fix(web): scope model list to provider and resolve composite catalog identity

Fix-in-lane for the two Task-4 review findings, red-first TDD, inside the
existing nine-file fence.

M-A: the composer now filters model <option>s to the intentionally selected
provider (harness.providerId). With no provider chosen the model select offers
only its placeholder, so a user can never pick a model that belongs to a
different provider.

M-B: the model <option> value is now the collision-safe composite
`${providerId}:${modelId}` (was the bare modelId), the controlled select value
mirrors that same identity so the exact catalog row highlights, and the change
handler resolves the composite back to the exact catalog row and persists that
row's own {harnessId, providerId, modelId}. selectModel(providerId, modelId) no
longer combines a bare model id with ambient provider state, so two providers
exposing the same modelId stay distinct.

New red-first tests prove: cross-provider models absent, identical modelIds
under two providers stay distinct and resolve to the intended tuple, a provider
change invalidates the old model and keeps send disabled until the new tuple
persists, and a model pick does not enable send until its exact PUT resolves.
The four existing anti-masking invariants and the 422 tuple-preservation test
are intact.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ESFAnh2t9HmLwng8oW95St

apps/web/src/lib/types.ts

@@ -1,3 +1,42 @@
+import type {
+  HarnessAuthState,
+  HarnessModelAvailability,
+  HarnessSelection,
+} from '@mosaicstack/types';
+
+// The exact harness/provider/model tuple and its closed enum companions are the
+// shared domain types — re-exported here so web consumers (and the runtime
+// guards) import one shape, never a divergent local redefinition.
+export type { HarnessSelection, HarnessAuthState, HarnessModelAvailability };
+
+/** Harness summary row from `GET /api/harnesses` (the `HarnessSummaryDto`). The
+ * harness id is kept distinct from any provider id — they are never merged. */
+export interface HarnessSummary {
+  id: string;
+  displayName: string;
+  capabilities: string[];
+}
+
+/** One selectable model in a harness catalog. Extends the `{harnessId,
+ * providerId, modelId}` tuple with the display/availability metadata the UI
+ * needs; `inputTypes` is kept as a plain `string[]` on the client boundary
+ * because it arrives from untrusted JSON and is only ever displayed. */
+export interface HarnessCatalogEntry extends HarnessSelection {
+  displayName: string;
+  reasoningCapability: boolean;
+  inputTypes: string[];
+  authState: HarnessAuthState;
+  availability: HarnessModelAvailability;
+}
+
+/** Harness-scoped catalog from `GET /api/harnesses/:harnessId/catalog`. */
+export interface HarnessCatalog {
+  harnessId: string;
+  version: string;
+  fingerprint: string;
+  models: HarnessCatalogEntry[];
+}
+
 /** Conversation returned by the gateway API. */
 export interface Conversation {
   id: string;

apps/web/src/spa/chat/chat-api.spec.ts

@@ -0,0 +1,195 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import {
+  fetchCatalog,
+  fetchHarnesses,
+  fetchPersistedSelection,
+  persistSelection,
+} from './chat-api';
+
+function json(body: unknown, status = 200): Response {
+  return new Response(JSON.stringify(body), {
+    status,
+    headers: { 'Content-Type': 'application/json' },
+  });
+}
+
+function stubFetch(): ReturnType<typeof vi.fn> {
+  const fetchMock = vi.fn();
+  vi.stubGlobal('fetch', fetchMock);
+  return fetchMock;
+}
+
+/** Every URL the client actually requested, across all calls. */
+function requestedUrls(fetchMock: ReturnType<typeof vi.fn>): string[] {
+  return fetchMock.mock.calls.map((call) => String(call[0]));
+}
+
+describe('chat-api', () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('fetchHarnesses GETs /api/harnesses and returns typed summaries (harness id separate from provider)', async () => {
+    const fetchMock = stubFetch();
+    fetchMock.mockResolvedValue(
+      json([
+        { id: 'pi', displayName: 'Pi', capabilities: ['chat', 'tools'] },
+        { id: 'openai', displayName: 'OpenAI', capabilities: ['chat'] },
+      ]),
+    );
+
+    const harnesses = await fetchHarnesses();
+
+    expect(fetchMock).toHaveBeenCalledOnce();
+    expect(String(fetchMock.mock.calls[0]?.[0])).toBe('/api/harnesses');
+    expect(harnesses).toEqual([
+      { id: 'pi', displayName: 'Pi', capabilities: ['chat', 'tools'] },
+      { id: 'openai', displayName: 'OpenAI', capabilities: ['chat'] },
+    ]);
+  });
+
+  it('fetchCatalog GETs the harness-scoped catalog and returns only its model entries', async () => {
+    const fetchMock = stubFetch();
+    fetchMock.mockResolvedValue(
+      json({
+        harnessId: 'pi',
+        version: '2026-08-11',
+        fingerprint: 'abc123',
+        models: [
+          {
+            harnessId: 'pi',
+            providerId: 'openai',
+            modelId: 'gpt-5',
+            displayName: 'GPT-5',
+            reasoningCapability: true,
+            inputTypes: ['text'],
+            authState: 'ready',
+            availability: 'available',
+          },
+        ],
+      }),
+    );
+
+    const result = await fetchCatalog('pi');
+
+    expect(String(fetchMock.mock.calls[0]?.[0])).toBe('/api/harnesses/pi/catalog');
+    expect(result.ok).toBe(true);
+    if (!result.ok) throw new Error('expected ok catalog');
+    expect(result.catalog.harnessId).toBe('pi');
+    expect(result.catalog.models).toHaveLength(1);
+    expect(result.catalog.models[0]).toMatchObject({
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+      availability: 'available',
+    });
+  });
+
+  it('normalizes a catalog 404 into a typed catalog_unavailable result without surfacing the raw body', async () => {
+    const fetchMock = stubFetch();
+    fetchMock.mockResolvedValue(
+      json(
+        {
+          code: 'adapter_unavailable',
+          message: 'raw gateway detail that must not leak verbatim',
+          harnessId: 'attacker-echo',
+          extra: { hostile: 'blob' },
+        },
+        404,
+      ),
+    );
+
+    const result = await fetchCatalog('ghost');
+
+    expect(result.ok).toBe(false);
+    if (result.ok) throw new Error('expected unavailable result');
+    expect(result.code).toBe('catalog_unavailable');
+    // harnessId comes from the request, never the (untrusted) response body.
+    expect(result.harnessId).toBe('ghost');
+    expect(typeof result.message).toBe('string');
+    // The raw response body is never rendered/returned verbatim.
+    expect(JSON.stringify(result)).not.toContain('hostile');
+    expect(JSON.stringify(result)).not.toContain('attacker-echo');
+  });
+
+  it('fetchPersistedSelection returns the stored tuple, or null when unset', async () => {
+    const fetchMock = stubFetch();
+    fetchMock.mockResolvedValueOnce(
+      json({ selection: { harnessId: 'pi', providerId: 'openai', modelId: 'gpt-5' } }),
+    );
+    await expect(fetchPersistedSelection()).resolves.toEqual({
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+    });
+    expect(String(fetchMock.mock.calls[0]?.[0])).toBe('/api/chat/preferences/selection');
+
+    fetchMock.mockResolvedValueOnce(json({ selection: null }));
+    await expect(fetchPersistedSelection()).resolves.toBeNull();
+  });
+
+  it('persistSelection PUTs the structured tuple (not free text) and returns the confirmed selection', async () => {
+    const fetchMock = stubFetch();
+    fetchMock.mockResolvedValue(
+      json({ selection: { harnessId: 'pi', providerId: 'openai', modelId: 'gpt-5' } }),
+    );
+
+    const result = await persistSelection({
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+    });
+
+    expect(result.ok).toBe(true);
+    const call = fetchMock.mock.calls[0];
+    expect(String(call?.[0])).toBe('/api/chat/preferences/selection');
+    const init = call?.[1] as RequestInit;
+    expect(String(init.method).toUpperCase()).toBe('PUT');
+    // The body is exactly the structured tuple — harness/provider/model kept distinct.
+    expect(JSON.parse(String(init.body))).toEqual({
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+    });
+  });
+
+  it('normalizes a selection 422 into a typed error preserving the requested tuple exactly', async () => {
+    const fetchMock = stubFetch();
+    fetchMock.mockResolvedValue(
+      json(
+        {
+          code: 'model_unavailable',
+          message: 'raw detail that must not leak',
+          selection: { harnessId: 'x', providerId: 'y', modelId: 'z' },
+        },
+        422,
+      ),
+    );
+
+    const requested = { harnessId: 'pi', providerId: 'openai', modelId: 'gpt-5' };
+    const result = await persistSelection(requested);
+
+    expect(result.ok).toBe(false);
+    if (result.ok) throw new Error('expected failed persist');
+    expect(['selection_invalid', 'model_unavailable']).toContain(result.code);
+    // The requested tuple is preserved unchanged — not replaced by the body's echo.
+    expect(result.requested).toEqual(requested);
+    expect(JSON.stringify(result)).not.toContain('raw detail');
+  });
+
+  it('never requests any /api/providers* endpoint', async () => {
+    const fetchMock = stubFetch();
+    fetchMock.mockResolvedValue(json([]));
+    await fetchHarnesses();
+    fetchMock.mockResolvedValue(
+      json({ harnessId: 'pi', version: '1', fingerprint: 'f', models: [] }),
+    );
+    await fetchCatalog('pi');
+    fetchMock.mockResolvedValue(json({ selection: null }));
+    await fetchPersistedSelection();
+
+    for (const url of requestedUrls(fetchMock)) {
+      expect(url).not.toContain('/api/providers');
+    }
+  });
+});

