRef = useRef(failure);
+  failureRef.current = failure;
+  const unverifiedRef = useRef(unverified);
+  unverifiedRef.current = unverified;
+
+  const runRef = useRef(0);
+  const abortRef = useRef<AbortController | null>(null);
+
+  const revalidate = useCallback(async (): Promise<void> => {
+    const current = optionsRef.current;
+    if (current.enabled === false) {
+      setValidating(false);
+      return;
+    }
+
+    const runId = ++runRef.current;
+    abortRef.current?.abort();
+    const controller = new AbortController();
+    abortRef.current = controller;
+    setValidating(true);
+
+    let value: unknown;
+    try {
+      value = await current.fetcher(controller.signal);
+    } catch (caught) {
+      if (runRef.current !== runId || controller.signal.aborted) return;
+      if (isAuthFailure(caught)) {
+        // An unauthenticated viewer must not keep (or be served) the
+        // previous user's last-known data.
+        setSnapshot(null);
+        setUnverified(false);
+        if (current.cacheKey) clearSnapshotCache(current.cacheKey);
+      }
+      setFailure({ kind: 'fetch', message: fetchFailureMessage(caught) });
+      setValidating(false);
+      return;
+    }
+
+    if (runRef.current !== runId) return;
+
+    const result = acceptSnapshot({
+      value,
+      validate: current.validate,
+      previous: snapshotRef.current,
+      policy: policyRef.current,
+      source: current.source,
+      now: clockRef.current(),
+    });
+
+    if (result.outcome === 'accepted') {
+      setSnapshot(result.snapshot);
+      setUnverified(false);
+      setFailure(null);
+      if (current.cacheKey) writeSnapshotCache(current.cacheKey, result.snapshot);
+    } else {
+      if (result.reason === 'cross-workspace') {
+        // Data verified for a different workspace must not linger as
+        // last-known situational awareness either.
+        setSnapshot(null);
+        setUnverified(false);
+      }
+      if (current.cacheKey) clearSnapshotCache(current.cacheKey);
+      setFailure({ kind: 'invalidated', reason: result.reason });
+    }
+    setValidating(false);
+  }, []);
+
+  // Restore the last-known snapshot (unverified) and run the first fetch.
+  useEffect(() => {
+    if (optionsRef.current.enabled === false) {
+      setValidating(false);
+      return;
+    }
+
+    const cacheKey = optionsRef.current.cacheKey;
+    if (cacheKey) {
+      const restored = readSnapshotCache<T>({
+        key: cacheKey,
+        workspace: policyRef.current.workspace,
+        policy: policyRef.current,
+        validate: optionsRef.current.validate,
+      });
+      if (restored.outcome === 'hit') {
+        setSnapshot(restored.snapshot);
+        setUnverified(true);
+      } else if (restored.outcome === 'invalidated') {
+        // A corrupted/foreign/regressed entry is dropped immediately; it must
+        // never surface as data. The fetch decides the visible state.
+        clearSnapshotCache(cacheKey);
+      }
+    }
+
+    void revalidate();
+
+    return () => {
+      abortRef.current?.abort();
+    };
+    // Mount-once by design: `revalidate` is stable and reads live options
+    // through refs, so it never needs to re-run when options change.
+    // Route-param pages remount this hook via an identity `key` instead.
+  }, [revalidate]);
+
+  // Aging tick: recomputes freshness as the snapshot ages past the policy.
+  useEffect(() => {
+    const interval = setInterval(
+      () => {
+        setNow(clockRef.current());
+      },
+      resolveTickMs(policyRef.current, optionsRef.current.tickMs),
+    );
+    return () => clearInterval(interval);
+  }, []);
+
+  const freshness = useMemo<FreshnessState>(() => {
+    if (snapshot === null) return validating ? 'unknown' : 'unavailable';
+    return computeFreshness({
+      snapshot,
+      policy,
+      now,
+      degraded: failure !== null || unverified,
+    });
+    // `now` from state covers age; refs inside computeFreshness are pure.
+  }, [snapshot, validating, failure, unverified, now, policy]);
+
+  const canMutate = freshness === 'current';
+
+  const mutate = useCallback(async <R>(operation: (data: T) => Promise<R>): Promise<R> => {
+    const currentSnapshot = snapshotRef.current;
+    // No verified snapshot at all: with nothing verified there is nothing
+    // current to mutate, regardless of the recorded failure.
+    if (currentSnapshot === null) throw new StaleMutationError('unavailable');
+    const state = computeFreshness({
+      snapshot: currentSnapshot,
+      policy: policyRef.current,
+      now: clockRef.current(),
+      degraded: failureRef.current !== null || unverifiedRef.current,
+    });
+    assertMutable(state);
+    return operation(currentSnapshot.data);
+  }, []);
+
+  return {
+    snapshot,
+    data: snapshot === null ? null : snapshot.data,
+    freshness,
+    validating,
+    failure,
+    canMutate,
+    revalidate,
+    mutate,
+  };
+}

apps/web/src/lib/freshness/validators.spec.ts

@@ -0,0 +1,103 @@
+import { describe, expect, it } from 'vitest';
+import type { Mission, Project, Task } from '@/lib/types';
+import {
+  validateMissionCollection,
+  validateProjectCollection,
+  validateProjectEntity,
+  validateTaskCollection,
+} from './validators';
+import { missionFixtures, projectFixtures, taskFixtures } from '@/spa/pages/page-fixtures';
+
+describe('validateTaskCollection', () => {
+  it('accepts a well-formed task collection', () => {
+    expect(validateTaskCollection(taskFixtures)).toEqual({
+      data: taskFixtures,
+      workspace: null,
+    });
+  });
+
+  it('accepts an empty collection (a healthy empty state is a valid payload)', () => {
+    expect(validateTaskCollection([])).toEqual({ data: [], workspace: null });
+  });
+
+  it.each([
+    ['not an array', { items: [] }],
+    ['item is not an object', ['nope']],
+    ['missing id', [{ ...(taskFixtures[0] as Task), id: undefined }]],
+    ['missing title', [{ ...(taskFixtures[0] as Task), title: undefined }]],
+    ['unknown status enum', [{ ...(taskFixtures[0] as Task), status: 'finished' }]],
+    ['unknown priority enum', [{ ...(taskFixtures[0] as Task), priority: 'urgent' }]],
+    ['tags of the wrong type', [{ ...(taskFixtures[0] as Task), tags: 'spa' }]],
+    ['metadata of the wrong type', [{ ...(taskFixtures[0] as Task), metadata: 'notes' }]],
+    ['createdAt of the wrong type', [{ ...(taskFixtures[0] as Task), createdAt: 1234 }]],
+    ['null sneaks past a required string', [{ ...(taskFixtures[0] as Task), title: null }]],
+  ])('rejects a malformed payload: %s', (_label, value) => {
+    expect(validateTaskCollection(value)).toBeNull();
+  });
+});
+
+describe('validateMissionCollection', () => {
+  it('accepts a well-formed mission collection', () => {
+    expect(validateMissionCollection(missionFixtures)).toEqual({
+      data: missionFixtures,
+      workspace: null,
+    });
+  });
+
+  it.each([
+    ['not an array', null],
+    ['item missing name', [{ ...(missionFixtures[0] as Mission), name: 42 }]],
+    ['unknown status enum', [{ ...(missionFixtures[0] as Mission), status: 'canceled' }]],
+    ['projectId of the wrong type', [{ ...(missionFixtures[0] as Mission), projectId: 7 }]],
+  ])('rejects a malformed payload: %s', (_label, value) => {
+    expect(validateMissionCollection(value)).toBeNull();
+  });
+});
+
+describe('validateProjectCollection', () => {
+  it('accepts a uniform workspace-scoped collection and reports its workspace', () => {
+    expect(validateProjectCollection(projectFixtures)).toEqual({
+      data: projectFixtures,
+      workspace: 'user-1',
+    });
+  });
+
+  it('accepts an empty collection with no workspace identity', () => {
+    expect(validateProjectCollection([])).toEqual({ data: [], workspace: null });
+  });
+
+  it.each([
+    ['not an array', 42],
+    ['item missing userId', [{ ...(projectFixtures[0] as Project), userId: undefined }]],
+    ['unknown status enum', [{ ...(projectFixtures[0] as Project), status: 'live' }]],
+    ['description of the wrong type', [{ ...(projectFixtures[0] as Project), description: 1 }]],
+  ])('rejects a malformed payload: %s', (_label, value) => {
+    expect(validateProjectCollection(value)).toBeNull();
+  });
+
+  it('rejects a collection mixing workspace identities (cross-workspace leak)', () => {
+    const mixed = [
+      projectFixtures[0] as Project,
+      { ...(projectFixtures[1] as Project), userId: 'user-2' },
+    ];
+    expect(validateProjectCollection(mixed)).toBeNull();
+  });
+});
+
+describe('validateProjectEntity', () => {
+  it('accepts a well-formed project and reports its workspace', () => {
+    expect(validateProjectEntity(projectFixtures[0])).toEqual({
+      data: projectFixtures[0],
+      workspace: 'user-1',
+    });
+  });
+
+  it.each([
+    ['not an object', 'project-1'],
+    ['null', null],
+    ['array', [projectFixtures[0]]],
+    ['missing userId', [{ ...(projectFixtures[0] as Project), userId: null }]],
+  ])('rejects a malformed entity: %s', (_label, value) => {
+    expect(validateProjectEntity(value)).toBeNull();
+  });
+});

apps/web/src/lib/freshness/validators.ts

@@ -0,0 +1,135 @@
+import type { Mission, Project, Task, MissionStatus, TaskPriority, TaskStatus } from '@/lib/types';
+import type { FreshPayload } from './model';
+
+/**
+ * Runtime schema validators for gateway collections (RI-5-001).
+ *
+ * `api<T>()` returns untrusted JSON cast to `T`; these validators are the
+ * seam where a malformed response becomes an explicit schema mismatch
+ * instead of flowing into the render path as if it were healthy data.
+ */
+
+const taskStatuses: readonly TaskStatus[] = [
+  'not-started',
+  'in-progress',
+  'blocked',
+  'done',
+  'cancelled',
+];
+const taskPriorities: readonly TaskPriority[] = ['critical', 'high', 'medium', 'low'];
+const missionStatuses: readonly MissionStatus[] = [
+  'planning',
+  'active',
+  'paused',
+  'completed',
+  'failed',
+];
+const projectStatuses: readonly Project['status'][] = ['active', 'paused', 'completed', 'archived'];
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null && !Array.isArray(value);
+}
+
+function isString(value: unknown): value is string {
+  return typeof value === 'string';
+}
+
+function isNullableString(value: unknown): value is string | null {
+  return value === null || typeof value === 'string';
+}
+
+function isOneOf<T extends string>(value: unknown, allowed: readonly T[]): value is T {
+  return typeof value === 'string' && (allowed as readonly string[]).includes(value);
+}
+
+function isNullableRecord(value: unknown): value is Record<string, unknown> | null {
+  return value === null || isRecord(value);
+}
+
+function isNullableStringArray(value: unknown): value is string[] | null {
+  if (value === null) return true;
+  if (!Array.isArray(value)) return false;
+  return value.every((item) => typeof item === 'string');
+}
+
+function isIsoLike(value: unknown): value is string {
+  return typeof value === 'string' && value.length > 0;
+}
+
+function isTask(value: unknown): value is Task {
+  if (!isRecord(value)) return false;
+  return (
+    isString(value['id']) &&
+    isString(value['title']) &&
+    isOneOf(value['status'], taskStatuses) &&
+    isOneOf(value['priority'], taskPriorities) &&
+    isNullableString(value['projectId']) &&
+    isNullableString(value['missionId']) &&
+    isNullableString(value['assignee']) &&
+    isNullableStringArray(value['tags']) &&
+    isNullableRecord(value['metadata']) &&
+    isNullableString(value['dueDate']) &&
+    isIsoLike(value['createdAt']) &&
+    isIsoLike(value['updatedAt'])
+  );
+}
+
+/** Tasks carry no workspace identity; scope falls back to the policy. */
+export function validateTaskCollection(value: unknown): FreshPayload<Task[]> | null {
+  if (!Array.isArray(value) || !value.every(isTask)) return null;
+  return { data: value as Task[], workspace: null };
+}
+
+function isMission(value: unknown): value is Mission {
+  if (!isRecord(value)) return false;
+  return (
+    isString(value['id']) &&
+    isString(value['name']) &&
+    isOneOf(value['status'], missionStatuses) &&
+    isNullableString(value['projectId']) &&
+    isNullableString(value['description']) &&
+    isNullableRecord(value['metadata']) &&
+    isIsoLike(value['createdAt']) &&
+    isIsoLike(value['updatedAt'])
+  );
+}
+
+/** Missions carry no workspace identity; scope falls back to the policy. */
+export function validateMissionCollection(value: unknown): FreshPayload<Mission[]> | null {
+  if (!Array.isArray(value) || !value.every(isMission)) return null;
+  return { data: value as Mission[], workspace: null };
+}
+
+function isProject(value: unknown): value is Project {
+  if (!isRecord(value)) return false;
+  return (
+    isString(value['id']) &&
+    isString(value['name']) &&
+    isOneOf(value['status'], projectStatuses) &&
+    isString(value['userId']) &&
+    isNullableString(value['description']) &&
+    isNullableRecord(value['metadata']) &&
+    isIsoLike(value['createdAt']) &&
+    isIsoLike(value['updatedAt'])
+  );
+}
+
+/**
+ * Projects are workspace-scoped: every item must carry the same `userId`.
+ * A collection mixing identities (cross-workspace leak) is a schema
+ * mismatch; the uniform `userId` becomes the snapshot workspace.
+ */
+export function validateProjectCollection(value: unknown): FreshPayload<Project[]> | null {
+  if (!Array.isArray(value) || !value.every(isProject)) return null;
+  const projects = value as Project[];
+  const workspaces = new Set(projects.map((project) => project.userId));
+  if (workspaces.size > 1) return null;
+  return { data: projects, workspace: projects.length > 0 ? projects[0]!.userId : null };
+}
+
+/** Single project entity (project detail primary collection). */
+export function validateProjectEntity(value: unknown): FreshPayload<Project> | null {
+  if (!isProject(value)) return null;
+  const project = value as Project;
+  return { data: project, workspace: project.userId };
+}

apps/web/src/spa/pages/project-detail.spec.tsx

@@ -35,6 +35,7 @@ afterEach(async () => {
   document.body.replaceChildren();
   root = null;
   apiMock.mockReset();
+  sessionStorage.clear();
 });
 
 async function renderProjectDetailPage(): Promise<ReturnType<typeof createMemoryRouter>> {
@@ -64,21 +65,49 @@ function clickButtonByText(text: string): void {
   button.dispatchEvent(new MouseEvent('click', { bubbles: true }));
 }
 
