docs(fleet): record per-agent model switch in north star

Per-agent model selection (operator-configurable, webUI switch over the
existing roster model_hint plumbing) recorded as a decision of record so
it isn't lost. Also confirms orchestrator/enhancer run Claude Opus 4.8 in
Claude Code; only workers run pi/gpt-5.5.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/fleet/north-star.md

@@ -353,6 +353,25 @@ re-evaluate if isolation or write-volume demands it.
 - **Docs as projections:** `docs/TASKS.md` and `MISSION-MANIFEST.md` become generated exports of the DB, not hand-maintained.
 - **Sub-decision pending:** dedicated schema in existing PG instance (recommended) vs. dedicated PG instance. Revisit if isolation or write-volume demands it.
 
+## Decisions of record (2026-06-24, with Jason)
+
+- **Per-agent model switch (operator-configurable, NOT a global lock):** model selection is
+  **per-agent**, never a host-global pin. Claude sessions MUST NOT be locked to a single model in
+  `~/.claude/settings.json`; each agent chooses its model independently. The plumbing already exists —
+  roster `model_hint` → `MOSAIC_AGENT_MODEL` → `start-agent-session.sh` appends `--model <hint>` to that
+  agent's harness (claude or pi); settable today via `mosaic fleet add|edit <agent> --model <hint>`.
+  **North-star target:** surface this as a **per-agent model switch in the webUI** (with CLI/TUI parity
+  per MVP-X1) — read the roster, expose a per-agent model dropdown, write `model_hint` back, and restart
+  that one agent to apply. Unset = inherit the harness default. This **composes with** the budget
+  downgrade ladder (opus → sonnet → haiku, then Claude → Codex): the operator sets the per-agent model
+  _intent/ceiling_; budget pacing may downgrade within policy. Tracked as a Fleet `TASKS.md` entry under
+  the Phase-5 webUI surface.
+- **Orchestrator runtime (confirmed live):** the **orchestrator and enhancer run Claude Opus 4.8 in the
+  Claude Code harness**; only workers (coder/reviewer) run pi/gpt-5.5. Consistent with the 2026-06-20
+  "Claude reserved for Claude Code only" decision (the orchestrator runs _in_ Claude Code, not an
+  alternate Claude harness). Pi/gpt-5.5 as the orchestrator is permitted **only if proven** at least as
+  satisfactory; absent that proof, the orchestrator stays on Claude Opus 4.8.
+
 ## Future enhancements (north-star, post-MVP — not on the MVP track)
 
 - **Mosaic Claude Discord Plugin** — a first-party Mosaic Discord connector that properly

packages/mosaic/framework/fleet/profiles/software-delivery.yaml

@@ -27,45 +27,49 @@ id: software-delivery
 title: Software Delivery
 description: >-
   The engineering fleet that turns ratified objectives into shipped, reviewed,
-  merged code. The lead (planner — the orchestrator seat) plans phased FRs into a
-  depends_on DAG, decomposition splits them into one-PR-each cards, coders execute
-  to green CI, and review / security-review / site-tester / merge-gate guard the
-  merge. This mirrors today's coding fleet.
-# NOTE: the canonical lead seat is the "orchestrator". In the persona library the
-# orchestrator IS the `planner` class (see roles/planner.md: "the planner role IS
-# the existing orchestrator class") — so the lead/floor reference `planner`, the
-# only class that actually resolves to a role contract.
-lead: planner
+  merged code. The lead (orchestrator) runs the supervisor loop and dispatches
+  ready work; it hands goal-decomposition to the planner, which plans phased FRs
+  into a depends_on DAG, decomposition splits them into one-PR-each cards, coders
+  execute to green CI, and review / security-review / site-tester / merge-gate
+  guard the merge. This mirrors today's coding fleet.
+# NOTE: the lead seat is the dedicated "orchestrator" — the always-on coordinator
+# that runs the supervisor tick, dispatches ready work, and routes PRs to the
+# merge-gate while holding only lean coordination state. The planner is now a
+# distinct seat (heavy goal-decomposition context) that reports to the
+# orchestrator. The two-agent floor is orchestrator + enhancer.
+lead: orchestrator
 floor:
-  - planner
+  - orchestrator
   - enhancer
 roster:
+  - class: orchestrator
   - class: board
-    reports_to: planner
+    reports_to: orchestrator
   - class: planner
+    reports_to: orchestrator
   - class: decomposition
     reports_to: planner
   - class: code
     reports_to: decomposition
     multiplicity: 2
   - class: review
-    reports_to: planner
+    reports_to: orchestrator
   - class: security-review
     reports_to: review
   - class: site-tester
     reports_to: review
   - class: documentation
-    reports_to: planner
+    reports_to: orchestrator
   - class: merge-gate
-    reports_to: planner
+    reports_to: orchestrator
   - class: rebase
     reports_to: merge-gate
   - class: operator
-    reports_to: planner
+    reports_to: orchestrator
   - class: session-review
-    reports_to: planner
+    reports_to: orchestrator
   - class: enhancer
-    reports_to: planner
+    reports_to: orchestrator
 notes: >-
-  Two-agent floor (orchestrator/planner + enhancer) is always staffed; every other
-  seat is added on demand.
+  Two-agent floor (orchestrator + enhancer) is always staffed; every other seat is
+  added on demand.