apps/web/src/spa/chat/chat-api.ts

@@ -0,0 +1,132 @@
+/**
+ * Typed fetch wrappers for the Task-3 harness HTTP contract the chat selection
+ * UI depends on. Every response body is untrusted and is normalized through the
+ * runtime guards before it reaches state — a 404 (catalog) and a 422 (selection)
+ * are mapped to typed, body-free error results so a raw gateway body is never
+ * rendered, and the caller's requested tuple is preserved verbatim on failure.
+ *
+ * This module talks ONLY to the harness/chat-preferences endpoints. It never
+ * calls `/api/providers*` — provider identity lives inside the harness catalog.
+ */
+import { asHarnessCatalog, asHarnessSelection, asHarnessSummaries } from './runtime-guards';
+import type { HarnessCatalog, HarnessSelection, HarnessSummary } from '@/lib/types';
+
+/** A catalog fetch either yields the typed catalog or a typed unavailability —
+ * never a thrown raw body. */
+export type CatalogResult =
+  | { ok: true; catalog: HarnessCatalog }
+  | { ok: false; code: 'catalog_unavailable'; harnessId: string; message: string };
+
+export type SelectionErrorCode = 'selection_invalid' | 'model_unavailable';
+
+/** A persist either confirms the stored tuple or reports a typed domain failure
+ * that echoes back the exact tuple the caller requested. */
+export type SelectionPersistResult =
+  | { ok: true; selection: HarnessSelection }
+  | { ok: false; code: SelectionErrorCode; message: string; requested: HarnessSelection };
+
+/** A safe, generic message for an unavailable catalog — the raw 404 body is
+ * never surfaced. */
+const CATALOG_UNAVAILABLE_MESSAGE = 'This harness catalog is currently unavailable.';
+
+/** A safe, generic message for a rejected selection. The untrusted 422 body's
+ * own `message` is deliberately NEVER surfaced — only this fixed copy — so a
+ * raw gateway detail can never leak into the UI. Only the closed `code` enum is
+ * read from the body. */
+const SELECTION_REJECTED_MESSAGE = 'This selection was rejected.';
+
+async function readJson(response: Response): Promise<unknown> {
+  return response.json().catch(() => null);
+}
+
+function safeSelectionCode(body: unknown): SelectionErrorCode {
+  if (typeof body === 'object' && body !== null && 'code' in body) {
+    const code = (body as { code: unknown }).code;
+    if (code === 'selection_invalid' || code === 'model_unavailable') return code;
+  }
+  // Default to the more conservative "invalid" classification for anything
+  // unrecognized rather than guessing "model_unavailable".
+  return 'selection_invalid';
+}
+
+/** `GET /api/harnesses` → the list of harness summaries. A non-OK response
+ * normalizes to an empty list (the UI then has no harness to select). */
+export async function fetchHarnesses(): Promise<HarnessSummary[]> {
+  const response = await fetch('/api/harnesses', {
+    credentials: 'include',
+    headers: { Accept: 'application/json' },
+  });
+  if (!response.ok) return [];
+  return asHarnessSummaries(await readJson(response));
+}
+
+/** `GET /api/harnesses/:harnessId/catalog` → the harness-scoped catalog. A 404
+ * (or any non-OK) becomes a typed `catalog_unavailable` result rather than a
+ * fallback catalog or a rendered raw body. */
+export async function fetchCatalog(harnessId: string): Promise<CatalogResult> {
+  const response = await fetch(`/api/harnesses/${encodeURIComponent(harnessId)}/catalog`, {
+    credentials: 'include',
+    headers: { Accept: 'application/json' },
+  });
+  if (!response.ok) {
+    return {
+      ok: false,
+      code: 'catalog_unavailable',
+      // Scoped to the requested harness id, never the untrusted body's echo.
+      harnessId,
+      message: CATALOG_UNAVAILABLE_MESSAGE,
+    };
+  }
+  return { ok: true, catalog: asHarnessCatalog(await readJson(response), harnessId) };
+}
+
+/** `GET /api/chat/preferences/selection` → the persisted tuple, or null when
+ * unset or malformed. */
+export async function fetchPersistedSelection(): Promise<HarnessSelection | null> {
+  const response = await fetch('/api/chat/preferences/selection', {
+    credentials: 'include',
+    headers: { Accept: 'application/json' },
+  });
+  if (!response.ok) return null;
+  const body = await readJson(response);
+  if (typeof body !== 'object' || body === null) return null;
+  return asHarnessSelection((body as { selection?: unknown }).selection);
+}
+
+/** `PUT /api/chat/preferences/selection` with the structured tuple as the body.
+ * On success returns the confirmed selection; on a typed domain failure (422)
+ * or validation error, returns a typed result carrying the EXACT requested
+ * tuple — never the body's echo — and never the raw body text. */
+export async function persistSelection(
+  selection: HarnessSelection,
+): Promise<SelectionPersistResult> {
+  const requested: HarnessSelection = {
+    harnessId: selection.harnessId,
+    providerId: selection.providerId,
+    modelId: selection.modelId,
+  };
+  const response = await fetch('/api/chat/preferences/selection', {
+    method: 'PUT',
+    credentials: 'include',
+    headers: { Accept: 'application/json', 'Content-Type': 'application/json' },
+    body: JSON.stringify(requested),
+  });
+  if (!response.ok) {
+    const body = await readJson(response);
+    return {
+      ok: false,
+      code: safeSelectionCode(body),
+      // Fixed copy only — the untrusted body's message is never surfaced.
+      message: SELECTION_REJECTED_MESSAGE,
+      requested,
+    };
+  }
+  const body = await readJson(response);
+  const confirmed =
+    typeof body === 'object' && body !== null
+      ? asHarnessSelection((body as { selection?: unknown }).selection)
+      : null;
+  // A malformed 2xx body is treated as a confirmation of exactly what we sent —
+  // the server accepted the tuple, so the requested tuple is the source of truth.
+  return { ok: true, selection: confirmed ?? requested };
+}