-describe('ProjectDetailPage', () => {
-  it('loads the project, tasks, missions, and optional PRD content for the active project', async () => {
+async function flushAct(): Promise<void> {
+  await act(async () => {
+    await Promise.resolve();
+  });
+}
+
+interface Deferred<T> {
+  promise: Promise<T>;
+  resolve: (value: T) => void;
+}
+
+function createDeferred<T>(): Deferred<T> {
+  let resolve!: (value: T) => void;
+  const promise = new Promise<T>((res) => {
+    resolve = res;
+  });
+  return { promise, resolve };
+}
+
+const projectOneTasks = taskFixtures.filter((task) => task.projectId === 'project-1');
+
+function mockHealthyLoad(): void {
   apiMock
     .mockResolvedValueOnce(projectFixtures[0])
     .mockResolvedValueOnce(missionFixtures)
-      .mockResolvedValueOnce(taskFixtures.filter((task) => task.projectId === 'project-1'));
+    .mockResolvedValueOnce(projectOneTasks);
+}
+
+describe('ProjectDetailPage', () => {
+  it('loads the project, tasks, missions, and optional PRD content for the active project', async () => {
+    mockHealthyLoad();
 
     await renderProjectDetailPage();
 
-    expect(apiMock.mock.calls).toEqual([
-      ['/api/projects/project-1'],
-      ['/api/missions'],
-      ['/api/tasks?projectId=project-1'],
+    expect(apiMock.mock.calls.map((call) => call[0])).toEqual([
+      '/api/projects/project-1',
+      '/api/missions',
+      '/api/tasks?projectId=project-1',
     ]);
 
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'current',
+    );
     expect(container.textContent).toContain('Mosaic Stack');
     expect(container.textContent).toContain('Route /projects/:id');
     expect(container.textContent).toContain('Tasks');
@@ -101,10 +130,7 @@ describe('ProjectDetailPage', () => {
   });
 
   it('opens and closes the existing read-only task modal from the tasks tab', async () => {
-    apiMock
-      .mockResolvedValueOnce(projectFixtures[0])
-      .mockResolvedValueOnce(missionFixtures)
-      .mockResolvedValueOnce(taskFixtures.filter((task) => task.projectId === 'project-1'));
+    mockHealthyLoad();
 
     await renderProjectDetailPage();
 
@@ -134,35 +160,153 @@ describe('ProjectDetailPage', () => {
     expect(container.querySelector('[role="dialog"]')).toBeNull();
   });
 
-  it('renders the project with an empty missions tab when the missions request fails', async () => {
-    apiMock
-      .mockResolvedValueOnce(projectFixtures[0])
-      .mockRejectedValueOnce(new Error('Missions request failed'))
-      .mockResolvedValueOnce(taskFixtures.filter((task) => task.projectId === 'project-1'));
+  it('shows verified completion verdicts when the task collection is current', async () => {
+    mockHealthyLoad();
 
     await renderProjectDetailPage();
 
+    const doneCard = [...container.querySelectorAll('div')].find(
+      (candidate) => candidate.textContent === 'Done1',
+    );
+    expect(doneCard).toBeTruthy();
+    const inProgressCard = [...container.querySelectorAll('div')].find(
+      (candidate) => candidate.textContent === 'In Progress1',
+    );
+    expect(inProgressCard).toBeTruthy();
+  });
+
+  it('renders an explicit unavailable missions tab when the missions request fails (partial, not empty)', async () => {
+    apiMock
+      .mockResolvedValueOnce(projectFixtures[0])
+      .mockRejectedValueOnce(new Error('Missions request failed'))
+      .mockResolvedValueOnce(projectOneTasks);
+
+    await renderProjectDetailPage();
+
+    // Secondary failure degrades the surface to partial; the project itself
+    // still renders.
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'partial',
+    );
     expect(container.textContent).toContain('Mosaic Stack');
-    expect(container.querySelector('[role="alert"]')).toBeNull();
+    const partial = container.querySelector('[role="status"]');
+    expect(partial?.textContent).toContain('Missions');
+    expect(partial?.textContent).toContain('unavailable');
 
     await act(async () => {
-      clickButtonByText('Missions (0)');
+      clickButtonByText('Missions (?)');
     });
 
-    expect(container.textContent).toContain('No missions for this project');
+    const alert = container.querySelector('[role="alert"]');
+    expect(alert?.textContent).toContain('Missions request failed');
+    // Negative control: a failed fetch must not look like an empty list.
+    expect(container.textContent).not.toContain('No missions for this project');
   });
 
-  it('renders a visible alert when the project request fails and lets the user navigate back', async () => {
+  it('marks derived verdicts unknown when the tasks collection is unavailable', async () => {
+    apiMock
+      .mockResolvedValueOnce(projectFixtures[0])
+      .mockResolvedValueOnce(missionFixtures)
+      .mockRejectedValueOnce(new Error('Tasks request failed'));
+
+    await renderProjectDetailPage();
+
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'partial',
+    );
+
+    // Completion verdicts become unknown ('?') — never green counts.
+    for (const label of ['Done', 'In Progress', 'Blocked', 'Tasks']) {
+      const unknownCard = [...container.querySelectorAll('div')].find(
+        (candidate) => candidate.textContent === `${label}?`,
+      );
+      expect(unknownCard, `expected ${label} card to render ?`).toBeTruthy();
+    }
+    // Negative control: no green "Done 1" verdict anywhere.
+    expect(
+      [...container.querySelectorAll('div')].some((candidate) => candidate.textContent === 'Done1'),
+    ).toBe(false);
+
+    await act(async () => {
+      clickButtonByText('Tasks (?)');
+    });
+
+    const alert = container.querySelector('[role="alert"]');
+    expect(alert?.textContent).toContain('Tasks request failed');
+    // Negative control: no healthy empty task list from a failed fetch.
+    expect(container.textContent).not.toContain('No tasks found');
+    expect(container.querySelector('table')).toBeNull();
+  });
+
+  it('recovers a partial surface to current after revalidation', async () => {
+    apiMock
+      .mockResolvedValueOnce(projectFixtures[0])
+      .mockResolvedValueOnce(missionFixtures)
+      .mockRejectedValueOnce(new Error('Tasks request failed'))
+      .mockResolvedValueOnce(projectFixtures[0])
+      .mockResolvedValueOnce(missionFixtures)
+      .mockResolvedValueOnce(projectOneTasks);
+
+    await renderProjectDetailPage();
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'partial',
+    );
+
+    await act(async () => {
+      clickButtonByText('Revalidate');
+    });
+    await flushAct();
+
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'current',
+    );
+    expect(
+      [...container.querySelectorAll('div')].some((candidate) => candidate.textContent === 'Done1'),
+    ).toBe(true);
+  });
+
+  it("never shows one project's data on another project's route after navigation", async () => {
+    mockHealthyLoad();
+
+    const router = await renderProjectDetailPage();
+    expect(container.textContent).toContain('Mosaic Stack');
+
+    const deferred = createDeferred<(typeof projectFixtures)[number]>();
+    apiMock
+      .mockResolvedValueOnce(deferred.promise)
+      .mockResolvedValueOnce([])
+      .mockResolvedValueOnce([]);
+
+    await act(async () => {
+      await router.navigate('/projects/project-2');
+    });
+
+    // While project-2 loads, nothing from project-1 may render on its route.
+    expect(container.textContent).toContain('Loading project...');
+    expect(container.textContent).not.toContain('Mosaic Stack');
+    expect(container.textContent).not.toContain('Route /projects/:id');
+
+    await act(async () => {
+      deferred.resolve(projectFixtures[1]!);
+      await deferred.promise;
+    });
+
+    expect(container.textContent).toContain('Agent Runtime');
+    expect(apiMock.mock.calls[3]?.[0]).toBe('/api/projects/project-2');
+  });
+
+  it('renders a visible unavailable state when the project request fails and lets the user navigate back', async () => {
     apiMock
       .mockRejectedValueOnce(new Error('Project request failed'))
       .mockResolvedValueOnce(missionFixtures)
-      .mockResolvedValueOnce(taskFixtures.filter((task) => task.projectId === 'project-1'));
+      .mockResolvedValueOnce(projectOneTasks);
 
     const router = await renderProjectDetailPage();
 
     const alert = container.querySelector('[role="alert"]');
     expect(alert).toBeTruthy();
     expect(alert?.textContent).toContain('Project request failed');
+    expect(alert?.textContent).toContain('not an empty result');
     expect(container.textContent).not.toContain('Mosaic Stack');
 
     await act(async () => {

apps/web/src/spa/pages/project-detail.tsx

@@ -1,14 +1,30 @@
-import { useEffect, useState, type ReactElement } from 'react';
+import { useState, type ReactElement } from 'react';
 import { useNavigate, useParams } from 'react-router-dom';
 import { MissionTimeline } from '@/components/projects/mission-timeline';
 import { PrdViewer } from '@/components/projects/prd-viewer';
 import { TaskDetailModal } from '@/components/tasks/task-detail-modal';
 import { TaskListView } from '@/components/tasks/task-list-view';
 import { TaskStatusSummary } from '@/components/tasks/task-status-summary';
+import {
+  PartialDataNotice,
+  StaleDataNotice,
+  UnavailableDataNotice,
+} from '@/components/freshness/freshness-notices';
 import { api } from '@/lib/api';
 import { cn } from '@/lib/cn';
 import type { Mission, Project, Task, TaskStatus } from '@/lib/types';
-import { getErrorMessage } from './page-errors';
+import {
+  combineFreshness,
+  UNKNOWN_VERDICT,
+  verdictValue,
+  type FreshSnapshot,
+} from '@/lib/freshness/model';
+import { describeFailure, useFreshCollection } from '@/lib/freshness/use-fresh-collection';
+import {
+  validateMissionCollection,
+  validateProjectEntity,
+  validateTaskCollection,
+} from '@/lib/freshness/validators';
 
 type Tab = 'overview' | 'tasks' | 'missions' | 'prd';
 
@@ -51,73 +67,62 @@ function TabButton({ id, label, activeTab, onClick }: TabButtonProps): ReactElem
   );
 }
 
+/** Remounts per project id so no state from one project renders for another. */
 export function ProjectDetailPage(): ReactElement {
   const { id = '' } = useParams();
+  return <ProjectDetail id={id} key={id} />;
+}
+
+function ProjectDetail({ id }: { id: string }): ReactElement {
   const navigate = useNavigate();
-  const [project, setProject] = useState<Project | null>(null);
-  const [missions, setMissions] = useState<Mission[]>([]);
-  const [tasks, setTasks] = useState<Task[]>([]);
-  const [loading, setLoading] = useState(true);
-  const [error, setError] = useState<string | null>(null);
+  const enabled = id.length > 0;
+
+  // Primary collection gates the surface; missions and tasks are secondaries
+  // whose failures degrade the surface to `partial` instead of rendering
+  // empty healthy lists.
+  const project = useFreshCollection<Project>({
+    source: `gateway:/api/projects/${id}`,
+    fetcher: (signal) => api<unknown>(`/api/projects/${id}`, { signal }),
+    validate: validateProjectEntity,
+    // No last-known restore: the entity carries workspace identity that
+    // cannot be scope-checked before display (see ProjectsPage note).
+    enabled,
+  });
+  const missions = useFreshCollection<Mission[]>({
+    source: 'gateway:/api/missions',
+    fetcher: (signal) => api<unknown>('/api/missions', { signal }),
+    validate: validateMissionCollection,
+    cacheKey: enabled ? 'missions' : null,
+    enabled,
+  });
+  const tasks = useFreshCollection<Task[]>({
+    source: `gateway:/api/tasks?projectId=${id}`,
+    fetcher: (signal) => api<unknown>(`/api/tasks?projectId=${id}`, { signal }),
+    validate: validateTaskCollection,
+    cacheKey: enabled ? `project-tasks:${id}` : null,
+    enabled,
+  });
+
   const [activeTab, setActiveTab] = useState<Tab>('overview');
   const [taskFilter, setTaskFilter] = useState<TaskStatus | 'all'>('all');
   const [selectedTask, setSelectedTask] = useState<Task | null>(null);
 
-  useEffect(() => {
-    if (!id) {
-      setError('Project id is missing.');
-      setLoading(false);
-      return;
-    }
+  const surface = combineFreshness(project.freshness, [missions.freshness, tasks.freshness]);
+  const tasksVerified = tasks.freshness === 'current';
+  const projectMissions = missions.data?.filter((mission) => mission.projectId === id) ?? null;
 
-    let cancelled = false;
-    setLoading(true);
-    setError(null);
-
-    void Promise.all([
-      api<Project>('/api/projects/' + id),
-      api<Mission[]>('/api/missions').catch(() => [] as Mission[]),
-      api<Task[]>('/api/tasks?projectId=' + id).catch(() => [] as Task[]),
-    ])
-      .then(([loadedProject, allMissions, loadedTasks]) => {
-        if (cancelled) return;
-        setProject(loadedProject);
-        setMissions(allMissions.filter((mission) => mission.projectId === id));
-        setTasks(loadedTasks);
-      })
-      .catch((caught: unknown) => {
-        if (cancelled) return;
-        setError(getErrorMessage(caught, 'Failed to load project.'));
-      })
-      .finally(() => {
-        if (cancelled) return;
-        setLoading(false);
-      });
-
-    return () => {
-      cancelled = true;
+  const retryAll = (): void => {
+    void Promise.all([project.revalidate(), missions.revalidate(), tasks.revalidate()]);
   };
-  }, [id]);
 
-  if (loading) {
-    return (
-      <div className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
-        <header className="mb-6 border-b px-1 pb-3">
-          <h1 className="text-2xl font-semibold">Project</h1>
-        </header>
-        <p className="py-16 text-center text-sm text-text-muted">Loading project...</p>
-      </div>
-    );
-  }
-
-  if (error || !project) {
+  if (!enabled) {
     return (
       <div className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
         <header className="mb-6 border-b px-1 pb-3">
           <h1 className="text-2xl font-semibold">Project</h1>
         </header>
         <div role="alert" className="rounded-lg border border-error/40 px-4 py-3 text-sm">
-          {error ?? 'Project not found.'}
+          Project id is missing.
         </div>
         <button
           type="button"
@@ -130,18 +135,81 @@ export function ProjectDetailPage(): ReactElement {
     );
   }
 