packages/mosaic/framework/fleet/roles/LIBRARY.md

@@ -19,6 +19,7 @@ their intro so tooling can group them.
 
 | Persona         | Purpose                                                                        |
 | --------------- | ------------------------------------------------------------------------------ |
+| orchestrator    | Always-on coordinator — runs the supervisor loop, dispatches ready work        |
 | board           | Multi-lens deliberation panel; owns the mission's direction, not its execution |
 | planner         | Turns ratified objectives into a phased FR plan wired into a `depends_on` DAG  |
 | decomposition   | Splits FRs into one-PR-each cards wired with `depends_on` edges                |

packages/mosaic/framework/fleet/roles/orchestrator.md

@@ -0,0 +1,46 @@
+# Orchestrator — fleet role definition
+
+The **orchestrator** is one half of the fleet's two-agent floor: every fleet runs,
+at minimum, an **orchestrator** and an **enhancer**. The orchestrator is the
+fleet's **always-on coordinator and dispatcher** (`class: orchestrator`,
+`persistent_persona: true`) — it owns fleet _movement_, not the work itself.
+
+It is a **core, always-on** agent, not an ephemeral per-lane worker.
+
+## Mandate
+
+1. **Run the supervisor tick** — perform the readiness scan each loop and keep the
+   two-agent floor (orchestrator + enhancer) healthy, restoring it the moment it
+   drops below the floor.
+2. **Dispatch ready work** — pick up cards whose `depends_on` edges are satisfied
+   and assign them via the backlog/claim, so no idle agent sits while ready work
+   exists.
+3. **Delegate decomposition, don't do it** — hand goal-decomposition work to the
+   **planner**, which it coordinates; the orchestrator tracks the resulting plan
+   but does not author the DAG itself.
+4. **Route PRs to the merge-gate** — push reviewed, ready-to-land PRs at the
+   **merge-gate** (the only merge path); it never approves or merges itself.
+5. **Interface with the operator/user** — be the fleet's coordination surface,
+   relaying status and accepting direction, while holding only coordination state.
+6. **Keep the loop turning** — re-dispatch on completion or failure so the fleet
+   keeps moving rather than stalling.
+
+## Boundaries
+
+- **Does NOT decompose goals into the DAG/cards** — that is the **planner**'s lane,
+  which the orchestrator dispatches to.
+- **Does NOT write product/source code** (coders), **review** (review), or
+  **approve merges itself** (merge-gate).
+- **Does NOT carry deep per-task context** — it delegates and tracks, keeping its
+  own context lean so the coordination loop stays fast.
+
+The orchestrator moves work; it never holds the heavy planning or execution
+context that the seats it dispatches to carry.
+
+## Persona
+
+A lean, decisive coordinator. It thinks in readiness and throughput, dispatches the
+next ready card the instant a dependency clears, and never lets an idle agent sit
+while ready work exists — keeping its own context minimal so the loop never slows.
+
+> Doctrine: `docs/fleet/north-star.md` (two-agent floor + role library).

packages/mosaic/framework/fleet/roles/planner.md

@@ -3,11 +3,11 @@
 The **planner** turns ratified objectives into an executable **plan** — phased
 functional requirements (FRs) wired into a `depends_on` DAG.
 