apps/web/src/spa/chat/composer.tsx

@@ -1,4 +1,5 @@
 import { useState, type KeyboardEvent, type ReactElement } from 'react';
+import type { HarnessSelectionValue } from './use-harness-selection';
 
 interface ComposerProps {
   onSend: (input: { content: string; provider?: string; modelId?: string }) => void;
@@ -9,6 +10,23 @@ interface ComposerProps {
    * pre-ack window where a second send could otherwise slip through. */
   sending: boolean;
   hasConversation: boolean;
+  /** Structured harness/provider/model selection state. The composer never
+   * accepts free-text provider/model — every sendable tuple is a validated,
+   * persisted catalog entry, and the send projection is derived from it. */
+  harness: HarnessSelectionValue;
+}
+
+/** The distinct provider ids present in the current catalog, in first-seen
+ * order — the provider select is catalog-derived, never a hardcoded list. */
+function providerOptions(harness: HarnessSelectionValue): string[] {
+  const seen = new Set<string>();
+  const out: string[] = [];
+  for (const model of harness.catalog?.models ?? []) {
+    if (seen.has(model.providerId)) continue;
+    seen.add(model.providerId);
+    out.push(model.providerId);
+  }
+  return out;
 }
 
 export function Composer({
@@ -17,21 +35,19 @@ export function Composer({
   streaming,
   sending,
   hasConversation,
+  harness,
 }: ComposerProps): ReactElement {
   const [content, setContent] = useState('');
-  const [provider, setProvider] = useState('');
-  const [modelId, setModelId] = useState('');
   const busy = streaming || sending;
 
   function submit(): void {
     if (busy) return;
+    // Send is gated on a validated, persisted catalog tuple — a draft or unset
+    // selection can never emit, so provider/model never travel as free text.
+    if (!harness.canSend) return;
     const trimmed = content.trim();
     if (!trimmed) return;
-    onSend({
-      content: trimmed,
-      provider: provider.trim() || undefined,
-      modelId: modelId.trim() || undefined,
-    });
+    onSend({ content: trimmed, ...harness.projection });
     setContent('');
   }
 
@@ -42,6 +58,19 @@ export function Composer({
     }
   }
 
+  // Scope the model options to the intentionally selected provider. With no
+  // provider chosen (`providerId === ''`) nothing matches, so the model select
+  // offers only the placeholder — never a cross-provider row.
+  const models = (harness.catalog?.models ?? []).filter(
+    (model) => model.providerId === harness.providerId,
+  );
+  // A collision-safe composite option identity covering the full provider+model
+  // tuple. The controlled select mirrors the same identity so the exact catalog
+  // row highlights (a bare modelId would collide across providers).
+  const modelOptionValue = (model: { providerId: string; modelId: string }): string =>
+    `${model.providerId}:${model.modelId}`;
+  const selectedModelValue = harness.modelId ? `${harness.providerId}:${harness.modelId}` : '';
+
   return (
     <form
       onSubmit={(event) => {
@@ -51,21 +80,68 @@ export function Composer({
       className="flex flex-col gap-2 border-t p-4"
     >
       <div className="flex flex-wrap gap-2">
-        <input
+        <select
+          aria-label="Harness"
+          value={harness.harnessId}
+          onChange={(event) => harness.selectHarness(event.target.value)}
+          className="rounded border px-2 py-1 text-xs"
+        >
+          <option value="">Select a harness…</option>
+          {harness.harnesses.map((item) => (
+            <option key={item.id} value={item.id}>
+              {item.displayName}
+            </option>
+          ))}
+        </select>
+        <select
           aria-label="Provider"
-          value={provider}
-          onChange={(event) => setProvider(event.target.value)}
-          placeholder="Provider (optional)"
+          value={harness.providerId}
+          onChange={(event) => harness.selectProvider(event.target.value)}
+          disabled={harness.catalogUnavailable || providerOptions(harness).length === 0}
           className="rounded border px-2 py-1 text-xs"
-        />
-        <input
+        >
+          <option value="">Select a provider…</option>
+          {providerOptions(harness).map((providerId) => (
+            <option key={providerId} value={providerId}>
+              {providerId}
+            </option>
+          ))}
+        </select>
+        <select
           aria-label="Model"
-          value={modelId}
-          onChange={(event) => setModelId(event.target.value)}
-          placeholder="Model (optional)"
+          value={selectedModelValue}
+          onChange={(event) => {
+            // Resolve the composite option identity back to the exact catalog
+            // row and persist that row's own provider+model — never a bare id.
+            const selected = models.find((model) => modelOptionValue(model) === event.target.value);
+            if (selected) harness.selectModel(selected.providerId, selected.modelId);
+          }}
+          disabled={harness.catalogUnavailable || models.length === 0}
           className="rounded border px-2 py-1 text-xs"
-        />
+        >
+          <option value="">Select a model…</option>
+          {models.map((model) => (
+            <option key={modelOptionValue(model)} value={modelOptionValue(model)}>
+              {model.displayName}
+            </option>
+          ))}
+        </select>
       </div>
+      {harness.catalogUnavailable ? (
+        <p role="status" className="text-xs opacity-70">
+          This harness catalog is currently unavailable.
+        </p>
+      ) : null}
+      {harness.isStale ? (
+        <p role="status" className="text-xs opacity-70">
+          The saved model is no longer available — pick another to continue.
+        </p>
+      ) : null}
+      {harness.persistError ? (
+        <p role="alert" className="text-xs">
+          {harness.persistError.message}
+        </p>
+      ) : null}
       <div className="flex items-end gap-2">
         <textarea
           aria-label="Message"
@@ -78,7 +154,7 @@ export function Composer({
         />
         <button
           type="submit"
-          disabled={!content.trim() || busy}
+          disabled={!content.trim() || busy || !harness.canSend}
           className="rounded px-3 py-2 text-sm font-medium"
         >
           Send

apps/web/src/spa/chat/runtime-guards.ts

@@ -5,6 +5,14 @@
  * a non-array, `.toFixed` on a non-number) or render an object as a React
  * child.
  */
+import type {
+  HarnessAuthState,
+  HarnessCatalog,
+  HarnessCatalogEntry,
+  HarnessModelAvailability,
+  HarnessSelection,
+  HarnessSummary,
+} from '@/lib/types';
 
 export function asString(value: unknown, fallback = ''): string {
   return typeof value === 'string' ? value : fallback;
@@ -38,6 +46,99 @@ export function isRecord(value: unknown): value is Record<string, unknown> {
   return typeof value === 'object' && value !== null;
 }
 