+  if (project.freshness === 'unknown') {
+    return (
+      <div className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
+        <header className="mb-6 border-b px-1 pb-3">
+          <h1 className="text-2xl font-semibold">Project</h1>
+        </header>
+        <p className="py-16 text-center text-sm text-text-muted">Loading project...</p>
+      </div>
+    );
+  }
+
+  if (project.freshness === 'unavailable' || project.data === null) {
+    return (
+      <div className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
+        <header className="mb-6 border-b px-1 pb-3">
+          <h1 className="text-2xl font-semibold">Project</h1>
+        </header>
+        <UnavailableDataNotice
+          title="This project"
+          detail={describeFailure(project.failure)}
+          onRetry={retryAll}
+        />
+        <button
+          type="button"
+          onClick={() => navigate('/projects')}
+          className="mt-4 w-fit text-sm underline"
+        >
+          Back to projects
+        </button>
+      </div>
+    );
+  }
+
+  const projectTasks = tasks.data ?? null;
   const filteredTasks =
-    taskFilter === 'all' ? tasks : tasks.filter((task) => task.status === taskFilter);
-  const prdContent = getPrdContent(project);
+    projectTasks === null
+      ? []
+      : taskFilter === 'all'
+        ? projectTasks
+        : projectTasks.filter((task) => task.status === taskFilter);
+
+  // Derived completion verdicts: unknown (never green) unless the task
+  // collection is verified current.
+  const doneCount = projectTasks?.filter((task) => task.status === 'done').length ?? 0;
+  const inProgressCount = projectTasks?.filter((task) => task.status === 'in-progress').length ?? 0;
+  const blockedCount = projectTasks?.filter((task) => task.status === 'blocked').length ?? 0;
+
+  const prdContent = getPrdContent(project.data);
   const tabs: Array<{ id: Tab; label: string }> = [
     { id: 'overview', label: 'Overview' },
-    { id: 'tasks', label: `Tasks (${tasks.length})` },
-    { id: 'missions', label: `Missions (${missions.length})` },
+    {
+      id: 'tasks',
+      label: `Tasks (${projectTasks === null ? UNKNOWN_VERDICT : projectTasks.length})`,
+    },
+    {
+      id: 'missions',
+      label: `Missions (${projectMissions === null ? UNKNOWN_VERDICT : projectMissions.length})`,
+    },
     ...(prdContent ? [{ id: 'prd' as const, label: 'PRD' }] : []),
   ];
 
+  const staleSnapshot: FreshSnapshot<unknown> | null =
+    project.freshness === 'stale'
+      ? project.snapshot
+      : missions.freshness === 'stale'
+        ? missions.snapshot
+        : tasks.freshness === 'stale'
+          ? tasks.snapshot
+          : null;
+  const missingSections: string[] = [];
+  if (missions.freshness === 'unavailable') missingSections.push('Missions');
+  if (tasks.freshness === 'unavailable') missingSections.push('Tasks');
+
   return (
-    <div className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
+    <div data-freshness={surface} className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
       <header className="mb-6 border-b px-1 pb-3">
         <nav className="mb-4 flex items-center gap-2 text-sm text-text-muted">
           <button
@@ -152,49 +220,64 @@ export function ProjectDetailPage(): ReactElement {
             Projects
           </button>
           <span>/</span>
-          <span className="text-text-primary">{project.name}</span>
+          <span className="text-text-primary">{project.data.name}</span>
         </nav>
 
         <div className="flex items-start justify-between gap-4">
           <div>
             <div className="flex items-center gap-3">
-              <h1 className="text-2xl font-semibold text-text-primary">{project.name}</h1>
+              <h1 className="text-2xl font-semibold text-text-primary">{project.data.name}</h1>
               <span
                 className={cn(
                   'rounded-full px-2 py-0.5 text-xs',
-                  projectStatusColors[project.status] ?? 'bg-gray-600/20 text-gray-400',
+                  projectStatusColors[project.data.status] ?? 'bg-gray-600/20 text-gray-400',
                 )}
               >
-                {project.status}
+                {project.data.status}
               </span>
             </div>
-            {project.description ? (
-              <p className="mt-1 text-sm text-text-muted">{project.description}</p>
+            {project.data.description ? (
+              <p className="mt-1 text-sm text-text-muted">{project.data.description}</p>
             ) : null}
             <p className="mt-2 text-xs text-text-muted">
-              Created {new Date(project.createdAt).toLocaleDateString()} · Updated{' '}
-              {new Date(project.updatedAt).toLocaleDateString()}
+              Created {new Date(project.data.createdAt).toLocaleDateString()} · Updated{' '}
+              {new Date(project.data.updatedAt).toLocaleDateString()}
             </p>
           </div>
         </div>
       </header>
 
+      {staleSnapshot !== null ? (
+        <div className="mb-6">
+          <StaleDataNotice label={staleSnapshot} onRetry={retryAll} />
+        </div>
+      ) : null}
+
+      {missingSections.length > 0 ? (
+        <div className="mb-6">
+          <PartialDataNotice missing={missingSections} onRetry={retryAll} />
+        </div>
+      ) : null}
+
       <div className="mb-6 grid grid-cols-2 gap-3 sm:grid-cols-4">
-        <StatCard label="Tasks" value={String(tasks.length)} />
+        <StatCard
+          label="Tasks"
+          value={projectTasks === null ? UNKNOWN_VERDICT : String(projectTasks.length)}
+        />
         <StatCard
           label="Done"
-          value={String(tasks.filter((task) => task.status === 'done').length)}
-          valueClass="text-success"
+          value={verdictValue(tasksVerified, String(doneCount))}
+          valueClass={tasksVerified ? 'text-success' : undefined}
         />
         <StatCard
           label="In Progress"
-          value={String(tasks.filter((task) => task.status === 'in-progress').length)}
-          valueClass="text-blue-400"
+          value={verdictValue(tasksVerified, String(inProgressCount))}
+          valueClass={tasksVerified ? 'text-blue-400' : undefined}
         />
         <StatCard
           label="Blocked"
-          value={String(tasks.filter((task) => task.status === 'blocked').length)}
-          valueClass={tasks.some((task) => task.status === 'blocked') ? 'text-error' : undefined}
+          value={verdictValue(tasksVerified, String(blockedCount))}
+          valueClass={tasksVerified && blockedCount > 0 ? 'text-error' : undefined}
         />
       </div>
 
@@ -211,23 +294,43 @@ export function ProjectDetailPage(): ReactElement {
       </div>
 
       {activeTab === 'overview' ? (
-        <OverviewTab project={project} missions={missions} tasks={tasks} />
+        <OverviewTab project={project.data} missions={projectMissions} tasks={projectTasks} />
       ) : null}
 
       {activeTab === 'tasks' ? (
         <div>
+          {projectTasks === null ? (
+            <UnavailableDataNotice
+              title="Tasks"
+              detail={describeFailure(tasks.failure)}
+              onRetry={retryAll}
+            />
+          ) : (
+            <>
               <div className="mb-4">
                 <TaskStatusSummary
-              tasks={tasks}
+                  tasks={projectTasks}
                   activeFilter={taskFilter}
                   onFilterChange={setTaskFilter}
                 />
               </div>
               <TaskListView tasks={filteredTasks} onTaskClick={setSelectedTask} />
+            </>
+          )}
         </div>
       ) : null}
 
-      {activeTab === 'missions' ? <MissionTimeline missions={missions} /> : null}
+      {activeTab === 'missions' ? (
+        projectMissions === null ? (
+          <UnavailableDataNotice
+            title="Missions"
+            detail={describeFailure(missions.failure)}
+            onRetry={retryAll}
+          />
+        ) : (
+          <MissionTimeline missions={projectMissions} />
+        )
+      ) : null}
 
       {activeTab === 'prd' && prdContent ? (
         <div className="rounded-lg border border-surface-border bg-surface-card p-6">
@@ -248,18 +351,26 @@ function OverviewTab({
   tasks,
 }: {
   project: Project;
-  missions: Mission[];
-  tasks: Task[];
+  missions: Mission[] | null;
+  tasks: Task[] | null;
 }): ReactElement {
-  const recentTasks = [...tasks]
-    .sort((left, right) => new Date(right.updatedAt).getTime() - new Date(left.updatedAt).getTime())
+  const recentTasks =
+    tasks === null
+      ? null
+      : [...tasks]
+          .sort(
+            (left, right) =>
+              new Date(right.updatedAt).getTime() - new Date(left.updatedAt).getTime(),
+          )
           .slice(0, 5);
 
   return (
     <div className="grid gap-6 lg:grid-cols-2">
       <section>
         <h2 className="mb-3 text-sm font-semibold text-text-secondary">Recent Tasks</h2>
-        {recentTasks.length === 0 ? (
+        {recentTasks === null ? (
+          <UnavailableDataNotice title="Tasks" />
+        ) : recentTasks.length === 0 ? (
           <div className="rounded-lg border border-surface-border bg-surface-card p-4 text-center">
             <p className="text-sm text-text-muted">No tasks yet</p>
           </div>
@@ -287,7 +398,9 @@ function OverviewTab({
 
       <section>
         <h2 className="mb-3 text-sm font-semibold text-text-secondary">Missions</h2>
-        {missions.length === 0 ? (
+        {missions === null ? (
+          <UnavailableDataNotice title="Missions" />
+        ) : missions.length === 0 ? (
           <div className="rounded-lg border border-surface-border bg-surface-card p-4 text-center">
             <p className="text-sm text-text-muted">No missions yet</p>
           </div>

apps/web/src/spa/pages/projects.spec.tsx

@@ -51,6 +51,7 @@ afterEach(async () => {
   document.body.replaceChildren();
   root = null;
   apiMock.mockReset();
+  sessionStorage.clear();
 });
 
 async function renderProjectsPage(): Promise<ReturnType<typeof createMemoryRouter>> {
@@ -71,6 +72,22 @@ async function renderProjectsPage(): Promise<ReturnType<typeof createMemoryRoute
   return router;
 }
 
+function clickButtonByText(text: string): void {
+  const button = [...container.querySelectorAll('button')].find((candidate) =>
+    candidate.textContent?.includes(text),
+  );
+  if (!button) {
+    throw new Error(`Button containing "${text}" not found`);
+  }
+  button.dispatchEvent(new MouseEvent('click', { bubbles: true }));
+}
+
+async function flushAct(): Promise<void> {
+  await act(async () => {
+    await Promise.resolve();
+  });
+}
+
 describe('ProjectsPage', () => {
   it('shows a visible loading state while the project request is in flight', async () => {
     const deferred = createDeferred<typeof projectFixtures>();
@@ -91,7 +108,7 @@ describe('ProjectsPage', () => {
 
     const router = await renderProjectsPage();
 
-    expect(apiMock).toHaveBeenCalledWith('/api/projects');
+    expect(apiMock.mock.calls[0]?.[0]).toBe('/api/projects');
     expect(container.textContent).toContain('Mosaic Stack');
     expect(container.textContent).toContain('Agent Runtime');
 
@@ -108,7 +125,7 @@ describe('ProjectsPage', () => {
     expect(container.textContent).toContain('Project detail target');
   });
 
-  it('renders the empty state when the API returns no projects', async () => {
+  it('renders the empty state only for a verified empty collection', async () => {
     apiMock.mockResolvedValueOnce([]);
 
     await renderProjectsPage();
@@ -117,9 +134,12 @@ describe('ProjectsPage', () => {
     expect(container.textContent).toContain(
       'Projects will appear here when created via the gateway API',
     );
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'current',
+    );
   });
 
-  it('renders a visible alert when the projects request fails', async () => {
+  it('renders a failed fetch as an explicit unavailable state, never an empty collection', async () => {
     apiMock.mockRejectedValueOnce(new Error('Projects are unavailable'));
 
     await renderProjectsPage();
@@ -127,5 +147,51 @@ describe('ProjectsPage', () => {
     const alert = container.querySelector('[role="alert"]');
     expect(alert).toBeTruthy();
     expect(alert?.textContent).toContain('Projects are unavailable');
+    expect(alert?.textContent).toContain('not an empty result');
+
+    // Negative controls: no healthy empty state and no project cards render
+    // from a failed fetch.
+    expect(container.textContent).not.toContain('No projects yet');
+    expect(container.textContent).not.toContain('Mosaic Stack');
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'unavailable',
+    );
+  });
+
+  it('renders an auth failure as unavailable and recovers after retry', async () => {
+    apiMock
+      .mockRejectedValueOnce(Object.assign(new Error('Unauthorized'), { statusCode: 401 }))
+      .mockResolvedValueOnce(projectFixtures);
+
+    await renderProjectsPage();
+
+    const alert = container.querySelector('[role="alert"]');
+    expect(alert?.textContent).toContain('Unauthorized');
+    expect(container.textContent).not.toContain('No projects yet');
+
+    await act(async () => {
+      clickButtonByText('Retry');
+    });
+    await flushAct();
+
+    expect(container.querySelector('[role="alert"]')).toBeNull();
+    expect(container.textContent).toContain('Mosaic Stack');
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'current',
+    );
+  });
+
+  it('renders a schema-mismatched response as unavailable, never as data', async () => {
+    apiMock.mockResolvedValueOnce({ results: projectFixtures });
+
+    await renderProjectsPage();
+
+    const alert = container.querySelector('[role="alert"]');
+    expect(alert?.textContent).toContain('not an empty result');
+    expect(container.textContent).not.toContain('Mosaic Stack');
+    expect(container.textContent).not.toContain('No projects yet');
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'unavailable',
+    );
   });
 });

apps/web/src/spa/pages/projects.tsx

@@ -1,53 +1,51 @@
-import { useEffect, useState, type ReactElement } from 'react';
+import { type ReactElement } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { ProjectCard } from '@/components/projects/project-card';
+import { StaleDataNotice, UnavailableDataNotice } from '@/components/freshness/freshness-notices';
 import { api } from '@/lib/api';
 import type { Project } from '@/lib/types';
-import { getErrorMessage } from './page-errors';
+import { useFreshCollection, describeFailure } from '@/lib/freshness/use-fresh-collection';
+import { validateProjectCollection } from '@/lib/freshness/validators';
 
 export function ProjectsPage(): ReactElement {
   const navigate = useNavigate();
-  const [projects, setProjects] = useState<Project[]>([]);
-  const [loading, setLoading] = useState(true);
-  const [error, setError] = useState<string | null>(null);
-
-  useEffect(() => {
-    let cancelled = false;
-
-    void api<Project[]>('/api/projects')
-      .then((response) => {
-        if (cancelled) return;
-        setProjects(response);
-      })
-      .catch((caught: unknown) => {
-        if (cancelled) return;
-        setError(getErrorMessage(caught, 'Failed to load projects.'));
-      })
-      .finally(() => {
-        if (cancelled) return;
-        setLoading(false);
+  const projects = useFreshCollection<Project[]>({
+    source: 'gateway:/api/projects',
+    fetcher: (signal) => api<unknown>('/api/projects', { signal }),
+    validate: validateProjectCollection,
+    // Projects carry workspace identity (userId) that is only knowable from
+    // the payload itself, so a restored entry cannot be scope-checked before
+    // display. Conservative choice: no last-known restore for this surface;
+    // cross-workspace switching is still invalidated at verification time.
   });
-
-    return () => {
-      cancelled = true;
+  const retry = (): void => {
+    void projects.revalidate();
   };
-  }, []);
 
   return (
-    <div className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
+    <div
+      data-freshness={projects.freshness}
+      className="flex min-h-screen flex-col px-4 py-6 sm:px-6"
+    >
       <header className="mb-6 border-b px-1 pb-3">
         <h1 className="text-2xl font-semibold">Projects</h1>
       </header>
 
-      {error ? (
-        <div role="alert" className="mb-6 rounded-lg border border-error/40 px-4 py-3 text-sm">
-          {error}
+      {projects.freshness === 'stale' && projects.snapshot ? (
+        <div className="mb-6">
+          <StaleDataNotice label={projects.snapshot} onRetry={retry} />
         </div>
       ) : null}
 