-> **Alias:** the planner role IS the existing **orchestrator** class. The
-> orchestrator _plays_ planner; this file documents the planning contract, it does
-> **not** introduce a competing class. The two-agent floor (orchestrator +
-> enhancer) is preserved — do not split planner into a separate persistent agent
-> that would break it.
+> **Reports to the orchestrator.** The planner is the goal-decomposition seat that
+> the **orchestrator** dispatches planning work to; it carries the heavy
+> goal-decomposition context, while the orchestrator holds only the lean
+> coordination state. The two-agent floor is **orchestrator + enhancer** — the
+> planner is added on demand, not part of the floor.
 
 It is a **front-office** role.
 
@@ -19,8 +19,8 @@ It is a **front-office** role.
    between FRs so downstream decomposition can parallelize safely.
 3. **Emit a plan, not tasks** — the planner's output is the phased FR/DAG
    document. Splitting FRs into one-PR-each cards is the **decomposition** role's job.
-4. **Re-plan on failure** — when execution diverges, the planner (orchestrator)
-   re-sequences the DAG rather than letting agents improvise.
+4. **Re-plan on failure** — when execution diverges, the planner re-sequences the
+   DAG rather than letting agents improvise.
 
 ## Boundaries
 
@@ -35,6 +35,7 @@ merge path.
 ## Persona
 
 The architect of the mission's shape. It thinks in phases and dependencies, hands
-a clean DAG to decomposition, and keeps the orchestrator/enhancer floor intact.
+a clean DAG to decomposition, and reports its plan back to the orchestrator that
+dispatched it.
 
 > Doctrine: `docs/fleet/north-star.md` (two-agent floor + role library).