+/**
+ * The HTTP harness/catalog/selection JSON bodies are as untrusted as the socket
+ * payloads above — a misbehaving or compromised gateway can send anything. The
+ * guards below normalize those bodies into the typed client shapes without ever
+ * rendering a raw body, so a 404/422/malformed response can never inject an
+ * object into React or a non-tuple into the selection state.
+ */
+
+/** Normalizes an untrusted `authState` to the closed set, defaulting to the
+ * safest value (`unavailable`) for anything unrecognized. */
+export function asHarnessAuthState(value: unknown): HarnessAuthState {
+  return value === 'ready' || value === 'auth_required' || value === 'unavailable'
+    ? value
+    : 'unavailable';
+}
+
+/** Normalizes an untrusted `availability` to the closed set, defaulting to
+ * `unavailable` so a malformed row can never present as sendable. */
+export function asHarnessAvailability(value: unknown): HarnessModelAvailability {
+  return value === 'available' ? 'available' : 'unavailable';
+}
+
+/** A tuple is valid only when all three ids are non-empty strings — a partial
+ * or malformed selection is rejected (null) rather than half-adopted. */
+export function asHarnessSelection(value: unknown): HarnessSelection | null {
+  if (!isRecord(value)) return null;
+  const harnessId = value.harnessId;
+  const providerId = value.providerId;
+  const modelId = value.modelId;
+  if (
+    typeof harnessId !== 'string' ||
+    typeof providerId !== 'string' ||
+    typeof modelId !== 'string' ||
+    harnessId.length === 0 ||
+    providerId.length === 0 ||
+    modelId.length === 0
+  ) {
+    return null;
+  }
+  return { harnessId, providerId, modelId };
+}
+
+/** Normalizes an untrusted array into typed harness summaries, dropping any row
+ * without a usable id. */
+export function asHarnessSummaries(value: unknown): HarnessSummary[] {
+  if (!Array.isArray(value)) return [];
+  const out: HarnessSummary[] = [];
+  for (const item of value) {
+    if (!isRecord(item)) continue;
+    const id = asString(item.id);
+    if (id.length === 0) continue;
+    out.push({
+      id,
+      displayName: asNonEmptyString(item.displayName, id),
+      capabilities: asStringArray(item.capabilities),
+    });
+  }
+  return out;
+}
+
+function asHarnessCatalogEntry(value: unknown): HarnessCatalogEntry | null {
+  const selection = asHarnessSelection(value);
+  if (selection === null || !isRecord(value)) return null;
+  return {
+    ...selection,
+    displayName: asNonEmptyString(value.displayName, selection.modelId),
+    reasoningCapability: value.reasoningCapability === true,
+    inputTypes: asStringArray(value.inputTypes),
+    authState: asHarnessAuthState(value.authState),
+    availability: asHarnessAvailability(value.availability),
+  };
+}
+
+/** Normalizes an untrusted catalog body into the typed client catalog. The
+ * caller supplies `harnessId` (from the request path) so the returned catalog
+ * is scoped to the harness that was actually requested, never a body-echoed id.
+ * Malformed model rows are dropped rather than invalidating the whole catalog. */
+export function asHarnessCatalog(value: unknown, harnessId: string): HarnessCatalog {
+  const record = isRecord(value) ? value : {};
+  const rawModels = Array.isArray(record.models) ? record.models : [];
+  const models: HarnessCatalogEntry[] = [];
+  for (const row of rawModels) {
+    const entry = asHarnessCatalogEntry(row);
+    if (entry !== null) models.push(entry);
+  }
+  return {
+    harnessId,
+    version: asString(record.version),
+    fingerprint: asString(record.fingerprint),
+    models,
+  };
+}
+
 /** The single point of truth for what counts as a valid conversation ID
  * anywhere a scoped server event may adopt one into state — a non-empty
  * string, nothing else. Every site that establishes or compares