-      {loading ? (
+      {projects.freshness === 'unknown' ? (
         <p className="py-8 text-center text-sm text-text-muted">Loading projects...</p>
-      ) : projects.length === 0 ? (
+      ) : projects.freshness === 'unavailable' ? (
+        <UnavailableDataNotice
+          title="Projects"
+          detail={describeFailure(projects.failure)}
+          onRetry={retry}
+        />
+      ) : projects.data !== null && projects.data.length === 0 ? (
         <div className="py-12 text-center">
           <h2 className="text-lg font-medium text-text-secondary">No projects yet</h2>
           <p className="mt-1 text-sm text-text-muted">
@@ -56,7 +54,7 @@ export function ProjectsPage(): ReactElement {
         </div>
       ) : (
         <div className="grid gap-4 sm:grid-cols-2 lg:grid-cols-3">
-          {projects.map((project) => (
+          {(projects.data ?? []).map((project) => (
             <ProjectCard
               key={project.id}
               project={project}

apps/web/src/spa/pages/tasks.spec.tsx

@@ -3,6 +3,9 @@ import { createRoot, type Root } from 'react-dom/client';
 import { createMemoryRouter, RouterProvider, type RouteObject } from 'react-router-dom';
 import { afterAll, afterEach, beforeAll, describe, expect, it, vi } from 'vitest';
 import { taskFixtures } from './page-fixtures';
+import { acceptSnapshot, DEFAULT_FRESHNESS_POLICY } from '@/lib/freshness/model';
+import { writeSnapshotCache } from '@/lib/freshness/snapshot-cache';
+import { validateTaskCollection } from '@/lib/freshness/validators';
 
 const { apiMock } = vi.hoisted(() => ({
   apiMock: vi.fn(),
@@ -48,6 +51,7 @@ afterEach(async () => {
   document.body.replaceChildren();
   root = null;
   apiMock.mockReset();
+  sessionStorage.clear();
 });
 
 async function renderTasksPage(): Promise<void> {
@@ -72,6 +76,13 @@ function clickButtonByText(text: string): void {
   button.dispatchEvent(new MouseEvent('click', { bubbles: true }));
 }
 
+/** Flush pending promise callbacks inside the act environment. */
+async function flushAct(): Promise<void> {
+  await act(async () => {
+    await Promise.resolve();
+  });
+}
+
 describe('TasksPage', () => {
   it('shows a visible loading state before the tasks request settles', async () => {
     const deferred = createDeferred<typeof taskFixtures>();
@@ -132,7 +143,7 @@ describe('TasksPage', () => {
     expect(container.textContent).toContain('Wire list and kanban modal interactions');
   });
 
-  it('renders a visible alert when the tasks request fails', async () => {
+  it('renders a failed fetch as an explicit unavailable state, never an empty healthy board', async () => {
     apiMock.mockRejectedValueOnce(new Error('Tasks request failed'));
 
     await renderTasksPage();
@@ -140,5 +151,80 @@ describe('TasksPage', () => {
     const alert = container.querySelector('[role="alert"]');
     expect(alert).toBeTruthy();
     expect(alert?.textContent).toContain('Tasks request failed');
+    expect(alert?.textContent).toContain('not an empty result');
+
+    // Negative controls: no board, no healthy empty-state markers, and the
+    // surface is marked unavailable rather than current.
+    expect(container.textContent).not.toContain('Not Started');
+    expect(container.textContent).not.toContain('No tasks');
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'unavailable',
+    );
+  });
+
+  it('recovers to a current board after retrying a failed fetch', async () => {
+    apiMock
+      .mockRejectedValueOnce(new Error('Tasks request failed'))
+      .mockResolvedValueOnce(taskFixtures);
+
+    await renderTasksPage();
+    expect(container.querySelector('[role="alert"]')).toBeTruthy();
+
+    await act(async () => {
+      clickButtonByText('Retry');
+    });
+    await flushAct();
+
+    expect(container.querySelector('[role="alert"]')).toBeNull();
+    expect(container.textContent).toContain('Not Started');
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'current',
+    );
+  });
+
+  it('labels restored last-known data as stale with source, version, and age until verified', async () => {
+    // Seed a last-known snapshot fetched five minutes ago; the page must
+    // render it only under an explicit staleness label while the fetch is
+    // still in flight.
+    const restored = acceptSnapshot({
+      value: taskFixtures,
+      validate: validateTaskCollection,
+      previous: null,
+      policy: DEFAULT_FRESHNESS_POLICY,
+      source: 'gateway:/api/tasks',
+      now: Date.now() - 5 * 60_000,
+    });
+    if (restored.outcome !== 'accepted') throw new Error('fixture setup failed');
+    writeSnapshotCache('tasks', restored.snapshot);
+
+    const deferred = createDeferred<typeof taskFixtures>();
+    apiMock.mockReturnValueOnce(deferred.promise);
+
+    await renderTasksPage();
+
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'stale',
+    );
+    const banner = container.querySelector('[role="status"]');
+    expect(banner?.textContent).toContain('last-known');
+    expect(banner?.textContent).toContain('may be out of date');
+    expect(banner?.textContent).toContain('gateway:/api/tasks');
+    expect(banner?.textContent).toContain('snapshot v1');
+    expect(banner?.textContent).toContain('5m ago');
+
+    // Last-known data still renders as situational awareness under the label.
+    expect(container.textContent).toContain('Route /tasks');
+    expect(container.textContent).not.toContain('Loading tasks...');
+
+    // Verification lands: the banner clears and the surface becomes current.
+    await act(async () => {
+      deferred.resolve(taskFixtures);
+      await deferred.promise;
+    });
+
+    expect(container.querySelector('[role="status"]')).toBeNull();
+    expect(container.querySelector('[data-freshness]')?.getAttribute('data-freshness')).toBe(
+      'current',
+    );
   });
 });

apps/web/src/spa/pages/tasks.tsx

@@ -1,45 +1,32 @@
-import { useEffect, useState, type ReactElement } from 'react';
+import { useState, type ReactElement } from 'react';
 import { KanbanBoard } from '@/components/tasks/kanban-board';
 import { TaskDetailModal } from '@/components/tasks/task-detail-modal';
 import { TaskListView } from '@/components/tasks/task-list-view';
+import { StaleDataNotice, UnavailableDataNotice } from '@/components/freshness/freshness-notices';
 import { api } from '@/lib/api';
 import { cn } from '@/lib/cn';
 import type { Task } from '@/lib/types';
-import { getErrorMessage } from './page-errors';
+import { useFreshCollection, describeFailure } from '@/lib/freshness/use-fresh-collection';
+import { validateTaskCollection } from '@/lib/freshness/validators';
 
 type ViewMode = 'list' | 'kanban';
 
 export function TasksPage(): ReactElement {
-  const [tasks, setTasks] = useState<Task[]>([]);
+  const tasks = useFreshCollection<Task[]>({
+    source: 'gateway:/api/tasks',
+    fetcher: (signal) => api<unknown>('/api/tasks', { signal }),
+    validate: validateTaskCollection,
+    cacheKey: 'tasks',
+  });
   const [view, setView] = useState<ViewMode>('kanban');
-  const [loading, setLoading] = useState(true);
-  const [error, setError] = useState<string | null>(null);
   const [selectedTask, setSelectedTask] = useState<Task | null>(null);
 
-  useEffect(() => {
-    let cancelled = false;
-
-    void api<Task[]>('/api/tasks')
-      .then((response) => {
-        if (cancelled) return;
-        setTasks(response);
-      })
-      .catch((caught: unknown) => {
-        if (cancelled) return;
-        setError(getErrorMessage(caught, 'Failed to load tasks.'));
-      })
-      .finally(() => {
-        if (cancelled) return;
-        setLoading(false);
-      });
-
-    return () => {
-      cancelled = true;
+  const retry = (): void => {
+    void tasks.revalidate();
   };
-  }, []);
 
   return (
-    <div className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
+    <div data-freshness={tasks.freshness} className="flex min-h-screen flex-col px-4 py-6 sm:px-6">
       <header className="mb-6 flex items-center justify-between gap-4 border-b px-1 pb-3">
         <h1 className="text-2xl font-semibold">Tasks</h1>
         <div className="flex rounded-lg border border-surface-border">
@@ -70,18 +57,24 @@ export function TasksPage(): ReactElement {
         </div>
       </header>
 
-      {error ? (
-        <div role="alert" className="mb-6 rounded-lg border border-error/40 px-4 py-3 text-sm">
-          {error}
+      {tasks.freshness === 'stale' && tasks.snapshot ? (
+        <div className="mb-6">
+          <StaleDataNotice label={tasks.snapshot} onRetry={retry} />
         </div>
       ) : null}
 
-      {loading ? (
+      {tasks.freshness === 'unknown' ? (
         <p className="py-8 text-center text-sm text-text-muted">Loading tasks...</p>
+      ) : tasks.freshness === 'unavailable' ? (
+        <UnavailableDataNotice
+          title="Tasks"
+          detail={describeFailure(tasks.failure)}
+          onRetry={retry}
+        />
       ) : view === 'kanban' ? (
-        <KanbanBoard tasks={tasks} onTaskClick={setSelectedTask} />
+        <KanbanBoard tasks={tasks.data ?? []} onTaskClick={setSelectedTask} />
       ) : (
-        <TaskListView tasks={tasks} onTaskClick={setSelectedTask} />
+        <TaskListView tasks={tasks.data ?? []} onTaskClick={setSelectedTask} />
       )}
 
       {selectedTask ? (

packages/mosaic/framework/install.sh

@@ -35,6 +35,18 @@ SOURCE_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 TARGET_DIR="${MOSAIC_HOME:-$HOME/.config/mosaic}"
 INSTALL_MODE="${MOSAIC_INSTALL_MODE:-prompt}"
 
+# Normalize the ambient umask so directory modes are a property of the installer
+# and not of whatever shell invoked it (#1236). Debian/Ubuntu ship umask 002, so
+# every `mkdir -p` below yielded 0775 — and the fleet env boundary rejects any
+# managed directory with `mode & 0o022`, which made `mosaic fleet init --write`
+# impossible on a stock install of those distros. Fedora/RHEL ship 022 and did
+# not trip it, so the product worked or did not depending on the operator's
+# login shell. 022 is what this script already assumes it produces: see the
+# umask note in make_durable_snapshot, which restores to the ambient value
+# precisely so "every later sync copy and new framework dir" gets 0644/0755.
+# Now that value is 022 rather than whatever was inherited.
+umask 022
+
 # Deliberately parsed from "$@" (a real, explicit, per-invocation argument) —
 # never an environment variable — so this opt-out can never sit silently
 # inherited in a shell profile. See #869 Point-1 C2.
@@ -696,6 +708,52 @@ sync_framework
 mkdir -p "$TARGET_DIR/memory"
 mkdir -p "$TARGET_DIR/credentials"
 
+# Three directories must be 0700, not merely not-group-writable (#1236).
+# The fleet code guards them with two different masks in two different
+# languages, and the strict one wins:
+#
+#   assertPrivateManagedDirectory (fleet-reconciler.js, `mode & 0o077`)
+#       -> MOSAIC_HOME and MOSAIC_HOME/fleet, checked before the roster lock is
+#          taken, so every mutating `mosaic fleet` command dies at 0755.
+#   assert_private_directory (tools/fleet/start-agent-session.sh, `mode & 077`)
+#       -> MOSAIC_HOME/fleet/agents, checked before a pane is ever spawned.
+#
+# Their laxer siblings (`mode & 0o022`) accept 0755, which is why normalizing
+# the umask above is necessary and not sufficient — a correct umask-022 install
+# still produces 0755 and still cannot run `mosaic fleet init --write`. Say the
+# strict modes outright rather than inferring them from a umask.
+#
+# Only these. The rest of the tree is content, stays 0755, and is only ever
+# reached by the 0o022 checks, which 0755 satisfies.
+chmod 700 "$TARGET_DIR" 2>/dev/null || \
+  warn "Could not set 0700 on $TARGET_DIR — 'mosaic fleet' mutations will fail as unsafe-permissions."
+if [[ -d "$TARGET_DIR/fleet" ]]; then
+  chmod 700 "$TARGET_DIR/fleet" 2>/dev/null || \
+    warn "Could not set 0700 on $TARGET_DIR/fleet — 'mosaic fleet' mutations will fail as unsafe-permissions."
+fi
+# fleet/agents does not exist on a first install — the CLI creates it 0700 on
+# demand. It is chmod'd here for the UPGRADE case: a tree built under umask 002
+# has it at 0775, and the repair sweep below cannot rescue it, because stripping
+# group/other write from 0755 leaves 0750 and `mode & 077` is still non-zero.
+if [[ -d "$TARGET_DIR/fleet/agents" ]]; then
+  chmod 700 "$TARGET_DIR/fleet/agents" 2>/dev/null || \
+    warn "Could not set 0700 on $TARGET_DIR/fleet/agents — agent sessions will fail to start as unsafe-permissions."
+fi
+# credentials/ holds secrets and was never meant to be group-readable either.
+# It is not on the fleet boundary, so a failure here breaks nothing — but it is
+# the one directory where a silently-failed chmod leaves secrets group-readable,
+# which is precisely the failure worth a line in the output.
+chmod 700 "$TARGET_DIR/credentials" 2>/dev/null || \
+  warn "Could not set 0700 on $TARGET_DIR/credentials — stored secrets may be readable by other users on this host."
+
+# Repair an existing tree. The umask above only governs directories this run
+# creates, so a host installed under umask 002 before this fix keeps its 0775
+# dirs through every upgrade and stays broken. Strips group/other WRITE only —
+# never read or execute — so it can repair the boundary violation without
+# changing who can traverse or read anything. Scoped to directories: file modes
+# are the manifest's business, not this fix's.
+find "$TARGET_DIR" -type d -perm /022 -exec chmod go-w {} + 2>/dev/null || true
+
 # Reconcile contract files from defaults/ into the framework root: framework-owned
 # files (CONSTITUTION/AGENTS/STANDARDS) are overwritten every upgrade (a divergent
 # copy is backed up once); user-seeded files (TOOLS) are written on first install only.

packages/mosaic/framework/systemd/user/mosaic-agent@.service

@@ -4,6 +4,14 @@ Documentation=https://git.mosaicstack.dev/mosaicstack/stack
 Requires=mosaic-tmux-holder.service
 After=mosaic-tmux-holder.service
 PartOf=mosaic-tmux-holder.service
+# Do not attempt a seat before its generated env exists. `install` enables this
+# unit (WantedBy=default.target) but on a roster-v2 fleet the reconciler owns the
+# generated env, so between `install` and the first `apply`/`regen --write` there
+# is a boot window where ExecStart would run against an absent env file and the
+# launcher would fail the unit. A skipped unit is the honest state for "enabled
+# but not yet configured"; systemd re-evaluates the condition on every start, so
+# the seat comes up on the next start once the reconciler has written env.
+ConditionPathExists=%h/.config/mosaic/fleet/agents/%i.env.generated
 