packages/mosaic/src/commands/compose-contract.spec.ts

@@ -164,4 +164,89 @@ describe('composeContract — overlay composer', () => {
     expect(composeContract('pi', fixture.home)).toContain('# pi runtime contract');
     expect(composeContract('codex', fixture.home)).not.toContain('# pi runtime contract');
   });
+
+  // ── Persona contract injection (A3b) ──────────────────────────────────────
+  // composeContract reads MOSAIC_AGENT_CLASS and injects the resolved persona
+  // (override-aware). Save/restore the env so these tests don't leak state.
+  describe('persona contract (A3b)', () => {
+    let prevClass: string | undefined;
+
+    beforeEach(() => {
+      prevClass = process.env['MOSAIC_AGENT_CLASS'];
+    });
+
+    afterEach(() => {
+      if (prevClass === undefined) delete process.env['MOSAIC_AGENT_CLASS'];
+      else process.env['MOSAIC_AGENT_CLASS'] = prevClass;
+    });
+
+    const seedBaseline = (klass: string, body: string): void => {
+      mkdirSync(join(fixture.home, 'fleet', 'roles'), { recursive: true });
+      writeFileSync(join(fixture.home, 'fleet', 'roles', `${klass}.md`), body);
+    };
+    const seedOverride = (klass: string, body: string): void => {
+      mkdirSync(join(fixture.home, 'fleet', 'roles.local'), { recursive: true });
+      writeFileSync(join(fixture.home, 'fleet', 'roles.local', `${klass}.md`), body);
+    };
+
+    it('injects the baseline persona when MOSAIC_AGENT_CLASS is set and a role file exists', () => {
+      seedBaseline('coder', '# Coder\n\n(`class: coder`)\n\nBASELINE-MANDATE: ship the lane.\n');
+      process.env['MOSAIC_AGENT_CLASS'] = 'coder';
+      const out = composeContract('claude', fixture.home);
+      expect(out).toContain('# Persona Contract (coder)');
+      expect(out).toContain('BASELINE-MANDATE');
+    });
+
+    it('OVERRIDE WINS at launch: roles.local persona is injected over baseline (AC-NS-7)', () => {
+      seedBaseline('coder', '# Coder\n\n(`class: coder`)\n\nBASELINE-MANDATE.\n');
+      seedOverride('coder', '# Coder (override)\n\n(`class: coder`)\n\nOVERRIDE-MANDATE.\n');
+      process.env['MOSAIC_AGENT_CLASS'] = 'coder';
+      const out = composeContract('claude', fixture.home);
+      expect(out).toContain('# Persona Contract (coder)');
+      expect(out).toContain('OVERRIDE-MANDATE');
+      expect(out).not.toContain('BASELINE-MANDATE');
+    });
+
+    it('does NOT inject a persona when MOSAIC_AGENT_CLASS is unset', () => {
+      seedBaseline('coder', '# Coder\n\n(`class: coder`)\n\nBASELINE-MANDATE.\n');
+      delete process.env['MOSAIC_AGENT_CLASS'];
+      const out = composeContract('claude', fixture.home);
+      expect(out).not.toContain('# Persona Contract');
+    });
+
+    it('does NOT inject (no throw) when MOSAIC_AGENT_CLASS names an unknown class', () => {
+      seedBaseline('coder', '# Coder\n\n(`class: coder`)\n\nBASELINE-MANDATE.\n');
+      process.env['MOSAIC_AGENT_CLASS'] = 'nonexistent';
+      expect(() => composeContract('claude', fixture.home)).not.toThrow();
+      expect(composeContract('claude', fixture.home)).not.toContain('# Persona Contract');
+    });
+
+    it('places the persona contract BEFORE the fleet comms block (identity, then peers)', () => {
+      seedBaseline('enhancer', '# Enhancer\n\n(`class: enhancer`)\n\nIMPROVE.\n');
+      mkdirSync(join(fixture.home, 'fleet'), { recursive: true });
+      writeFileSync(
+        join(fixture.home, 'fleet', 'roster.yaml'),
+        [
+          'agents:',
+          '  - name: orchestrator',
+          '    class: orchestrator',
+          '  - name: enhancer',
+          '    class: enhancer',
+          '',
+        ].join('\n'),
+      );
+      const prevName = process.env['MOSAIC_AGENT_NAME'];
+      try {
+        process.env['MOSAIC_AGENT_CLASS'] = 'enhancer';
+        process.env['MOSAIC_AGENT_NAME'] = 'enhancer';
+        const out = composeContract('claude', fixture.home);
+        expect(out).toContain('# Persona Contract (enhancer)');
+        expect(out).toContain('# Fleet Comms');
+        expect(out.indexOf('# Persona Contract')).toBeLessThan(out.indexOf('# Fleet Comms'));
+      } finally {
+        if (prevName === undefined) delete process.env['MOSAIC_AGENT_NAME'];
+        else process.env['MOSAIC_AGENT_NAME'] = prevName;
+      }
+    });
+  });
 });

packages/mosaic/src/commands/fleet-personas.ts

@@ -25,6 +25,7 @@
  * can reference a customized or user-added persona.
  */
 
+import { readFileSync, readdirSync } from 'node:fs';
 import { readFile, readdir } from 'node:fs/promises';
 import { homedir } from 'node:os';
 import { basename, join } from 'node:path';
@@ -88,13 +89,12 @@ export interface DirClasses {
  * classes still appear in `classes` for membership checks.
  */
 export async function extractClassesFromDir(dir: string): Promise<DirClasses> {
-  const classes = new Set<string>();
-  const byClass = new Map<string, PersonaFile>();
+  const acc: DirClasses = { classes: new Set<string>(), byClass: new Map<string, PersonaFile>() };
   let entries: string[];
   try {
     entries = await readdir(dir);
   } catch {
-    return { classes, byClass };
+    return acc;
   }
 