apps/web/src/spa/chat/use-harness-selection.spec.tsx

@@ -0,0 +1,445 @@
+import { act, type ReactElement } from 'react';
+import { createRoot, type Root } from 'react-dom/client';
+import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, it, vi } from 'vitest';
+import { useHarnessSelection, type HarnessSelectionValue } from './use-harness-selection';
+
+function json(body: unknown, status = 200): Response {
+  return new Response(JSON.stringify(body), {
+    status,
+    headers: { 'Content-Type': 'application/json' },
+  });
+}
+
+interface Scenario {
+  harnesses?: unknown;
+  catalog?: { body: unknown; status?: number };
+  selection?: unknown;
+  /** When set, the PUT resolves only when this is called (for race tests). */
+  deferPut?: boolean;
+}
+
+interface Deferred<T> {
+  promise: Promise<T>;
+  resolve: (value: T) => void;
+}
+
+function defer<T>(): Deferred<T> {
+  let resolve!: (value: T) => void;
+  const promise = new Promise<T>((r) => {
+    resolve = r;
+  });
+  return { promise, resolve };
+}
+
+let putBodies: unknown[] = [];
+let putDeferred: Deferred<Response> | null = null;
+
+function installFetch(scenario: Scenario): ReturnType<typeof vi.fn> {
+  putBodies = [];
+  putDeferred = scenario.deferPut ? defer<Response>() : null;
+  const fetchMock = vi.fn(async (input: unknown, init?: RequestInit) => {
+    const url = String(input);
+    const method = String(init?.method ?? 'GET').toUpperCase();
+    if (url === '/api/harnesses') return json(scenario.harnesses ?? []);
+    if (url.startsWith('/api/harnesses/') && url.endsWith('/catalog')) {
+      const spec = scenario.catalog ?? {
+        body: { harnessId: 'pi', version: '1', fingerprint: 'f', models: [] },
+      };
+      return json(spec.body, spec.status ?? 200);
+    }
+    if (url === '/api/chat/preferences/selection' && method === 'GET') {
+      return json({ selection: scenario.selection ?? null });
+    }
+    if (url === '/api/chat/preferences/selection' && method === 'PUT') {
+      putBodies.push(JSON.parse(String(init?.body)));
+      const ok = json({ selection: JSON.parse(String(init?.body)) });
+      if (putDeferred) return putDeferred.promise;
+      return ok;
+    }
+    return new Response('not found', { status: 404 });
+  });
+  vi.stubGlobal('fetch', fetchMock);
+  return fetchMock;
+}
+
+let latest: HarnessSelectionValue | null = null;
+
+function Probe(): ReactElement | null {
+  latest = useHarnessSelection();
+  return null;
+}
+
+let root: Root | null;
+let container: HTMLElement;
+
+beforeAll(() => {
+  Object.defineProperty(globalThis, 'IS_REACT_ACT_ENVIRONMENT', {
+    configurable: true,
+    value: true,
+  });
+});
+
+afterAll(() => {
+  Reflect.deleteProperty(globalThis, 'IS_REACT_ACT_ENVIRONMENT');
+});
+
+beforeEach(() => {
+  latest = null;
+  container = document.createElement('div');
+  document.body.append(container);
+  root = createRoot(container);
+});
+
+afterEach(async () => {
+  await act(async () => {
+    root?.unmount();
+  });
+  document.body.replaceChildren();
+  vi.unstubAllGlobals();
+});
+
+async function mount(): Promise<void> {
+  await act(async () => {
+    root?.render(<Probe />);
+  });
+  await flush();
+}
+
+async function flush(times = 5): Promise<void> {
+  for (let i = 0; i < times; i += 1) {
+    await act(async () => {
+      await Promise.resolve();
+    });
+  }
+}
+
+function value(): HarnessSelectionValue {
+  if (!latest) throw new Error('hook value not captured');
+  return latest;
+}
+
+const PI_CATALOG = {
+  harnessId: 'pi',
+  version: '2026-08-11',
+  fingerprint: 'fp',
+  models: [
+    {
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+      displayName: 'GPT-5',
+      reasoningCapability: true,
+      inputTypes: ['text'],
+      authState: 'ready',
+      availability: 'available',
+    },
+    {
+      harnessId: 'pi',
+      providerId: 'anthropic',
+      modelId: 'claude',
+      displayName: 'Claude',
+      reasoningCapability: true,
+      inputTypes: ['text'],
+      authState: 'ready',
+      availability: 'available',
+    },
+  ],
+};
+
+describe('useHarnessSelection', () => {
+  it('loads harnesses and, once a harness is chosen, the model options come only from its catalog', async () => {
+    installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: { body: PI_CATALOG },
+      selection: null,
+    });
+    await mount();
+
+    expect(value().harnesses).toEqual([{ id: 'pi', displayName: 'Pi', capabilities: [] }]);
+    expect(value().catalog).toBeNull();
+
+    await act(async () => {
+      value().selectHarness('pi');
+    });
+    await flush();
+
+    expect(value().catalog?.harnessId).toBe('pi');
+    expect(value().catalog?.models.map((m) => m.modelId)).toEqual(['gpt-5', 'claude']);
+  });
+
+  it('does not auto-select any catalog row when there is no persisted selection (no first-row fallback)', async () => {
+    const fetchMock = installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: { body: PI_CATALOG },
+      selection: null,
+    });
+    await mount();
+    await act(async () => {
+      value().selectHarness('pi');
+    });
+    await flush();
+
+    expect(value().modelId).toBe('');
+    expect(value().persistedSelection).toBeNull();
+    expect(value().canSend).toBe(false);
+    // Nothing was persisted — no PUT fired for an unset selection.
+    const putCalls = fetchMock.mock.calls.filter(
+      (c) => String((c[1] as RequestInit)?.method).toUpperCase() === 'PUT',
+    );
+    expect(putCalls).toHaveLength(0);
+  });
+
+  it('persists the structured tuple and only enables send AFTER the PUT resolves (no race ahead of persistence)', async () => {
+    installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: { body: PI_CATALOG },
+      selection: null,
+      deferPut: true,
+    });
+    await mount();
+    await act(async () => {
+      value().selectHarness('pi');
+    });
+    await flush();
+    await act(async () => {
+      value().selectProvider('openai');
+    });
+    await act(async () => {
+      value().selectModel('openai', 'gpt-5');
+    });
+    await flush();
+
+    // PUT is in flight (deferred) — send MUST NOT be enabled yet.
+    expect(value().canSend).toBe(false);
+
+    await act(async () => {
+      putDeferred?.resolve(
+        json({ selection: { harnessId: 'pi', providerId: 'openai', modelId: 'gpt-5' } }),
+      );
+    });
+    await flush();
+
+    expect(putBodies).toContainEqual({ harnessId: 'pi', providerId: 'openai', modelId: 'gpt-5' });
+    expect(value().persistedSelection).toEqual({
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+    });
+    expect(value().canSend).toBe(true);
+    expect(value().projection).toEqual({ provider: 'openai', modelId: 'gpt-5' });
+  });
+
+  it('keeps a stale/unavailable persisted selection visibly displayed rather than silently dropping it', async () => {
+    installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: { body: PI_CATALOG },
+      selection: { harnessId: 'pi', providerId: 'openai', modelId: 'retired-model' },
+    });
+    await mount();
+
+    // The persisted tuple is displayed even though its model is gone from the catalog.
+    expect(value().persistedSelection).toEqual({
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'retired-model',
+    });
+    expect(value().modelId).toBe('retired-model');
+    expect(value().isStale).toBe(true);
+    // A stale model is not a valid catalog option, so send stays disabled.
+    expect(value().canSend).toBe(false);
+  });
+
+  it('disables send for an empty catalog (no viable model) and never fabricates one', async () => {
+    installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: { body: { harnessId: 'pi', version: '1', fingerprint: 'f', models: [] } },
+      selection: null,
+    });
+    await mount();
+    await act(async () => {
+      value().selectHarness('pi');
+    });
+    await flush();
+
+    expect(value().catalog?.models ?? []).toHaveLength(0);
+    expect(value().canSend).toBe(false);
+  });
+
+  it('marks the catalog unavailable and disables send when the catalog request 404s', async () => {
+    installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: {
+        body: { code: 'adapter_unavailable', message: 'x', harnessId: 'pi' },
+        status: 404,
+      },
+      selection: null,
+    });
+    await mount();
+    await act(async () => {
+      value().selectHarness('pi');
+    });
+    await flush();
+
+    expect(value().catalogUnavailable).toBe(true);
+    expect(value().canSend).toBe(false);
+  });
+
+  it('on a 422 persist, keeps the requested tuple visible, surfaces a typed error, and leaves send disabled', async () => {
+    installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: {
+        body: {
+          ...PI_CATALOG,
+          models: [{ ...PI_CATALOG.models[0], availability: 'unavailable' }],
+        },
+      },
+      selection: null,
+    });
+    // Override PUT to 422.
+    const fetchMock = vi.fn(async (input: unknown, init?: RequestInit) => {
+      const url = String(input);
+      const method = String(init?.method ?? 'GET').toUpperCase();
+      if (url === '/api/harnesses')
+        return json([{ id: 'pi', displayName: 'Pi', capabilities: [] }]);
+      if (url.endsWith('/catalog')) return json(PI_CATALOG);
+      if (url === '/api/chat/preferences/selection' && method === 'GET')
+        return json({ selection: null });
+      if (url === '/api/chat/preferences/selection' && method === 'PUT') {
+        return json(
+          {
+            code: 'model_unavailable',
+            message: 'nope',
+            selection: { harnessId: 'a', providerId: 'b', modelId: 'c' },
+          },
+          422,
+        );
+      }
+      return new Response('nf', { status: 404 });
+    });
+    vi.stubGlobal('fetch', fetchMock);
+
+    await mount();
+    await act(async () => {
+      value().selectHarness('pi');
+    });
+    await flush();
+    await act(async () => {
+      value().selectProvider('openai');
+    });
+    await act(async () => {
+      value().selectModel('openai', 'gpt-5');
+    });
+    await flush();
+
+    expect(value().modelId).toBe('gpt-5');
+    expect(value().persistError?.code).toBe('model_unavailable');
+    expect(value().persistError?.requested).toEqual({
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+    });
+    expect(value().persistedSelection).toBeNull();
+    expect(value().canSend).toBe(false);
+  });
+
+  it('invalidates the model on a provider change and keeps send disabled until the new tuple persists', async () => {
+    installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: { body: PI_CATALOG },
+      selection: null,
+    });
+    await mount();
+    await act(async () => {
+      value().selectHarness('pi');
+    });
+    await flush();
+    await act(async () => {
+      value().selectProvider('openai');
+    });
+    await act(async () => {
+      value().selectModel('openai', 'gpt-5');
+    });
+    await flush();
+    // A valid provider-A tuple has persisted.
+    expect(value().canSend).toBe(true);
+    expect(value().persistedSelection).toEqual({
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+    });
+
+    // Switching provider clears the model that no longer belongs to it.
+    await act(async () => {
+      value().selectProvider('anthropic');
+    });
+    expect(value().modelId).toBe('');
+    expect(value().canSend).toBe(false);
+
+    // Send stays disabled until the new exact provider-B tuple persists.
+    await act(async () => {
+      value().selectModel('anthropic', 'claude');
+    });
+    await flush();
+    expect(value().canSend).toBe(true);
+    expect(value().persistedSelection).toEqual({
+      harnessId: 'pi',
+      providerId: 'anthropic',
+      modelId: 'claude',
+    });
+    expect(value().projection).toEqual({ provider: 'anthropic', modelId: 'claude' });
+  });
+
+  it('does not enable send on a model pick until the PUT for that exact new tuple resolves', async () => {
+    installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: { body: PI_CATALOG },
+      selection: { harnessId: 'pi', providerId: 'openai', modelId: 'gpt-5' },
+      deferPut: true,
+    });
+    await mount();
+    // The persisted, in-catalog tuple is sendable after mount (no PUT needed).
+    expect(value().canSend).toBe(true);
+
+    await act(async () => {
+      value().selectProvider('anthropic');
+    });
+    expect(value().modelId).toBe('');
+    expect(value().canSend).toBe(false);
+
+    await act(async () => {
+      value().selectModel('anthropic', 'claude');
+    });
+    await flush();
+    // PUT for the new tuple is still in flight — send MUST stay disabled.
+    expect(value().canSend).toBe(false);
+
+    await act(async () => {
+      putDeferred?.resolve(
+        json({ selection: { harnessId: 'pi', providerId: 'anthropic', modelId: 'claude' } }),
+      );
+    });
+    await flush();
+    expect(value().canSend).toBe(true);
+    expect(value().projection).toEqual({ provider: 'anthropic', modelId: 'claude' });
+  });
+
+  it('never requests any /api/providers* endpoint across the whole flow', async () => {
+    const fetchMock = installFetch({
+      harnesses: [{ id: 'pi', displayName: 'Pi', capabilities: [] }],
+      catalog: { body: PI_CATALOG },
+      selection: { harnessId: 'pi', providerId: 'openai', modelId: 'gpt-5' },
+    });
+    await mount();
+    await act(async () => {
+      value().selectProvider('anthropic');
+    });
+    await act(async () => {
+      value().selectModel('anthropic', 'claude');
+    });
+    await flush();
+
+    for (const call of fetchMock.mock.calls) {
+      expect(String(call[0])).not.toContain('/api/providers');
+    }
+  });
+});