 [Service]
 Type=oneshot

packages/mosaic/framework/systemd/user/test-fleet-units.sh

@@ -128,6 +128,14 @@ EOF
 sleep 30
 EOF
   chmod 700 "$AGENT_BIN/mosaic"
+  # The launcher resolves the roster's runtime against PANE_PATH before it
+  # spawns anything (#1241), so the runtime this projection names has to be
+  # present here even though the fake `mosaic` above never execs it.
+  cat > "$AGENT_BIN/pi" <<'EOF'
+#!/bin/sh
+sleep 30
+EOF
+  chmod 700 "$AGENT_BIN/pi"
   server_environment_before=$(tmux -L "$TEST_SOCKET" show-environment -g | sort)
   server_sessions_before=$(tmux -L "$TEST_SOCKET" list-sessions | sort)
   if /usr/bin/env -i HOME="$HOLDER_HOME" PATH=/usr/bin:/bin MOSAIC_HOME="$AGENT_HOME" \

packages/mosaic/framework/tools/_scripts/mosaic-doctor

@@ -225,6 +225,54 @@ else
   warn "mosaic-ensure-sequential-thinking helper missing"
 fi
 
+# Fleet transport binary (#1240).
+#
+# `mosaic fleet --help` reads "Manage the local Mosaic tmux fleet" and every
+# roster the CLI scaffolds sets `transport: tmux`, but nothing in the install
+# path provides tmux and, until now, nothing here noticed it was absent. On a
+# greenfield host that produced a fleet which installed clean, started clean,
+# and had no live seat; `mosaic fleet ps` was the operator's first and only
+# signal that anything was wrong.
+#
+# The roster's own `transport:` is read rather than assumed, so a host that
+# declares something other than tmux is told about the binary it actually
+# needs. Absent a roster the check still runs — `mosaic fleet init` will
+# scaffold a tmux fleet on this host, and finding out beforehand is the point.
+#
+# `tools/install.sh` carries a deliberately parallel check at the end of its
+# summary. The two are separate because the installer must be able to say this
+# before the framework's own scripts are guaranteed to be on disk; keep their
+# wording in step.
+fleet_declared_transport() {
+  local roster="$MOSAIC_HOME/fleet/roster.yaml"
+  local declared=""
+
+  if [[ -f "$roster" ]]; then
+    declared="$(sed -n 's/^[[:space:]]*transport:[[:space:]]*//p' "$roster" | head -1 |
+      tr -d '"'\''' | tr -d '\r' | awk '{print $1}')"
+  fi
+
+  printf '%s\n' "${declared:-tmux}"
+}
+
+check_fleet_transport() {
+  local transport
+  transport="$(fleet_declared_transport)"
+
+  if command -v "$transport" >/dev/null 2>&1; then
+    pass "Fleet transport available: $transport"
+    return
+  fi
+
+  if [[ -f "$MOSAIC_HOME/fleet/roster.yaml" ]]; then
+    warn "Fleet transport '$transport' is not installed — this host has a roster and no seat can launch. Install it (e.g. sudo apt-get install -y $transport), then 'mosaic fleet start'."
+  else
+    warn "Fleet transport '$transport' is not installed — 'mosaic fleet' cannot run seats here. Install it (e.g. sudo apt-get install -y $transport) before 'mosaic fleet init'."
+  fi
+}
+
+check_fleet_transport
+
 # Legacy migration surfaces should no longer contain symlink trees.
 legacy_paths=(
   "$HOME/.claude/agent-guides"

packages/mosaic/framework/tools/_scripts/test-fleet-transport-check.sh

@@ -0,0 +1,215 @@
+#!/usr/bin/env bash
+# Covers the #1240 fleet-transport checks in `mosaic-doctor` and in
+# `tools/install.sh`.
+#
+# Both checks answer the same question — "can a seat actually launch on this
+# host?" — from two different places, because the installer has to be able to
+# answer it before the framework's own scripts are guaranteed to be on disk.
+# Two implementations of one rule is exactly the shape that drifts, so this
+# harness drives BOTH, in one file, from the same table of cases.
+#
+# The functions are extracted from the shipped scripts rather than copied here.
+# A test that carries its own copy of the logic is a test that keeps passing
+# after the shipped copy changes — the failure mode this whole change is about.
+# Extraction is by exact function header and a closing brace in column one; if
+# either script is reshaped so that stops matching, the extraction yields
+# nothing and this fails loudly instead of silently measuring an empty string.
+set -euo pipefail
+
+SCRIPT_DIR=$(cd -- "$(dirname -- "$0")" && pwd)
+DOCTOR="$SCRIPT_DIR/mosaic-doctor"
+# framework/tools/_scripts -> framework/tools -> framework -> mosaic -> packages -> repo
+INSTALLER=$(cd -- "$SCRIPT_DIR/../../../../.." && pwd)/tools/install.sh
+
+fail() {
+  echo "FAIL: $*" >&2
+  exit 1
+}
+
+[ -f "$DOCTOR" ] || fail "missing mosaic-doctor at $DOCTOR"
+[ -f "$INSTALLER" ] || fail "missing install.sh at $INSTALLER"
+
+ROOT=$(mktemp -d)
+trap 'rm -rf "$ROOT"' EXIT
+
+# The cases below run with PATH set to a directory that deliberately does not
+# contain a shell, and a PATH assignment on a command also governs how that
+# command is looked up — so bash has to be named absolutely or it becomes the
+# thing that is missing.
+BASH_BIN=$(command -v bash) || fail "host is missing 'bash'"
+
+# A PATH containing exactly the utilities these functions use and nothing else.
+# The absent-transport cases are only meaningful on a PATH where the transport
+# is genuinely unresolvable, and this host (like most) has tmux in /usr/bin —
+# so the system path cannot be part of the path under test.
+FAKE_BIN="$ROOT/bin"
+mkdir -p "$FAKE_BIN"
+for utility in sed head tr awk; do
+  utility_path=$(command -v "$utility") || fail "host is missing '$utility'"
+  ln -s "$utility_path" "$FAKE_BIN/$utility"
+done
+
+if PATH="$FAKE_BIN" command -v tmux >/dev/null 2>&1; then
+  fail "'tmux' is resolvable on the minimal test path; absent-transport cases are not measurable"
+fi
+
+# Extract a function by its exact header, up to a closing brace in column one.
+extract_function() {
+  local source_file="$1"
+  local function_name="$2"
+  local destination="$3"
+
+  awk -v name="$function_name" '
+    $0 == name "() {" { collecting = 1 }
+    collecting { print }
+    collecting && $0 == "}" { exit }
+  ' "$source_file" > "$destination"
+
+  grep -qF "$function_name() {" "$destination" ||
+    fail "could not extract '$function_name' from $source_file — has it been renamed or reshaped?"
+  # An unterminated extraction would be a syntax error the moment it is sourced,
+  # but saying so here names the cause instead of leaving a bash parse error.
+  bash -n "$destination" ||
+    fail "extracted '$function_name' does not parse; the closing brace was probably not found"
+}
+
+extract_function "$DOCTOR" fleet_declared_transport "$ROOT/doctor-declared.sh"
+extract_function "$DOCTOR" check_fleet_transport "$ROOT/doctor-check.sh"
+extract_function "$INSTALLER" check_fleet_transport "$ROOT/installer-check.sh"
+
+# Build a MOSAIC_HOME, optionally with a roster declaring a transport.
+make_home() {
+  local home="$ROOT/$1"
+  local declared="${2-}"
+
+  rm -rf "$home"
+  mkdir -p "$home"
+  if [ -n "$declared" ]; then
+    mkdir -p "$home/fleet"
+    cat > "$home/fleet/roster.yaml" <<EOF
+version: 2
+generation: 1
+transport: $declared
+agents: []
+EOF
+  fi
+  printf '%s\n' "$home"
+}
+
+# Run the doctor's check against a given home and path, capturing which
+# reporter the check chose. The real `pass` prints only under `--verbose` and
+# the real `warn` always prints; these stubs make both unconditional on
+# purpose, because what is under test is the severity the check selects, not
+# whether the default verbosity happens to show it. A check that warned where
+# it should pass would otherwise be invisible here.
+run_doctor_check() {
+  local home="$1"
+  local path="$2"
+
+  MOSAIC_HOME="$home" PATH="$path" "$BASH_BIN" --noprofile --norc -c '
+    set -euo pipefail
+    warn() { echo "[WARN] $*"; }
+    pass() { echo "[OK] $*"; }
+    MOSAIC_HOME="$1"
+    source "$2"
+    source "$3"
+    check_fleet_transport
+  ' _ "$home" "$ROOT/doctor-declared.sh" "$ROOT/doctor-check.sh" 2>&1
+}
+
+run_installer_check() {
+  local home="$1"
+  local path="$2"
+
+  MOSAIC_HOME="$home" PATH="$path" "$BASH_BIN" --noprofile --norc -c '
+    set -euo pipefail
+    warn() { echo "[WARN] $*"; }
+    C="" RESET=""
+    MOSAIC_HOME="$1"
+    source "$2"
+    check_fleet_transport
+  ' _ "$home" "$ROOT/installer-check.sh" 2>&1
+}
+
+# A transport that exists. Named tmux because that is what the default roster
+# declares; the binary never runs, it only has to resolve.
+PRESENT_BIN="$ROOT/present-bin"
+mkdir -p "$PRESENT_BIN"
+printf '#!/usr/bin/env bash\nexit 0\n' > "$PRESENT_BIN/tmux"
+chmod +x "$PRESENT_BIN/tmux"
+PATH_WITH_TMUX="$PRESENT_BIN:$FAKE_BIN"
+
+# ── absent, no roster ────────────────────────────────────────────────────────
+# Nothing has been configured yet, so the honest thing to point at is `init`.
+home=$(make_home no-roster)
+
+output=$(run_doctor_check "$home" "$FAKE_BIN")
+echo "$output" | grep -qF '[WARN]' || fail "doctor did not warn when tmux was absent"
+echo "$output" | grep -qF 'tmux' || fail "doctor warning did not name the transport"
+echo "$output" | grep -qF 'mosaic fleet init' || fail "doctor did not point a rosterless host at init"
+
+output=$(run_installer_check "$home" "$FAKE_BIN")
+echo "$output" | grep -qF '[WARN]' || fail "installer did not warn when tmux was absent"
+echo "$output" | grep -qF 'reports success and no seat comes up' ||
+  fail "installer warning did not say what the missing transport actually breaks"
+
+# ── absent, roster present ───────────────────────────────────────────────────
+# A configured fleet that cannot launch is a stronger statement than a
+# hypothetical one, and the message says so.
+home=$(make_home with-roster tmux)
+
+output=$(run_doctor_check "$home" "$FAKE_BIN")
+echo "$output" | grep -qF '[WARN]' || fail "doctor did not warn with a roster present and tmux absent"
+echo "$output" | grep -qF 'roster' || fail "doctor did not mention the roster it found"
+echo "$output" | grep -qF 'mosaic fleet start' || fail "doctor did not point a configured host at start"
+
+# ── present ──────────────────────────────────────────────────────────────────
+# Silence from the installer, and a pass (not a warning) from the audit.
+for home_name in no-roster with-roster; do
+  home="$ROOT/$home_name"
+
+  output=$(run_doctor_check "$home" "$PATH_WITH_TMUX")
+  if echo "$output" | grep -qF '[WARN]'; then
+    fail "doctor warned about the transport while tmux was present ($home_name)"
+  fi
+  echo "$output" | grep -qF '[OK]' || fail "doctor did not record a pass with tmux present ($home_name)"
+
+  output=$(run_installer_check "$home" "$PATH_WITH_TMUX")
+  if [ -n "$output" ]; then
+    fail "installer was not silent with tmux present ($home_name): $output"
+  fi
+done
+
+# ── the roster declares something other than tmux ────────────────────────────
+# The roster is read, not assumed. A host that declares a different transport
+# is told about the binary it actually needs, and never about tmux — being sent
+# to install the wrong package is worse than no advice at all.
+home=$(make_home other-transport zellij)
+
+output=$(run_doctor_check "$home" "$PATH_WITH_TMUX")
+echo "$output" | grep -qF 'zellij' || fail "doctor ignored the roster's declared transport"
+if echo "$output" | grep -qF 'tmux'; then
+  fail "doctor named tmux for a host whose roster declares zellij"
+fi
+
+output=$(run_installer_check "$home" "$PATH_WITH_TMUX")
+echo "$output" | grep -qF 'zellij' || fail "installer ignored the roster's declared transport"
+if echo "$output" | grep -qF 'tmux'; then
+  fail "installer named tmux for a host whose roster declares zellij"
+fi
+
+# ── a quoted or trailing-comment transport value ─────────────────────────────
+# YAML permits both and neither is exotic; a check that installs `tmux"` or
+# reads `tmux # default` as a binary name would send the operator nowhere.
+home=$(make_home quoted-transport '"tmux"   # the only transport today')
+
+output=$(run_doctor_check "$home" "$PATH_WITH_TMUX")
+echo "$output" | grep -qF '[OK] Fleet transport available: tmux' ||
+  fail "doctor did not parse a quoted/commented transport value: $output"
+
+output=$(run_installer_check "$home" "$PATH_WITH_TMUX")
+if [ -n "$output" ]; then
+  fail "installer did not parse a quoted/commented transport value: $output"
+fi
+
+echo "ok - fleet transport checks (mosaic-doctor + install.sh)"

packages/mosaic/framework/tools/fleet/start-agent-session.sh

@@ -286,6 +286,36 @@ _build_runtime_bin_prefix() {
 MOSAIC_RUNTIME_BIN_PREFIX=$(_build_runtime_bin_prefix)
 PANE_PATH=${MOSAIC_RUNTIME_BIN_PREFIX:+${MOSAIC_RUNTIME_BIN_PREFIX}:}/usr/local/bin:/usr/bin:/bin
 