   for (const entry of entries) {
@@ -105,36 +105,75 @@ export async function extractClassesFromDir(dir: string): Promise<DirClasses> {
     } catch {
       continue;
     }
-    if (entry === 'LIBRARY.md') {
-      for (const m of text.matchAll(LIBRARY_ROW)) {
-        const name = m[1];
-        if (name && name !== 'persona') classes.add(name);
-      }
+    accumulateEntry(acc, dir, entry, text);
+  }
+  return acc;
+}
+
+/**
+ * Synchronous twin of {@link extractClassesFromDir}. Identical extraction
+ * semantics (same markers, same union of marker/filename/LIBRARY sources) on
+ * sync fs, for the synchronous launch-time prompt path (composeContract) which
+ * cannot await. Missing dir / unreadable files degrade gracefully.
+ */
+export function extractClassesFromDirSync(dir: string): DirClasses {
+  const acc: DirClasses = { classes: new Set<string>(), byClass: new Map<string, PersonaFile>() };
+  let entries: string[];
+  try {
+    entries = readdirSync(dir);
+  } catch {
+    return acc;
+  }
+
+  for (const entry of entries) {
+    if (!entry.endsWith('.md')) continue;
+    let text: string;
+    try {
+      text = readFileSync(join(dir, entry), 'utf8');
+    } catch {
       continue;
     }
-    // The filename stem is itself a valid class (covers marker-less alias docs).
-    const stem = basename(entry, '.md');
-    classes.add(stem);
-    const domainMatch = DOMAIN_MARKER.exec(text);
-    const domain = domainMatch?.[1];
-    let markedClassForFile: string | undefined;
-    for (const m of text.matchAll(CLASS_MARKER)) {
-      const klass = m[1];
-      if (!klass) continue;
-      classes.add(klass);
-      // Record the FIRST marker as the file's defining class (the prose names
-      // the persona's own class up top; later mentions reference siblings).
-      if (!markedClassForFile) {
-        markedClassForFile = klass;
-        byClass.set(klass, { klass, file: join(dir, entry), ...(domain ? { domain } : {}) });
-      }
+    accumulateEntry(acc, dir, entry, text);
+  }
+  return acc;
+}
+
+/**
+ * Apply the class-extraction rules for ONE role file's text into `acc`. Pure
+ * over already-read content, so the async and sync directory scanners share a
+ * single definition of "what classes a file contributes" (DRY — no semantic
+ * drift between the launch-time and command-time paths).
+ */
+function accumulateEntry(acc: DirClasses, dir: string, entry: string, text: string): void {
+  const { classes, byClass } = acc;
+  if (entry === 'LIBRARY.md') {
+    for (const m of text.matchAll(LIBRARY_ROW)) {
+      const name = m[1];
+      if (name && name !== 'persona') classes.add(name);
     }
-    // A marker-less file still maps its stem to itself (no domain known).
-    if (!markedClassForFile && !byClass.has(stem)) {
-      byClass.set(stem, { klass: stem, file: join(dir, entry) });
+    return;
+  }
+  // The filename stem is itself a valid class (covers marker-less alias docs).
+  const stem = basename(entry, '.md');
+  classes.add(stem);
+  const domainMatch = DOMAIN_MARKER.exec(text);
+  const domain = domainMatch?.[1];
+  let markedClassForFile: string | undefined;
+  for (const m of text.matchAll(CLASS_MARKER)) {
+    const klass = m[1];
+    if (!klass) continue;
+    classes.add(klass);
+    // Record the FIRST marker as the file's defining class (the prose names
+    // the persona's own class up top; later mentions reference siblings).
+    if (!markedClassForFile) {
+      markedClassForFile = klass;
+      byClass.set(klass, { klass, file: join(dir, entry), ...(domain ? { domain } : {}) });
     }
   }
-  return { classes, byClass };
+  // A marker-less file still maps its stem to itself (no domain known).
+  if (!markedClassForFile && !byClass.has(stem)) {
+    byClass.set(stem, { klass: stem, file: join(dir, entry) });
+  }
 }
 
 export interface PersonaDirs {
@@ -229,6 +268,51 @@ export async function resolvePersona(
   );
 }
 