apps/web/src/spa/chat/use-harness-selection.ts

@@ -0,0 +1,216 @@
+import { useCallback, useEffect, useRef, useState } from 'react';
+import {
+  fetchCatalog,
+  fetchHarnesses,
+  fetchPersistedSelection,
+  persistSelection,
+  type SelectionErrorCode,
+} from './chat-api';
+import type { HarnessCatalog, HarnessSelection, HarnessSummary } from '@/lib/types';
+
+export interface HarnessPersistError {
+  code: SelectionErrorCode;
+  message: string;
+  /** The exact tuple the user requested — preserved so the failed selection
+   * stays visible rather than being silently dropped. */
+  requested: HarnessSelection;
+}
+
+export interface HarnessSelectionValue {
+  harnesses: HarnessSummary[];
+  catalog: HarnessCatalog | null;
+  /** True when the selected harness has no usable catalog (404/error). */
+  catalogUnavailable: boolean;
+  /** The working (displayed) selection, kept as three distinct ids. Empty
+   * strings mean "not chosen yet" — there is deliberately no first-row default. */
+  harnessId: string;
+  providerId: string;
+  modelId: string;
+  /** The last tuple confirmed persisted by the server, or null. */
+  persistedSelection: HarnessSelection | null;
+  /** True when a persisted selection references a model no longer present as an
+   * available catalog entry — it stays visibly displayed rather than dropped. */
+  isStale: boolean;
+  /** True ONLY once a full tuple has been confirmed persisted AND it is a
+   * currently-available catalog entry. Send stays disabled otherwise, so a send
+   * can never race ahead of successful persistence. */
+  canSend: boolean;
+  persistError: HarnessPersistError | null;
+  selectHarness: (harnessId: string) => void;
+  selectProvider: (providerId: string) => void;
+  /** Persist the EXACT catalog row's `{providerId, modelId}` — the caller
+   * resolves the composite option identity to the real entry and passes both
+   * ids, so a bare model id is never combined with ambient provider state. */
+  selectModel: (providerId: string, modelId: string) => void;
+  /** The compatibility `{provider, modelId}` projection for the legacy socket
+   * send path — derived ONLY from the validated persisted tuple, never from any
+   * free-text or unpersisted draft. Empty when nothing is sendable. */
+  projection: { provider?: string; modelId?: string };
+}
+
+/** A tuple is a currently-usable catalog option only when the catalog holds a
+ * matching, available entry — the single gate that keeps a stale/unavailable
+ * model from ever counting as sendable. */
+function isAvailableInCatalog(
+  selection: HarnessSelection | null,
+  catalog: HarnessCatalog | null,
+): boolean {
+  if (selection === null || catalog === null) return false;
+  return catalog.models.some(
+    (model) =>
+      model.providerId === selection.providerId &&
+      model.modelId === selection.modelId &&
+      model.availability === 'available',
+  );
+}
+
+function tuplesEqual(a: HarnessSelection | null, b: HarnessSelection | null): boolean {
+  if (a === null || b === null) return a === b;
+  return a.harnessId === b.harnessId && a.providerId === b.providerId && a.modelId === b.modelId;
+}
+
+/**
+ * Owns the harness/catalog/selection state for the chat composer: loads the
+ * harness list and any persisted tuple on mount, loads a harness's catalog when
+ * chosen, and PUT-persists the full `{harnessId, providerId, modelId}` tuple
+ * when a model is picked. It never auto-selects a catalog row, keeps a
+ * stale/unavailable persisted tuple visible, and only reports `canSend` true
+ * once a full tuple has actually persisted as an available catalog entry.
+ */
+export function useHarnessSelection(): HarnessSelectionValue {
+  const [harnesses, setHarnesses] = useState<HarnessSummary[]>([]);
+  const [catalog, setCatalog] = useState<HarnessCatalog | null>(null);
+  const [catalogUnavailable, setCatalogUnavailable] = useState(false);
+  const [harnessId, setHarnessId] = useState('');
+  const [providerId, setProviderId] = useState('');
+  const [modelId, setModelId] = useState('');
+  const [persistedSelection, setPersistedSelection] = useState<HarnessSelection | null>(null);
+  const [persistError, setPersistError] = useState<HarnessPersistError | null>(null);
+
+  // Monotonic request ids so a slow in-flight catalog/persist response can never
+  // overwrite the result of a newer request the user has since triggered.
+  const catalogRequestRef = useRef(0);
+  const persistRequestRef = useRef(0);
+
+  const loadCatalog = useCallback(async (id: string): Promise<void> => {
+    const requestId = catalogRequestRef.current + 1;
+    catalogRequestRef.current = requestId;
+    setCatalog(null);
+    setCatalogUnavailable(false);
+    const result = await fetchCatalog(id);
+    if (catalogRequestRef.current !== requestId) return;
+    if (result.ok) {
+      setCatalog(result.catalog);
+      setCatalogUnavailable(false);
+    } else {
+      setCatalog(null);
+      setCatalogUnavailable(true);
+    }
+  }, []);
+
+  useEffect(() => {
+    let active = true;
+    void (async (): Promise<void> => {
+      const [list, persisted] = await Promise.all([fetchHarnesses(), fetchPersistedSelection()]);
+      if (!active) return;
+      setHarnesses(list);
+      if (persisted !== null) {
+        // Adopt the persisted tuple as the displayed selection and load its
+        // catalog. If the model has since been retired, it still shows (stale).
+        setHarnessId(persisted.harnessId);
+        setProviderId(persisted.providerId);
+        setModelId(persisted.modelId);
+        setPersistedSelection(persisted);
+        await loadCatalog(persisted.harnessId);
+      }
+      // No persisted selection → nothing is auto-selected; the user must choose.
+    })();
+    return () => {
+      active = false;
+    };
+  }, [loadCatalog]);
+
+  const selectHarness = useCallback(
+    (id: string): void => {
+      setHarnessId(id);
+      // Changing harness invalidates the provider/model draft — never carry a
+      // model across harnesses.
+      setProviderId('');
+      setModelId('');
+      setPersistError(null);
+      void loadCatalog(id);
+    },
+    [loadCatalog],
+  );
+
+  const selectProvider = useCallback((id: string): void => {
+    setProviderId(id);
+    // A new provider invalidates the chosen model — no cross-provider carryover.
+    setModelId('');
+    setPersistError(null);
+  }, []);
+
+  const selectModel = useCallback(
+    (selectedProviderId: string, selectedModelId: string): void => {
+      // Bind the model to the EXACT catalog row's provider — never to ambient
+      // provider state — so two providers exposing the same modelId can never
+      // collide or mis-resolve. Keep the displayed provider consistent with the
+      // resolved row.
+      setProviderId(selectedProviderId);
+      setModelId(selectedModelId);
+      setPersistError(null);
+      const requested: HarnessSelection = {
+        harnessId,
+        providerId: selectedProviderId,
+        modelId: selectedModelId,
+      };
+      const requestId = persistRequestRef.current + 1;
+      persistRequestRef.current = requestId;
+      void (async (): Promise<void> => {
+        const result = await persistSelection(requested);
+        if (persistRequestRef.current !== requestId) return;
+        if (result.ok) {
+          setPersistedSelection(result.selection);
+          setPersistError(null);
+        } else {
+          // Leave persistedSelection unchanged (send stays disabled) and surface
+          // the typed error carrying the exact requested tuple.
+          setPersistError({
+            code: result.code,
+            message: result.message,
+            requested: result.requested,
+          });
+        }
+      })();
+    },
+    [harnessId],
+  );
+
+  const draft: HarnessSelection = { harnessId, providerId, modelId };
+  const isStale = persistedSelection !== null && !isAvailableInCatalog(persistedSelection, catalog);
+  const canSend =
+    persistedSelection !== null &&
+    !catalogUnavailable &&
+    tuplesEqual(draft, persistedSelection) &&
+    isAvailableInCatalog(persistedSelection, catalog);
+  const projection: { provider?: string; modelId?: string } = canSend
+    ? { provider: persistedSelection.providerId, modelId: persistedSelection.modelId }
+    : {};
+
+  return {
+    harnesses,
+    catalog,
+    catalogUnavailable,
+    harnessId,
+    providerId,
+    modelId,
+    persistedSelection,
+    isStale,
+    canSend,
+    persistError,
+    selectHarness,
+    selectProvider,
+    selectModel,
+    projection,
+  };
+}