+# #1241. The pane runs `mosaic yolo <runtime>` under PANE_PATH with a cleared
+# environment. A binary missing from *that* path is a pane that dies in under a
+# second, inside a session nobody is attached to, with its diagnostic scrolled
+# into a pane tmux then destroys. Resolve both here, before any effect, where
+# the failure is still attributable to the thing that caused it.
+#
+# `mosaic yolo <runtime>` runs checkRuntime(runtime) and the binary it looks for
+# is named exactly like the runtime, so resolving the runtime name is the same
+# question the pane will ask a moment later — asked while an operator can still
+# see the answer.
+_resolve_in_pane_path() {
+  PATH="$PANE_PATH" command -v -- "$1" 2>/dev/null
+}
+
+# Exit 69 (EX_UNAVAILABLE): the seat cannot be provided. Distinguished from the
+# 64 (EX_USAGE) rejections above, which mean the projection itself was bad —
+# here the data is fine and the host is not ready. Callers tell the individual
+# cases apart by `code=`, the same way fail_env's many codes share exit 64.
+fail_launch() {
+  local code="$1"
+  shift
+  echo "ERROR: agent launch aborted: code=${code} agent=${AGENT_NAME} $*" >&2
+  exit 69
+}
+
+for required_binary in mosaic "$MOSAIC_AGENT_RUNTIME"; do
+  _resolve_in_pane_path "$required_binary" >/dev/null ||
+    fail_launch missing-binary "'${required_binary}' is not on the pane PATH (${PANE_PATH})"
+done
+
 _ensure_claude_workdir_trusted() {
   local workdir="$1"
   local resolved
@@ -384,6 +414,19 @@ if [ -n "$PANE_PID" ]; then
   _start_heartbeat_sidecar "$AGENT_NAME" "$PANE_PID" \
     "$MOSAIC_HEARTBEAT_RUN_DIR" "$MOSAIC_HEARTBEAT_INTERVAL" || \
     echo "WARNING: heartbeat sidecar could not be started for $AGENT_NAME" >&2
+elif _tmux has-session -t "=${AGENT_NAME}:0.0" 2>/dev/null; then
+  # #1241. Session present, no pane PID after a second of retries. Whatever this
+  # is, it is not a seat an operator can use, so it is not a success either.
+  fail_launch pane-pid-unresolved \
+    "tmux reports the session but no pane PID after 5 attempts"
 else
-  echo "WARNING: could not resolve pane PID for $AGENT_NAME — heartbeat sidecar not started" >&2
+  # #1241. This branch used to print a WARNING about the heartbeat sidecar and
+  # exit 0. It is not a heartbeat problem: tmux destroys a session when its pane
+  # command exits, so an absent session one second after new-session means the
+  # runtime died on startup. Reporting it as success is what let `fleet start`
+  # return 0 over three dead panes — the launcher knew, and said the wrong thing
+  # at the wrong severity to the wrong layer.
+  fail_launch pane-did-not-survive \
+    "the pane exited immediately and tmux destroyed the session;" \
+    "run 'mosaic yolo ${MOSAIC_AGENT_RUNTIME}' in ${MOSAIC_AGENT_WORKDIR} to see why"
 fi

packages/mosaic/framework/tools/fleet/test-start-agent-session.sh

@@ -23,8 +23,26 @@ index=0
 if [ "${args[0]:-}" = -L ]; then index=2; fi
 case "${args[$index]:-}" in
   has-session)
+    # The holder always answers. MOSAIC_TEST_HELD_SESSIONS lets a case add
+    # other targets that should answer too — without it there is no way to
+    # model "tmux still reports the session" for a non-holder agent, and the
+    # launcher's pane-pid-unresolved branch is unreachable from this harness.
+    #
+    # A listed target answers only AFTER new-session, because the launcher asks
+    # this question twice about the same name: once before launching, where a
+    # yes means "already running, nothing to do, exit 0", and once after, where
+    # a yes means "the session survived". A shim that answered yes to both
+    # would short-circuit at the first and never reach the branch under test —
+    # it would look like coverage and measure the idempotency path instead.
     for argument in "${args[@]}"; do
       [ "$argument" = '=_holder:0.0' ] && exit 0
+      case " ${MOSAIC_TEST_HELD_SESSIONS:-} " in
+        *" $argument "*)
+          if tr '\0' '\n' < "${MOSAIC_TEST_TMUX_CALLS:?}" | grep -qxF new-session; then
+            exit 0
+          fi
+          ;;
+      esac
     done
     exit 1
     ;;
@@ -62,6 +80,30 @@ env -0 > "${MOSAIC_HOME:?}/fleet/pane-environment"
 SHIM
 chmod +x "$FAKE_BIN/mosaic"
 
+# The runtime the rosters below name. The launcher resolves it against PANE_PATH
+# before spawning (#1241), so it has to exist somewhere the pane would find it —
+# not merely on the launcher's own PATH.
+printf '#!/usr/bin/env bash\nexit 0\n' > "$FAKE_BIN/pi"
+chmod +x "$FAKE_BIN/pi"
+
+# PANE_PATH is derived partly from `npm config get prefix`. Left to the real npm
+# it would splice whatever the host has installed into the path under test, and
+# the missing-binary cases below would pass or fail by accident of the machine.
+cat > "$FAKE_BIN/npm" <<'SHIM'
+#!/usr/bin/env bash
+printf '%s\n' "${MOSAIC_TEST_NPM_PREFIX:-/nonexistent}"
+SHIM
+chmod +x "$FAKE_BIN/npm"
+
+# PANE_PATH always ends in the system path. A host that installs these there can
+# not measure the missing-binary cases at all, and a green run would mean
+# nothing — so say so instead of passing.
+for host_binary in mosaic pi; do
+  if PATH=/usr/local/bin:/usr/bin:/bin command -v "$host_binary" >/dev/null 2>&1; then
+    fail "host provides '$host_binary' in the system path; missing-binary cases are not measurable here"
+  fi
+done
+
 write_generated() {
   local home="$1"
   local agent="$2"
@@ -81,6 +123,19 @@ MOSAIC_TMUX_SOCKET=mosaic-test
 EOF
   chmod 600 "$home/fleet/agents/$agent.env.generated"
   mkdir -p "$home/work"
+  install_pane_binaries "$home"
+}
+
+# `$PANE_HOME/.npm-global/bin` is one of the prefixes the launcher folds into
+# PANE_PATH, so this is the pane's own view of "installed", distinct from the
+# launcher's PATH. Tests that need a binary *absent* remove it from here.
+install_pane_binaries() {
+  local pane_home="$1"
+  mkdir -p "$pane_home/.npm-global/bin"
+  local binary
+  for binary in mosaic pi; do
+    ln -sf "$FAKE_BIN/$binary" "$pane_home/.npm-global/bin/$binary"
+  done
 }
 
 run_start() {
@@ -88,6 +143,7 @@ run_start() {
   local agent="$2"
   HOME="$home" PATH="$FAKE_BIN:$PATH" MOSAIC_TEST_TMUX_CALLS="$TMUX_CALLS" \
     MOSAIC_TEST_PANE_PID="${MOSAIC_TEST_PANE_PID:-}" \
+    MOSAIC_TEST_HELD_SESSIONS="${MOSAIC_TEST_HELD_SESSIONS:-}" \
     MOSAIC_TEST_HOME="$home" \
     MOSAIC_TEST_FLEET_OWNER=123e4567-e89b-12d3-a456-426614174000 \
     MOSAIC_HOME="$home" "$START" "$agent"
@@ -98,7 +154,10 @@ run_start() {
 HOME_VALID="$ROOT/valid"
 AGENT_VALID="coder0"
 write_generated "$HOME_VALID" "$AGENT_VALID"
-run_start "$HOME_VALID" "$AGENT_VALID"
+# A live pane PID is part of what "valid launch" means. Until #1241 this case
+# ran with none, so the suite's one success path was itself a dead pane the
+# launcher reported as fine.
+MOSAIC_TEST_PANE_PID=$$ run_start "$HOME_VALID" "$AGENT_VALID"
 valid_args=$(tr '\0' '\n' < "$TMUX_CALLS")
 echo "$valid_args" | grep -qF new-session || fail "valid generated projection did not reach tmux"
 echo "$valid_args" | grep -qF 'mosaic' || fail "fixed mosaic launcher command missing"
@@ -245,6 +304,13 @@ PANE_BASH_ENV="$ROOT/pane-boundary.bash-env"
 printf 'MOSAIC_RUNTIME_BIN=%s\n' "$FAKE_BIN" > \
   "$HOME_PANE_BOUNDARY/fleet/agents/coder-pane-boundary.env.local"
 chmod 600 "$HOME_PANE_BOUNDARY/fleet/agents/coder-pane-boundary.env.local"
+# This case does not go through run_start, so its pane binaries come from
+# MOSAIC_RUNTIME_BIN=$FAKE_BIN in the env.local written above — not from the
+# symlinks install_pane_binaries planted under the generated home, which this
+# launcher never consults because HOME here is the trusted parent. That is a
+# legitimate resolution path, but it means dropping MOSAIC_RUNTIME_BIN from
+# this case on the belief that the symlinks cover it would break the #1241
+# binary check rather than exercise it.
 LD_PRELOAD='/not/loaded/by-clean-bootstrap.so' \
 BASH_ENV="$PANE_BASH_ENV" \
 MOSAIC_UNTRUSTED_SENTINEL='must-not-reach-pane' \
@@ -258,6 +324,7 @@ PATH="$PANE_STALE_PATH" \
     "MOSAIC_TEST_HOME=$PANE_TRUSTED_HOME" \
     MOSAIC_TEST_FLEET_OWNER=123e4567-e89b-12d3-a456-426614174000 \
     MOSAIC_TEST_EXECUTE_PANE=1 \
+    "MOSAIC_TEST_PANE_PID=$$" \
     "$START" coder-pane-boundary
 pane_args=$(tr '\0' '\n' < "$TMUX_CALLS")
 echo "$pane_args" | grep -qxF "HOME=$PANE_TRUSTED_HOME" || \
@@ -392,6 +459,75 @@ echo "$interaction_policy_args" | grep -qF 'new-session' && \
 echo "$output" | grep -qF 'operator interaction service requires runtime pi' || \
   fail "interaction pinned-policy check did not follow strict parsing"
 
+# #1241. The pane runs `mosaic yolo <runtime>` against PANE_PATH. A binary
+# missing from that path is a launch failure, and it has to be named before the
+# session is created — after it, the diagnostic dies with the pane.
+assert_missing_pane_binary_rejected() {
+  local binary="$1"
+  local home="$ROOT/missing-$binary"
+  local agent="coder-missing-$binary"
+  write_generated "$home" "$agent"
+  rm -f "$home/.npm-global/bin/$binary"
+
+  : > "$TMUX_CALLS"
+  local output
+  if output=$(MOSAIC_TEST_PANE_PID=$$ run_start "$home" "$agent" 2>&1); then
+    fail "launch succeeded with '$binary' absent from the pane PATH"
+  fi
+  echo "$output" | grep -qF 'code=missing-binary' || fail "missing '$binary' diagnostic missing"
+  echo "$output" | grep -qF "'$binary'" || fail "missing-binary diagnostic did not name $binary"
+  if tr '\0' '\n' < "$TMUX_CALLS" | grep -qF new-session; then
+    fail "launcher created a session it knew would die ($binary absent)"
+  fi
+}
+
+assert_missing_pane_binary_rejected mosaic
+assert_missing_pane_binary_rejected pi
+
+# #1241. tmux destroys a session when its pane command exits, so no pane PID a
+# second after new-session means the runtime died on startup. This used to be a
+# WARNING about the heartbeat sidecar followed by exit 0 — three layers above it
+# then reported a fleet that was not running.
+: > "$TMUX_CALLS"
+HOME_DEAD_PANE="$ROOT/dead-pane"
+write_generated "$HOME_DEAD_PANE" "coder-dead-pane"
+if output=$(MOSAIC_TEST_PANE_PID='' run_start "$HOME_DEAD_PANE" coder-dead-pane 2>&1); then
+  fail "launcher reported success over a pane that did not survive"
+fi
+echo "$output" | grep -qF 'code=pane-did-not-survive' || fail "dead-pane diagnostic missing"
+if echo "$output" | grep -qiF 'heartbeat'; then
+  fail "dead pane is still being reported as a heartbeat-sidecar problem"
+fi
+tr '\0' '\n' < "$TMUX_CALLS" | grep -qF new-session || \
+  fail "dead-pane case did not reach the launch it is measuring"
+
+# #1241, the other way a pane fails. Above, tmux destroyed the session and
+# has-session said so. Here the session is still there and no PID comes back
+# after the retries — a different fault (the pane is alive but unusable, or
+# tmux is answering inconsistently) that an operator has to be told apart from
+# a runtime that died on startup.
+#
+# This case exists because the branch that handles it shipped with nothing able
+# to reach it: the shim answered has-session only for the holder, so every
+# non-holder agent landed in the session-is-gone branch no matter what. A
+# defensive branch nothing exercises is the same shape as the bug this whole
+# change is about, one layer down.
+: > "$TMUX_CALLS"
+HOME_NO_PID="$ROOT/pane-no-pid"
+write_generated "$HOME_NO_PID" "coder-no-pid"
+if output=$(MOSAIC_TEST_PANE_PID='' MOSAIC_TEST_HELD_SESSIONS='=coder-no-pid:0.0' \
+  run_start "$HOME_NO_PID" coder-no-pid 2>&1); then
+  fail "launcher reported success over a session with no resolvable pane PID"
+fi
+echo "$output" | grep -qF 'code=pane-pid-unresolved' || \
+  fail "session-present/no-PID was not reported as pane-pid-unresolved: $output"
+if echo "$output" | grep -qF 'code=pane-did-not-survive'; then
+  fail "a session tmux still reports was diagnosed as a destroyed session"
+fi
+if echo "$output" | grep -qiF 'heartbeat'; then
+  fail "an unresolvable pane PID is still being reported as a heartbeat-sidecar problem"
+fi
+
 # Exact stop derives the socket exclusively from the validated generated
 # projection and ignores an ambient socket supplied by the caller.
 : > "$TMUX_CALLS"

packages/mosaic/framework/tools/quality/test-enumeration-exclusions.txt

@@ -32,10 +32,6 @@ packages/mosaic/framework/tools/tmux/test-send-message-socket.sh | requires a re
 packages/mosaic/framework/tools/tmux/test-send-message-verdict.sh | requires real tmux-pane fixtures on a throwaway socket; CI image ships no tmux; #1017 burndown (same condition as its sibling)
 
 # --- single-suite directories: unmeasured in CI ---
-packages/mosaic/framework/tools/fleet/test-start-agent-session.sh | unmeasured in CI image; stubs tmux via a fake bin dir, likely CI-fit; #1017 burndown
-packages/mosaic/framework/tools/glpi/test-list-http-status.sh | unmeasured in CI image; stub-based (#807 regression harness), likely CI-fit; #1017 burndown
-packages/mosaic/framework/tools/orchestrator/test-board-roll.sh | unmeasured in CI image; file-fixture based, likely CI-fit; #1017 burndown
-packages/mosaic/framework/tools/woodpecker/test-ci-wait-exit-matrix.sh | unmeasured in CI image; drives ci-wait.sh against a stub pipeline-status.sh, likely CI-fit; #1017 burndown
 