+/**
+ * Synchronous twin of {@link resolvePersona} — same override-wins precedence
+ * (roles.local/ beats roles/, by marker first then filename stem), returning
+ * null if neither layer defines the class. Exists for the synchronous launch
+ * prompt path (composeContract → readPersonaContractBlock) which cannot await.
+ * Keeping it here, beside the async resolver, keeps the resolution semantics in
+ * one module so the launch-time and command-time resolutions never diverge.
+ */
+export function resolvePersonaSync(
+  klass: string,
+  opts: PersonaDirs = {},
+): PersonaResolution | null {
+  const { rolesDir, overrideDir } = resolveDirs(opts);
+  const base = extractClassesFromDirSync(rolesDir);
+  const over = extractClassesFromDirSync(overrideDir);
+
+  const fromLayer = (
+    dir: string,
+    extracted: DirClasses,
+    layer: PersonaLayer,
+  ): PersonaResolution | null => {
+    // Prefer the marker-defined file; fall back to the filename stem.
+    const pf = extracted.byClass.get(klass);
+    if (!pf) {
+      if (!extracted.classes.has(klass)) return null;
+      const byName = join(dir, `${klass}.md`);
+      try {
+        const content = readFileSync(byName, 'utf8');
+        const dm = DOMAIN_MARKER.exec(content);
+        return { klass, layer, file: byName, content, ...(dm?.[1] ? { domain: dm[1] } : {}) };
+      } catch {
+        return null;
+      }
+    }
+    try {
+      const content = readFileSync(pf.file, 'utf8');
+      return { klass, layer, file: pf.file, content, ...(pf.domain ? { domain: pf.domain } : {}) };
+    } catch {
+      return null;
+    }
+  };
+
+  return fromLayer(overrideDir, over, 'override') ?? fromLayer(rolesDir, base, 'baseline');
+}
+
 export interface PersonaStatusEntry {
   klass: string;
   status: PersonaStatus;

packages/mosaic/src/commands/fleet-profiles.spec.ts

@@ -46,10 +46,12 @@ describe('listPersonaClasses (real role library)', () => {
 
   it('covers marker-less engineering personas via filename + LIBRARY index', async () => {
     const classes = await listPersonaClasses(rolesDir);
-    // planner/decomposition have a role file but no inline marker (planner aliases
-    // the orchestrator class) — they resolve from the filename + LIBRARY.md row.
+    // planner/decomposition have a role file but no inline marker — they resolve
+    // from the filename + LIBRARY.md row.
     expect(classes.has('planner')).toBe(true);
     expect(classes.has('decomposition')).toBe(true);
+    // The dedicated orchestrator persona resolves (inline marker + filename + row).
+    expect(classes.has('orchestrator')).toBe(true);
   });
 
   it('returns an empty set for a missing roles dir (graceful)', async () => {
@@ -75,11 +77,17 @@ describe('baseline profiles (real library)', () => {
 
   it('software-delivery has the expected lead, floor, and roster shape', async () => {
     const profile = await loadProfile('software-delivery', realLib);
-    expect(profile.lead).toBe('planner');
-    expect(profile.floor).toEqual(['planner', 'enhancer']);
+    expect(profile.lead).toBe('orchestrator');
+    expect(profile.floor).toEqual(['orchestrator', 'enhancer']);
     const code = profile.roster.find((r) => r.class === 'code');
     expect(code?.multiplicity).toBe(2);
     expect(code?.reportsTo).toBe('decomposition');
+    // The dedicated orchestrator is the lead seat (no reports_to); the planner is
+    // now a distinct seat that reports to it.
+    const orchestrator = profile.roster.find((r) => r.class === 'orchestrator');
+    expect(orchestrator?.reportsTo).toBeUndefined();
+    const planner = profile.roster.find((r) => r.class === 'planner');
+    expect(planner?.reportsTo).toBe('orchestrator');
   });
 