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

@@ -38,6 +38,76 @@ function findButton(container: HTMLElement, text: string): HTMLButtonElement {
   return button;
 }
 
+function jsonResponse(body: unknown, status = 200): Response {
+  return new Response(JSON.stringify(body), {
+    status,
+    headers: { 'Content-Type': 'application/json' },
+  });
+}
+
+const DEFAULT_CATALOG = {
+  harnessId: 'pi',
+  version: '2026-08-11',
+  fingerprint: 'fp',
+  models: [
+    {
+      harnessId: 'pi',
+      providerId: 'openai',
+      modelId: 'gpt-5',
+      displayName: 'GPT-5',
+      reasoningCapability: true,
+      inputTypes: ['text'],
+      authState: 'ready',
+      availability: 'available',
+    },
+    {
+      harnessId: 'pi',
+      providerId: 'anthropic',
+      modelId: 'claude',
+      displayName: 'Claude',
+      reasoningCapability: true,
+      inputTypes: ['text'],
+      authState: 'ready',
+      availability: 'available',
+    },
+  ],
+};
+
+/** A harness/catalog/selection HTTP stub for the chat-api the selection hook
+ * drives. `selection` seeds the persisted tuple returned by the GET (a valid
+ * in-catalog tuple by default, so `canSend` settles true after mount). */
+function harnessFetch(
+  selection: unknown = { harnessId: 'pi', providerId: 'openai', modelId: 'gpt-5' },
+): typeof fetch {
+  return vi.fn(async (input: unknown, init?: RequestInit) => {
+    const url = String(input);
+    const method = String(init?.method ?? 'GET').toUpperCase();
+    if (url === '/api/harnesses') {
+      return jsonResponse([{ id: 'pi', displayName: 'Pi', capabilities: [] }]);
+    }
+    if (url.startsWith('/api/harnesses/') && url.endsWith('/catalog')) {
+      return jsonResponse(DEFAULT_CATALOG);
+    }
+    if (url === '/api/chat/preferences/selection' && method === 'GET') {
+      return jsonResponse({ selection });
+    }
+    if (url === '/api/chat/preferences/selection' && method === 'PUT') {
+      return jsonResponse({ selection: JSON.parse(String(init?.body)) });
+    }
+    return new Response('not found', { status: 404 });
+  }) as unknown as typeof fetch;
+}
+
+/** Drains the selection hook's chained mount fetches (harnesses → selection →
+ * catalog) and any pending PUT so derived `canSend` settles before assertions. */
+async function flushAsync(times = 5): Promise<void> {
+  for (let i = 0; i < times; i += 1) {
+    await act(async () => {
+      await Promise.resolve();
+    });
+  }
+}
+
 let fake: ReturnType<typeof createFakeChatSocket>;
 let root: Root | null;
 let container: HTMLElement;
@@ -57,12 +127,16 @@ beforeEach(async () => {
   fake = createFakeChatSocket();
   getSocketMock.mockReset().mockReturnValue(fake.socket);
   destroySocketMock.mockReset();
+  vi.stubGlobal('fetch', harnessFetch());
   container = document.createElement('div');
   document.body.append(container);
   root = createRoot(container);
   await act(async () => {
     root?.render(<ChatPage />);
   });
+  // Settle the selection hook's mount fetches so the default in-catalog tuple
+  // persists and `canSend` is true for the existing send-path tests.
+  await flushAsync();
 });
 
 afterEach(async () => {
@@ -70,8 +144,23 @@ afterEach(async () => {
     root?.unmount();
   });
   document.body.replaceChildren();
+  vi.unstubAllGlobals();
 });
 
+/** Re-mounts ChatPage against a custom fetch stub (e.g. an unset selection) for
+ * tests that need a non-default selection scenario. */
+async function remountWithFetch(fetchImpl: typeof fetch): Promise<void> {
+  await act(async () => {
+    root?.unmount();
+  });
+  vi.stubGlobal('fetch', fetchImpl);
+  root = createRoot(container);
+  await act(async () => {
+    root?.render(<ChatPage />);
+  });
+  await flushAsync();
+}
+
 describe('ChatPage', () => {
   it('streams agent:text and agent:thinking, shows tool status, and finalizes on agent:end with usage', async () => {
     await act(async () => {
@@ -390,24 +479,62 @@ describe('ChatPage', () => {
     expect(container.querySelector('[role="alert"]')).toBeTruthy();
   });
 
-  it('sends a message with optional provider/model fields and emits abort from the Stop control', async () => {
+  it('renders harness and provider as separate selects (not merged) and no free-text provider/model inputs', async () => {
+    // The old free-text inputs are gone.
+    expect(container.querySelector('input[aria-label="Provider"]')).toBeNull();
+    expect(container.querySelector('input[aria-label="Model"]')).toBeNull();
+
+    const harnessSelect = container.querySelector(
+      'select[aria-label="Harness"]',
+    ) as HTMLSelectElement;
+    const providerSelect = container.querySelector(
+      'select[aria-label="Provider"]',
+    ) as HTMLSelectElement;
+    const modelSelect = container.querySelector('select[aria-label="Model"]') as HTMLSelectElement;
+    expect(harnessSelect).toBeTruthy();
+    expect(providerSelect).toBeTruthy();
+    expect(modelSelect).toBeTruthy();
+    // Harness and provider are distinct controls carrying distinct identifiers.
+    expect(harnessSelect).not.toBe(providerSelect);
+    expect([...harnessSelect.options].map((o) => o.value)).toContain('pi');
+    expect([...providerSelect.options].map((o) => o.value)).toContain('openai');
+    expect([...providerSelect.options].map((o) => o.value)).toContain('anthropic');
+    // The model options are catalog-derived (not hardcoded) and scoped to the
+    // selected provider (openai, from the persisted tuple) using a collision-safe
+    // composite identity — the anthropic row is absent, not a bare 'claude'.
+    const modelValues = [...modelSelect.options].map((o) => o.value);
+    expect(modelValues).toContain('openai:gpt-5');
+    expect(modelValues).not.toContain('anthropic:claude');
+    expect(modelValues).not.toContain('claude');
+  });
+
+  it('sends provider/model derived from the persisted catalog tuple (never free text) and emits abort from Stop', async () => {
     const textarea = container.querySelector(
       'textarea[aria-label="Message"]',
     ) as HTMLTextAreaElement;
-    const providerInput = container.querySelector(
-      'input[aria-label="Provider"]',
-    ) as HTMLInputElement;
-    const modelInput = container.querySelector('input[aria-label="Model"]') as HTMLInputElement;
 
     const stopButtonBefore = container.querySelector(
       'button[aria-label="Stop"]',
     ) as HTMLButtonElement;
     expect(stopButtonBefore.disabled).toBe(true);
 
+    // Choose a fresh tuple from the catalog and let it persist.
+    const providerSelect = container.querySelector(
+      'select[aria-label="Provider"]',
+    ) as HTMLSelectElement;
+    await act(async () => {
+      selectValue(providerSelect, 'anthropic');
+    });
+    const modelSelect = container.querySelector('select[aria-label="Model"]') as HTMLSelectElement;
+    await act(async () => {
+      // Composite provider+model option identity (provider was switched to
+      // anthropic above); the bare 'claude' no longer identifies an option.
+      selectValue(modelSelect, 'anthropic:claude');
+    });
+    await flushAsync();
+
     await act(async () => {
       setValue(textarea, 'hello there');
-      setValue(providerInput, 'anthropic');
-      setValue(modelInput, 'claude');
     });
     await act(async () => {
       textarea.dispatchEvent(
@@ -415,6 +542,7 @@ describe('ChatPage', () => {
       );
     });
 