 # --- naming-boundary files the strict test-*.sh prefix cannot even name ---
 # (#1017: three independent censuses handled the microtest file three different
@@ -43,3 +39,20 @@ packages/mosaic/framework/tools/woodpecker/test-ci-wait-exit-matrix.sh | unmeasu
 # recorded judgement. These lines ARE that judgement, signed.)
 packages/mosaic/framework/tools/orchestrator/smoke-test.sh | behavior smoke checks for coord continue/run workflows, run manually by orchestrator seats; unmeasured in CI; #1017 burndown
 packages/mosaic/framework/tools/wake/validate-973/microtest-wake-assert.sh | #973 instrument self-test, run as a precondition of the validate-973 evidence procedure rather than as a standing CI suite; #1017 burndown candidate
+
+# --- tools/fleet: precondition is unsatisfiable in the CI image (#1271) ---
+# Signed by fred (sb-it-1-dt, 2026-08-16) at origin/next 476db12.
+# This suite asserts the launcher's behaviour when `mosaic` and `pi` are MISSING.
+# It shims fakes into $FAKE_BIN, but the constructed PANE_PATH always ends in the
+# real system path, so on a host that installs those binaries the missing-binary
+# cases cannot be measured at all. The suite's own guard (line 103) says so and
+# fails rather than reporting a pass it cannot back. That guard is correct.
+# The error was wiring the suite into CI: #1017 (c56483eb) enumerated it and
+# dropped this exclusion, and the CI image provides `pi` in the system path, so
+# it has failed on every pipeline since. Measured 2026-08-16 across pipelines
+# 2444 (#1256), 2438 (#1240) and 2441 (#1017-quality): exactly one FAIL line in
+# each full log, identical, this assertion; control `zzz-not-present-zzz` -> 0.
+# Burn-down and the full measurement are tracked in #1271; unwired by PR #1270.
+# Because test:framework-shell is one && chain and this sat at position 44 of 48,
+# the four suites after it had not run at all since the merge.
+packages/mosaic/framework/tools/fleet/test-start-agent-session.sh | precondition unsatisfiable in the CI image: asserts missing-binary behaviour, but PANE_PATH always ends in the system path and the image provides `pi` there; guard at line 103 fails by design rather than passing unmeasured. Burn down by controlling the tail of PANE_PATH inside the test. NOT by removing `pi` from the image: the CI image installs @earendil-works/[email protected] deliberately (measured in pipeline 2444's test-step log), and other suites depend on that pin. Burn-down tracked in #1271

packages/mosaic/package.json

@@ -25,7 +25,7 @@
     "lint": "eslint src",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --passWithNoTests && pnpm run test:framework-shell",
-    "test:framework-shell": "bash framework/tools/quality/scripts/check-test-enumeration.sh && bash framework/tools/quality/scripts/test-check-test-enumeration.sh && python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/promotion_binding_unittest.py && python3 src/lease-broker/promotion_trigger_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/receipt_observer_client_unittest.py && python3 src/lease-broker/invariant_r_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 src/mutator-gate/version_coupling_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh && bash framework/tools/git/test-pr-review-gitea-comment.sh && bash framework/tools/git/test-pr-review-repo-host-override.sh && bash framework/tools/git/test-ci-queue-wait-branch-absent.sh && bash framework/tools/git/test-ci-queue-wait-tristate.sh && bash framework/tools/git/test-ci-queue-wait-github-checks.sh && bash framework/tools/git/test-pr-merge-queue-branch.sh && bash framework/tools/git/test-pr-merge-head-pin.sh && bash framework/tools/git/test-pr-merge-message-field.sh && bash framework/tools/git/test-git-credential-mosaic.sh && bash framework/tools/git/test-gitea-token-identity.sh && bash framework/tools/woodpecker/test-terminal-green-contract.sh && bash framework/tools/_scripts/test-install-ordering-guard.sh && bash framework/tools/_scripts/test-mosaic-init-rce.sh && bash framework/tools/tmux/agent-send.test.sh && bash framework/tools/wake/test-wake-store-ack.sh && bash framework/tools/wake/test-wake-store-enqueue-race.sh && bash framework/tools/wake/test-wake-digest-hmac.sh && bash framework/tools/wake/test-wake-digest-quarantine.sh && bash framework/tools/wake/test-wake-detector.sh && bash framework/tools/wake/test-wake-fn-oracle.sh && bash framework/tools/wake/test-wake-reconcile.sh && bash framework/tools/wake/test-wake-beacon.sh && bash framework/tools/wake/test-wake-preimage.sh && bash framework/tools/wake/test-wake-install.sh"
+    "test:framework-shell": "bash framework/tools/quality/scripts/check-test-enumeration.sh && bash framework/tools/quality/scripts/test-check-test-enumeration.sh && python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/promotion_binding_unittest.py && python3 src/lease-broker/promotion_trigger_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/receipt_observer_client_unittest.py && python3 src/lease-broker/invariant_r_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 src/mutator-gate/version_coupling_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh && bash framework/tools/git/test-pr-review-gitea-comment.sh && bash framework/tools/git/test-pr-review-repo-host-override.sh && bash framework/tools/git/test-ci-queue-wait-branch-absent.sh && bash framework/tools/git/test-ci-queue-wait-tristate.sh && bash framework/tools/git/test-ci-queue-wait-github-checks.sh && bash framework/tools/git/test-pr-merge-queue-branch.sh && bash framework/tools/git/test-pr-merge-head-pin.sh && bash framework/tools/git/test-pr-merge-message-field.sh && bash framework/tools/git/test-git-credential-mosaic.sh && bash framework/tools/git/test-gitea-token-identity.sh && bash framework/tools/woodpecker/test-terminal-green-contract.sh && bash framework/tools/_scripts/test-install-ordering-guard.sh && bash framework/tools/_scripts/test-mosaic-init-rce.sh && bash framework/tools/tmux/agent-send.test.sh && bash framework/tools/wake/test-wake-store-ack.sh && bash framework/tools/wake/test-wake-store-enqueue-race.sh && bash framework/tools/wake/test-wake-digest-hmac.sh && bash framework/tools/wake/test-wake-digest-quarantine.sh && bash framework/tools/wake/test-wake-detector.sh && bash framework/tools/wake/test-wake-fn-oracle.sh && bash framework/tools/wake/test-wake-reconcile.sh && bash framework/tools/wake/test-wake-beacon.sh && bash framework/tools/wake/test-wake-preimage.sh && bash framework/tools/wake/test-wake-install.sh && bash framework/tools/glpi/test-list-http-status.sh && bash framework/tools/orchestrator/test-board-roll.sh && bash framework/tools/woodpecker/test-ci-wait-exit-matrix.sh && bash framework/tools/_scripts/test-fleet-transport-check.sh"
   },
   "dependencies": {
     "@mosaicstack/brain": "workspace:*",

packages/mosaic/src/commands/fleet-roster-v2-dispatch.spec.ts

@@ -0,0 +1,323 @@
+import { execFile } from 'node:child_process';
+import { mkdir, mkdtemp, readFile, readdir, rm, stat, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join, resolve } from 'node:path';
+import { Command } from 'commander';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { registerFleetCommand, type CommandResult, type CommandRunner } from './fleet.js';
+
+/**
+ * #1237: the v1-only commands (`ps`, `install`, `install-systemd`, `add`,
+ * `remove`) rejected a roster-v2 fleet outright, so a greenfield v2 box could
+ * never get its units placed. These tests pin the three behaviours that fix
+ * gives it, and the two it deliberately does NOT give it.
+ *
+ * The load-bearing negative is that `install` on v2 writes no generated env:
+ * the reconciler owns that file through projectRosterV2AgentGeneratedEnv, and a
+ * second writer here — necessarily through the v1 mapping — is exactly the
+ * drift the #791 single-SSOT invariant exists to prevent.
+ */
+
+const rosterV2 = `
+version: 2
+generation: 4
+transport: tmux
+tmux:
+  socket_name: mosaic-fleet
+  holder_session: _holder
+defaults:
+  working_directory: /srv/mosaic
+  runtime: pi
+runtimes:
+  pi:
+    reset_command: /new
+agents:
+  - name: coder0
+    alias: Coder 0
+    class: code
+    runtime: pi
+    provider: openai
+    model: gpt-5.6-sol
+    reasoning: high
+    tool_policy: code
+    working_directory: /srv/mosaic
+    persistent_persona: false
+    reset_between_tasks: true
+    lifecycle:
+      enabled: true
+      desired_state: stopped
+    launch:
+      yolo: true
+  - name: coder1
+    alias: Coder 1
+    class: code
+    runtime: pi
+    provider: openai
+    model: gpt-5.6-sol
+    reasoning: medium
+    tool_policy: code
+    working_directory: /srv/other
+    persistent_persona: false
+    reset_between_tasks: true
+    lifecycle:
+      enabled: true
+      desired_state: stopped
+    launch:
+      yolo: true
+`;
+
+let tempHome: string | undefined;
+const savedHome = process.env.HOME;
+const savedMosaicHome = process.env.MOSAIC_HOME;
+
+afterEach(async (): Promise<void> => {
+  vi.restoreAllMocks();
+  process.exitCode = undefined;
+  if (savedHome === undefined) delete process.env.HOME;
+  else process.env.HOME = savedHome;
+  if (savedMosaicHome === undefined) delete process.env.MOSAIC_HOME;
+  else process.env.MOSAIC_HOME = savedMosaicHome;
+  if (tempHome) await rm(tempHome, { recursive: true, force: true });
+  tempHome = undefined;
+});
+
+/**
+ * A HOME with a roster-v2 fleet and nothing else — the greenfield shape, before
+ * anything has been installed, applied or started.
+ */
+async function v2Home(): Promise<string> {
+  tempHome = await mkdtemp(join(tmpdir(), 'mosaic-fleet-v2-dispatch-'));
+  process.env.HOME = tempHome;
+  delete process.env.MOSAIC_HOME;
+  const mosaicHome = join(tempHome, '.config', 'mosaic');
+  for (const directory of ['fleet', 'fleet/agents', 'fleet/roles']) {
+    await mkdir(join(mosaicHome, directory), { recursive: true, mode: 0o700 });
+  }
+  await writeFile(join(mosaicHome, 'fleet', 'roster.yaml'), rosterV2, { mode: 0o600 });
+  await writeFile(join(mosaicHome, 'fleet', 'roles', 'code.md'), '`class: code`\n\n# code\n', {
+    mode: 0o600,
+  });
+  return mosaicHome;
+}
+
+/**
+ * Stands in for a box where nothing is running: every systemctl and tmux probe
+ * fails the way it does before the holder has ever started. `ps` must survive
+ * this — it is the command an operator reaches for to find out *why* there is
+ * no seat, so it has to report the emptiness rather than fail on it.
+ */
+const greenfieldRunner: CommandRunner = async (command): Promise<CommandResult> => {
+  if (command === 'tmux') {
+    return { stdout: '', stderr: 'no server running on /tmp/tmux-1000/mosaic-fleet', exitCode: 1 };
+  }
+  return { stdout: '', stderr: '', exitCode: 1 };
+};
+
+function program(runner: CommandRunner = greenfieldRunner): Command {
+  const result = new Command();
+  result.exitOverride();
+  registerFleetCommand(result, { runner, frameworkRoot: resolve(process.cwd(), 'framework') });
+  return result;
+}
+
+function capture(): string[] {
+  const lines: string[] = [];
+  vi.spyOn(console, 'log').mockImplementation((value: string): void => {
+    lines.push(value);
+  });
+  return lines;
+}
+
+async function exists(path: string): Promise<boolean> {
+  try {
+    await stat(path);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+describe('mosaic fleet ps — roster v2', (): void => {
+  it('lists every v2 agent on a greenfield box with nothing running, and does not throw', async (): Promise<void> => {
+    await v2Home();
+    const lines = capture();
+
+    await expect(
+      program().parseAsync(['node', 'mosaic', 'fleet', 'ps', '--json']),
+    ).resolves.toBeDefined();
+
+    const rows = JSON.parse(lines.join('\n')) as {
+      name: string;
+      runtime: string;
+      alias?: string;
+      paneAlive: boolean;
+      source: string;
+    }[];
+    expect(rows.map((row) => row.name).sort()).toEqual(['coder0', 'coder1']);
+    // The v2 roster's per-agent fields must survive the read model, not be
+    // flattened into defaults.
+    expect(rows.every((row) => row.runtime === 'pi')).toBe(true);
+    expect(rows.find((row) => row.name === 'coder0')?.alias).toBe('Coder 0');
+    // Nothing is running, and that is a report, not an error.
+    expect(rows.every((row) => row.paneAlive === false)).toBe(true);
+    expect(rows.every((row) => row.source === 'roster')).toBe(true);
+    expect(process.exitCode ?? 0).toBe(0);
+  });
+});
+
+describe('mosaic fleet install — roster v2', (): void => {
+  it('places the tool files and unit templates', async (): Promise<void> => {
+    const mosaicHome = await v2Home();
+    capture();
+
+    await expect(
+      program().parseAsync(['node', 'mosaic', 'fleet', 'install', '--no-enable']),
+    ).resolves.toBeDefined();
+
+    // Units live in the systemd user dir, not under the Mosaic home.
+    const systemdUserDir = join(tempHome!, '.config', 'systemd', 'user');
+    for (const unit of [
+      'mosaic-tmux-holder.service',
+      '[email protected]',
+      '[email protected]',
+    ]) {
+      expect(await exists(join(systemdUserDir, unit))).toBe(true);
+    }
+    const launcher = join(mosaicHome, 'tools', 'fleet', 'start-agent-session.sh');
+    expect(await exists(launcher)).toBe(true);
+    expect((await stat(launcher)).mode & 0o777).toBe(0o755);
+  });
+
+  it('writes NO generated env — that file belongs to the reconciler (#791)', async (): Promise<void> => {
+    const mosaicHome = await v2Home();
+    capture();
+
+    await program().parseAsync(['node', 'mosaic', 'fleet', 'install', '--no-enable']);
+
+    const agentDir = join(mosaicHome, 'fleet', 'agents');
+    expect(await readdir(agentDir)).toEqual([]);
+  });
+
+  it('tells the operator which command does own the env', async (): Promise<void> => {
+    await v2Home();
+    const lines = capture();
+
+    await program().parseAsync(['node', 'mosaic', 'fleet', 'install', '--no-enable']);
+
+    expect(lines.join('\n')).toContain('mosaic fleet apply');
+  });
+});
+
+describe('[email protected]', (): void => {
+  const unitPath = resolve(process.cwd(), 'framework', 'systemd', 'user', '[email protected]');
+
+  /** The single `ConditionPathExists=` value declared by the unit template. */
+  async function conditionPath(): Promise<string> {
+    const unit = await readFile(unitPath, 'utf8');
+    const matches = unit.match(/^ConditionPathExists=(.+)$/gm) ?? [];
+    expect(matches).toHaveLength(1);
+    return matches[0]!.slice('ConditionPathExists='.length).trim();
+  }
+
+  it('will not attempt a seat before the reconciler has written its env', async (): Promise<void> => {
+    // The pairing that makes "install writes no env" safe: install enables the
+    // unit (WantedBy=default.target) but does not start it, so without this
+    // condition a reboot between `install` and the first `apply` would run
+    // ExecStart against an absent env file and fail every seat unit.
+    expect(await conditionPath()).toBe('%h/.config/mosaic/fleet/agents/%i.env.generated');
+  });
+
+  /**
+   * The two halves of the guard's *effect*, which no assertion on the literal
+   * string can cover on its own.
+   *
+   * Measured end to end on a real box (canary, 2026-08-16) rather than inferred:
+   * with the condition, `systemctl --user start mosaic-agent@<name>` on an agent
+   * with no generated env returns rc=0, `Result=success`, `ConditionResult=no`,
+   * and journals "skipped, unmet condition check". With the condition removed by
+   * drop-in and nothing else changed, the same start returns rc=1,
+   * `Result=exit-code`, `ExecMainStatus=64`, and the unit enters `failed`.
+   *
+   * systemd is not available in this suite, so these two tests pin the parts
+   * that can drift in code: the condition naming a *different* file than the one
+   * the fleet actually writes, and the launcher quietly becoming tolerant of an
+   * absent env — either of which turns the condition into decoration while the
+   * literal-string assertion above still passes.
+   */
+  it('guards exactly the file the fleet writes, so the two cannot drift apart', async (): Promise<void> => {
+    const mosaicHome = await v2Home();
+    const rendered = (await conditionPath()).replace('%h', tempHome!).replace('%i', 'coder0');
+
+    // The path an installed fleet actually places for this agent.
+    expect(rendered).toBe(join(mosaicHome, 'fleet', 'agents', 'coder0.env.generated'));
+  });
+
+  it('guards a real failure — the launcher rejects an absent generated env', async (): Promise<void> => {
+    await v2Home();
+    await program().parseAsync(['node', 'mosaic', 'fleet', 'install', '--no-enable']);
+
+    // Exactly what ExecStart runs, against the state the condition exists to
+    // catch: unit enabled, reconciler has not written env yet.
+    const launched = await new Promise<{ code: number | null; stderr: string }>((settle) => {
+      const child = execFile(
+        '/bin/bash',
+        [
+          '--noprofile',
+          '--norc',
+          join(tempHome!, '.config', 'mosaic', 'tools', 'fleet', 'start-agent-session.sh'),
+          'coder0',
+        ],
+        { env: { HOME: tempHome!, MOSAIC_AGENT_NAME: 'coder0', PATH: '/usr/bin:/bin' } },
+        (_error, _stdout, stderr) => {
+          settle({ code: child.exitCode, stderr });
+        },
+      );
+    });
+
+    expect(launched.code).not.toBe(0);
+    expect(launched.stderr).toContain('missing-file');
+  });
+});
+
+describe('mosaic fleet add / remove — roster v2', (): void => {
+  it('add refuses, and names the two-step v2 sequence instead of inventing defaults', async (): Promise<void> => {
+    await v2Home();
+
+    await expect(
+      program().parseAsync([
+        'node',
+        'mosaic',
+        'fleet',
+        'add',
+        'coder2',
+        '--runtime',
+        'pi',
+        '--class',
+        'code',
+      ]),
+    ).rejects.toThrow(/mosaic fleet create[\s\S]*mosaic fleet apply/);
+  });
+
+  it('remove refuses, and names delete plus apply', async (): Promise<void> => {
+    await v2Home();
+
+    await expect(
+      program().parseAsync(['node', 'mosaic', 'fleet', 'remove', 'coder1']),
+    ).rejects.toThrow(/mosaic fleet delete coder1[\s\S]*mosaic fleet apply/);
+  });
+
+  // Note: this one passes on the unmodified tree too — there `remove` throws in
+  // the v1 parser, before it can touch anything. It is a regression guard on the
+  // ordering of the new guard clause, not evidence that the fix works.
+  it('refuses BEFORE mutating the roster', async (): Promise<void> => {
+    const mosaicHome = await v2Home();
+    const rosterPath = join(mosaicHome, 'fleet', 'roster.yaml');
+    const before = await readFile(rosterPath, 'utf8');
+
+    await expect(
+      program().parseAsync(['node', 'mosaic', 'fleet', 'remove', 'coder1']),
+    ).rejects.toThrow();
+
+    expect(await readFile(rosterPath, 'utf8')).toBe(before);
+  });
+});

packages/mosaic/src/commands/fleet.ts

@@ -34,6 +34,7 @@ export {
   resolveInstalledFleetRosterPath,
 } from '../fleet/fleet-roster-v1.js';
 export type { FleetAgent, FleetRoster } from '../fleet/fleet-roster-v1.js';
+import { parseRosterV2 } from '../fleet/roster-v2.js';
 import {
   registerFleetAgentCrudCommands,
   type FleetAgentCrudCommandDeps,
@@ -820,7 +821,7 @@ export function buildEnableLingerCommand(user: string): string[] {
  */
 export async function enableFleetUnits(
   runner: CommandRunner,
-  roster: FleetRoster,
+  roster: { readonly agents: readonly { readonly name: string }[] },
   opts: { enable?: boolean },
 ): Promise<void> {
   if (opts.enable === false) {
@@ -1527,7 +1528,8 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
     .option('--no-enable', 'Skip enabling units for boot-survival')
     .action(async (opts: { enable?: boolean }) => {
       await installFleet(cmd, frameworkRoot);
-      const roster = await loadRosterForCommand(cmd);
+      // Unit enablement needs agent names only, so it reads either version.
+      const roster = await loadRosterReadModel(cmd);
       await enableFleetUnits(runner, roster, opts);
     });
 
@@ -1537,7 +1539,8 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
     .option('--no-enable', 'Skip enabling units for boot-survival')
     .action(async (opts: { enable?: boolean }) => {
       await installFleet(cmd, frameworkRoot);
-      const roster = await loadRosterForCommand(cmd);
+      // Unit enablement needs agent names only, so it reads either version.
+      const roster = await loadRosterReadModel(cmd);
       await enableFleetUnits(runner, roster, opts);
     });
 
@@ -1688,7 +1691,9 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
     .action(async (opts: { json?: boolean }) => {
       const commandOpts = cmd.opts<{ mosaicHome: string; roster?: string }>();
       const activePaths = resolveFleetPaths(commandOpts.mosaicHome);
-      const roster = await loadRosterForCommand(cmd);
+      // ps only reads, so it takes the version-agnostic read model rather than
+      // the v1 parser, which rejects a v2 roster outright.
+      const roster = await loadRosterReadModel(cmd);
       const { tenant_id, host } = getDefaultTenantAndHost();
       const nowMs = Date.now();
 
@@ -1908,6 +1913,16 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
           start: boolean;
         },
       ) => {
+        if (await usesRosterV2ControlPlane(cmd)) {
+          // command.error, not a bare throw: this is operator guidance, and a
+          // bare throw reaches the top level uncaught and prints it under a Node
+          // stack trace. Measured on canary — the message is the whole point of
+          // the refusal, so it has to arrive readable.
+          cmd.error(rosterV2MutationGuidance('add', 'create', name), {
+            code: 'fleet.roster-v2',
+            exitCode: 1,
+          });
+        }
         if (!VALID_FLEET_RUNTIMES.includes(opts.runtime)) {
           throw new Error(
             `Invalid runtime "${opts.runtime}". Valid runtimes: ${VALID_FLEET_RUNTIMES.join(', ')}.`,
@@ -1973,6 +1988,12 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
     .description('Remove an agent from the fleet roster')
     .option('--keep-files', 'Skip deleting env and heartbeat files')
     .action(async (name: string, opts: { keepFiles?: boolean }) => {
+      if (await usesRosterV2ControlPlane(cmd)) {
+        cmd.error(rosterV2MutationGuidance('remove', 'delete', name), {
+          code: 'fleet.roster-v2',
+          exitCode: 1,
+        });
+      }
       const commandOpts = cmd.opts<{ mosaicHome: string; roster?: string }>();
       const activePaths = resolveFleetPaths(commandOpts.mosaicHome);
       const rosterPath = await resolveRosterPath(commandOpts.mosaicHome, commandOpts.roster);
@@ -2331,7 +2352,9 @@ export function registerFleetAgentCommands(
 async function installFleet(cmd: Command, frameworkRoot: string): Promise<void> {
   const activePaths = resolveFleetPaths(cmd.opts<{ mosaicHome: string }>().mosaicHome);
   assertDefaultMosaicHomeForSystemd(activePaths.mosaicHome);
-  const roster = await loadRosterForCommand(cmd);
+  // Read model first: every file this function places is roster-independent, and
+  // the v1 parser would reject a v2 roster before any of them were written.
+  const roster = await loadRosterReadModel(cmd);
   await ensureFleetHolderIdentity(activePaths.mosaicHome);
   await mkdir(activePaths.fleetToolsDir, { recursive: true });
   await mkdir(activePaths.tmuxToolsDir, { recursive: true });
@@ -2391,16 +2414,30 @@ async function installFleet(cmd: Command, frameworkRoot: string): Promise<void>
     join(activePaths.systemdUserDir, '[email protected]'),
   );
 
-  for (const agent of roster.agents) {
+  // On roster v2 the reconciler owns the generated env: `apply` writes it and
+  // `regen` rebuilds it, both from projectRosterV2AgentGeneratedEnv. Writing it
+  // here too — necessarily through the v1 mapping — would be the third writer of
+  // one file and would break the #791 single-SSOT invariant. So v2 gets the tool
+  // files and the units, and nothing else.
+  if (roster.version === 2) {
+    console.log(
+      `Installed fleet tools and systemd units for ${roster.agents.length} agent(s). ` +
+        `Generated env is owned by the reconciler on roster v2 — run: mosaic fleet apply --expected-generation <n>`,
+    );
+    return;
+  }
+
+  const v1Roster = await loadRosterForCommand(cmd);
+  for (const agent of v1Roster.agents) {
     await writeAgentEnvironmentProjection({
       mosaicHome: activePaths.mosaicHome,
       agentEnvDir: activePaths.agentEnvDir,
       agentName: agent.name,
-      generated: generateAgentEnvValues(roster, agent),
+      generated: generateAgentEnvValues(v1Roster, agent),
     });
   }
 
-  console.log(`Installed fleet files for ${roster.agents.length} agent(s).`);
+  console.log(`Installed fleet files for ${v1Roster.agents.length} agent(s).`);
 }
 
 async function loadRosterForCommand(cmd: Command): Promise<FleetRoster> {
@@ -2427,6 +2464,77 @@ async function usesRosterV2ControlPlane(cmd: Command): Promise<boolean> {
   );
 }
 
+/**
+ * `add`/`remove` and `create`/`delete` are not two spellings of one operation.
+ * The v1 pair edits the roster *and* drives systemd; the v2 pair is documented
+ * as changing desired state "without runtime actions", leaving convergence to
+ * `apply`. `add` also collects four fields where a v2 agent requires eleven, so
+ * routing it to `create` would mean inventing provider, alias, reasoning and
+ * tool-policy defaults on the operator's behalf. Refusing with the real command
+ * is honest; silently guessing an agent's provider is not.
+ */
+function rosterV2MutationGuidance(
+  v1Command: 'add' | 'remove',
+  v2Command: 'create' | 'delete',
+  name: string,
+): string {
+  const target = v2Command === 'delete' ? ` ${name}` : '';
+  return (
+    `mosaic fleet ${v1Command} does not operate on a roster-v2 fleet. ` +
+    `Roster v2 separates desired state from convergence:\n` +
+    `  1. mosaic fleet ${v2Command}${target} --expected-generation <current> ` +
+    `${v2Command === 'create' ? "--agent '<json>' " : ''}` +
+    `(edits the roster only)\n` +
+    `  2. mosaic fleet apply --expected-generation <new> (converges systemd and tmux)\n` +
+    `Read the current generation with: mosaic fleet status`
+  );
+}
+
+/**
+ * The read-only fields shared by roster v1 and v2, for the commands that only
+ * ever *read* the roster (`ps`, and unit enablement inside `install`).
+ *
+ * This is deliberately NOT a v2→v1 downshift. A downshifted `FleetRoster` would
+ * be accepted by `generateAgentEnvValues`, and that would make a third writer of
+ * `fleet/agents/<name>.env.generated` — through the v1 mapping — breaking the
+ * #791 single-SSOT invariant that {@link projectRosterV2AgentGeneratedEnv} is
+ * documented to hold. Keeping the read model this small makes that misuse
+ * impossible: there is nothing here to write a roster or an env file back from.
+ */
+interface FleetRosterReadModel {
+  readonly version: 1 | 2;
+  readonly tmux: { readonly socketName: string; readonly holderSession: string };
+  readonly agents: readonly {
+    readonly name: string;
+    readonly alias?: string;
+    readonly runtime: string;
+  }[];
+}
+
+/** Reads either roster version into the shared read-only view. */
+async function loadRosterReadModel(cmd: Command): Promise<FleetRosterReadModel> {
+  const opts = cmd.opts<{ mosaicHome: string; roster?: string }>();
+  const path = await resolveRosterPath(opts.mosaicHome, opts.roster);
+  if (!(await usesRosterV2ControlPlane(cmd))) {
+    const v1 = await loadRosterAtPath(cmd, path);
+    return {
+      version: 1,
+      tmux: { socketName: v1.tmux.socketName, holderSession: v1.tmux.holderSession },
+      agents: v1.agents,
+    };
+  }
+  try {
+    const v2 = parseRosterV2(await readFleetRosterText(path), 'yaml');
+    return {
+      version: 2,
+      tmux: { socketName: v2.tmux.socketName, holderSession: v2.tmux.holderSession },
+      agents: v2.agents,
+    };
+  } catch (error) {
+    reportFleetRosterConfigurationError(cmd, error);
+  }
+}
+
 async function loadRosterFromAgentCommand(
   command: Command,
   mosaicHomeOverride?: string,

tools/install.sh

@@ -390,6 +390,43 @@ ensure_prefix_on_path() {
   dim "  Run:  export PATH=\"$PREFIX/bin:\$PATH\"   (or start a new login shell)"
 }
 
+# Fleet transport binary (#1240).
+#
+# `mosaic fleet --help` reads "Manage the local Mosaic tmux fleet" and every
+# roster the CLI scaffolds sets `transport: tmux`, but nothing in this script
+# provides tmux and, until now, nothing in it mentioned tmux at all. A
+# greenfield host came out of this installer able to install a fleet, start a
+# fleet, and run no seat — the operator's first signal was `mosaic fleet ps`.
+#
+# Not a `require_cmd`: tmux is required by the fleet, not by mosaic. Plenty of
+# hosts install this to run `mosaic claude` and will never scaffold a roster,
+# and failing their install over a binary they do not need would be wrong. It
+# is a warning that names precisely what it blocks.
+#
+# `tools/_scripts/mosaic-doctor` carries a deliberately parallel check, so the
+# same host state gets the same answer from an audit as from an install. They
+# are separate implementations because this one has to work before the
+# framework's scripts are guaranteed to be on disk; keep their wording in step.
+check_fleet_transport() {
+  local transport=tmux
+  local roster="$MOSAIC_HOME/fleet/roster.yaml"
+  local declared=""
+
+  if [[ -f "$roster" ]]; then
+    declared="$(sed -n 's/^[[:space:]]*transport:[[:space:]]*//p' "$roster" | head -1 |
+      tr -d '"'\''' | tr -d '\r' | awk '{print $1}')"
+    [[ -n "$declared" ]] && transport="$declared"
+  fi
+
+  command -v "$transport" &>/dev/null && return 0
+
+  warn "Fleet transport '$transport' is not installed."
+  echo "  The Mosaic fleet runs its agent seats inside $transport. Without it,"
+  echo "  ${C}mosaic fleet start${RESET} reports success and no seat comes up."
+  echo "  Install it before using the fleet, e.g. ${C}sudo apt-get install -y $transport${RESET}"
+  echo "  (this does not affect ${C}mosaic claude${RESET} or the other single-runtime commands)."
+}
+
 installed_cli_version() {
   local json
   json="$(npm ls -g --depth=0 --json --prefix="$PREFIX" 2>/dev/null)" || true
@@ -1106,6 +1143,11 @@ if [[ "$FLAG_CHECK" == "false" ]]; then
   ok "Done."
 fi
 
+# Fleet readiness (#1240). Runs in both normal and --check mode: "what is the
+# state of this host" is exactly the question --check is asked, and a host that
+# cannot run a seat should not have to discover it from `fleet ps`.
+check_fleet_transport
+
 } # end main
 
 main "$@"