+    // The projected provider/model come from the validated persisted tuple.
     expect(fake.emitted).toContainEqual({
       event: 'message',
       payload: {
@@ -443,6 +571,28 @@ describe('ChatPage', () => {
     expect(fake.emitted).toContainEqual({ event: 'abort', payload: { conversationId: 'c1' } });
   });
 
+  it('disables send until a selection has persisted — no send with an unset selection', async () => {
+    await remountWithFetch(harnessFetch(null));
+
+    const sendButton = findButton(container, 'Send');
+    const textarea = container.querySelector(
+      'textarea[aria-label="Message"]',
+    ) as HTMLTextAreaElement;
+
+    await act(async () => {
+      setValue(textarea, 'should not send');
+    });
+    // Content present, but no selection persisted → Send stays disabled.
+    expect(sendButton.disabled).toBe(true);
+
+    await act(async () => {
+      textarea.dispatchEvent(
+        new KeyboardEvent('keydown', { key: 'Enter', bubbles: true, cancelable: true }),
+      );
+    });
+    expect(fake.emitted.filter((e) => e.event === 'message')).toHaveLength(0);
+  });
+
   it('renders the session panel from a pre-ack session:info and keeps it visible after the later ack', async () => {
     const textarea = container.querySelector(
       'textarea[aria-label="Message"]',
@@ -620,6 +770,134 @@ describe('ChatPage', () => {
     expect(fake.emitted.filter((e) => e.event === 'message')).toHaveLength(1);
   });
 
+  it('scopes the model options to the intentionally selected provider (cross-provider models absent)', async () => {
+    await remountWithFetch(harnessFetch(null));
+
+    const harnessSelect = container.querySelector(
+      'select[aria-label="Harness"]',
+    ) as HTMLSelectElement;
+    await act(async () => {
+      selectValue(harnessSelect, 'pi');
+    });
+    await flushAsync();
+
+    const providerSelect = container.querySelector(
+      'select[aria-label="Provider"]',
+    ) as HTMLSelectElement;
+    await act(async () => {
+      selectValue(providerSelect, 'openai');
+    });
+
+    const modelSelect = container.querySelector('select[aria-label="Model"]') as HTMLSelectElement;
+    const optionValues = [...modelSelect.options].map((o) => o.value).filter((v) => v !== '');
+    // Only the selected provider's models are offered — provider B's model
+    // (anthropic:claude) is absent, so a user cannot pick across providers.
+    expect(optionValues).toEqual(['openai:gpt-5']);
+    expect(optionValues).not.toContain('anthropic:claude');
+  });
+
+  it('keeps identical modelIds under two providers distinct and resolves the pick to the exact tuple', async () => {
+    const COLLIDING_CATALOG = {
+      harnessId: 'pi',
+      version: '2026-08-11',
+      fingerprint: 'fp',
+      models: [
+        {
+          harnessId: 'pi',
+          providerId: 'alpha',
+          modelId: 'gpt-x',
+          displayName: 'Alpha GPT-X',
+          reasoningCapability: true,
+          inputTypes: ['text'],
+          authState: 'ready',
+          availability: 'available',
+        },
+        {
+          harnessId: 'pi',
+          providerId: 'beta',
+          modelId: 'gpt-x',
+          displayName: 'Beta GPT-X',
+          reasoningCapability: true,
+          inputTypes: ['text'],
+          authState: 'ready',
+          availability: 'available',
+        },
+      ],
+    };
+    const collidingFetch = vi.fn(async (input: unknown, init?: RequestInit) => {
+      const url = String(input);
+      const method = String(init?.method ?? 'GET').toUpperCase();
+      if (url === '/api/harnesses') {
+        return jsonResponse([{ id: 'pi', displayName: 'Pi', capabilities: [] }]);
+      }
+      if (url.startsWith('/api/harnesses/') && url.endsWith('/catalog')) {
+        return jsonResponse(COLLIDING_CATALOG);
+      }
+      if (url === '/api/chat/preferences/selection' && method === 'GET') {
+        return jsonResponse({ selection: null });
+      }
+      if (url === '/api/chat/preferences/selection' && method === 'PUT') {
+        return jsonResponse({ selection: JSON.parse(String(init?.body)) });
+      }
+      return new Response('not found', { status: 404 });
+    }) as unknown as typeof fetch;
+
+    await remountWithFetch(collidingFetch);
+
+    const harnessSelect = container.querySelector(
+      'select[aria-label="Harness"]',
+    ) as HTMLSelectElement;
+    await act(async () => {
+      selectValue(harnessSelect, 'pi');
+    });
+    await flushAsync();
+
+    const providerSelect = container.querySelector(
+      'select[aria-label="Provider"]',
+    ) as HTMLSelectElement;
+    await act(async () => {
+      selectValue(providerSelect, 'alpha');
+    });
+
+    const modelSelect = container.querySelector('select[aria-label="Model"]') as HTMLSelectElement;
+    // The colliding modelId is provider-qualified in the option value, never a
+    // bare id, so the two providers' 'gpt-x' rows are uniquely identifiable.
+    const optionValues = [...modelSelect.options].map((o) => o.value).filter((v) => v !== '');
+    expect(optionValues).toEqual(['alpha:gpt-x']);
+
+    await act(async () => {
+      selectValue(modelSelect, 'alpha:gpt-x');
+    });
+    await flushAsync();
+
+    // The controlled select highlights the alpha row via the composite identity.
+    expect(modelSelect.value).toBe('alpha:gpt-x');
+
+    const textarea = container.querySelector(
+      'textarea[aria-label="Message"]',
+    ) as HTMLTextAreaElement;
+    await act(async () => {
+      setValue(textarea, 'ping');
+    });
+    await act(async () => {
+      textarea.dispatchEvent(
+        new KeyboardEvent('keydown', { key: 'Enter', bubbles: true, cancelable: true }),
+      );
+    });
+
+    // The persisted/sent tuple resolves to provider alpha — NOT beta — even
+    // though the bare modelId 'gpt-x' exists under both providers.
+    expect(fake.emitted).toContainEqual({
+      event: 'message',
+      payload: {
+        conversationId: undefined,
+        content: 'ping',
+        provider: 'alpha',
+        modelId: 'gpt-x',
+      },
+    });
+  });
+
   it('removes socket handlers and tears down the socket on unmount, with no network calls', async () => {
     expect(fake.listeners.size).toBeGreaterThan(0);
 

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

@@ -6,6 +6,7 @@ import { asFiniteNumberOrNull, asString } from '@/spa/chat/runtime-guards';
 import { SessionPanel } from '@/spa/chat/session-panel';
 import { ToolCallList } from '@/spa/chat/tool-call-list';
 import { useChatConnection } from '@/spa/chat/use-chat-connection';
+import { useHarnessSelection } from '@/spa/chat/use-harness-selection';
 
 /** Renders a real value normally, but an honest "unavailable" label instead
  * of a fabricated `0` for a missing/malformed count — a real `0 tokens` and
@@ -23,6 +24,7 @@ function formatCost(value: unknown): string {
 
 export function ChatPage(): ReactElement {
   const { state, actions } = useChatConnection();
+  const harness = useHarnessSelection();
   const hasConversation = state.conversationId !== null;
 
   return (
@@ -86,6 +88,7 @@ export function ChatPage(): ReactElement {
         streaming={state.streaming}
         sending={state.sending}
         hasConversation={hasConversation}
+        harness={harness}
       />
     </div>
   );