   it('loadProfile throws on an unknown id', async () => {

packages/mosaic/src/commands/fleet.spec.ts

@@ -246,6 +246,8 @@ describe('fleet roster parsing', () => {
     expect(generateAgentEnv(roster, getRosterAgent(roster, 'coder0'))).toBe(
       [
         'MOSAIC_AGENT_NAME=coder0',
+        // Reflects the roster's non-default `class: implementer` (A3a).
+        'MOSAIC_AGENT_CLASS=implementer',
         'MOSAIC_AGENT_RUNTIME=codex',
         'MOSAIC_AGENT_MODEL=',
         'MOSAIC_AGENT_WORKDIR=/srv/mosaic',
@@ -255,6 +257,40 @@ describe('fleet roster parsing', () => {
     );
   });
 
+  it('emits MOSAIC_AGENT_CLASS=worker for an agent that declares no class', async () => {
+    cleanup = await tempDir();
+    const rosterPath = join(cleanup, 'roster.json');
+    await writeFile(
+      rosterPath,
+      JSON.stringify({
+        version: 1,
+        transport: 'tmux',
+        agents: [{ name: 'coder0', runtime: 'codex' }],
+      }),
+    );
+    const roster = await loadFleetRoster(rosterPath);
+    expect(generateAgentEnv(roster, getRosterAgent(roster, 'coder0'))).toContain(
+      'MOSAIC_AGENT_CLASS=worker\n',
+    );
+  });
+
+  it('shell-escapes MOSAIC_AGENT_CLASS so a launcher reads it verbatim', async () => {
+    cleanup = await tempDir();
+    const rosterPath = join(cleanup, 'roster.json');
+    await writeFile(
+      rosterPath,
+      JSON.stringify({
+        version: 1,
+        transport: 'tmux',
+        agents: [{ name: 'coder0', runtime: 'codex', class: 'orchestrator' }],
+      }),
+    );
+    const roster = await loadFleetRoster(rosterPath);
+    expect(generateAgentEnv(roster, getRosterAgent(roster, 'coder0'))).toContain(
+      'MOSAIC_AGENT_CLASS=orchestrator\n',
+    );
+  });
+
   it('preserves site-owned agent EnvironmentFile overrides while refreshing roster keys', () => {
     const generated = [
       'MOSAIC_AGENT_NAME=coder0',
@@ -286,6 +322,28 @@ describe('fleet roster parsing', () => {
     );
   });
 
+  it('updates (does not duplicate) MOSAIC_AGENT_CLASS on re-launch', () => {
+    const generated = [
+      'MOSAIC_AGENT_NAME=coder0',
+      'MOSAIC_AGENT_CLASS=orchestrator',
+      'MOSAIC_AGENT_RUNTIME=codex',
+      '',
+    ].join('\n');
+    const existing = [
+      'MOSAIC_AGENT_NAME=coder0',
+      'MOSAIC_AGENT_CLASS=worker',
+      'MOSAIC_AGENT_RUNTIME=codex',
+      '',
+    ].join('\n');
+
+    const merged = mergeAgentEnv(generated, existing);
+    // mergeAgentEnv keys by VAR name, so the regenerated CLASS wins and there is
+    // exactly one MOSAIC_AGENT_CLASS line (no stale worker value left behind).
+    expect(merged).toContain('MOSAIC_AGENT_CLASS=orchestrator');
+    expect(merged).not.toContain('MOSAIC_AGENT_CLASS=worker');
+    expect(merged.match(/^MOSAIC_AGENT_CLASS=/gm)).toHaveLength(1);
+  });
+
   it('rejects unknown roster fields instead of silently defaulting', async () => {
     cleanup = await tempDir();
     const rosterPath = join(cleanup, 'roster.yaml');

packages/mosaic/src/commands/fleet.ts

@@ -490,6 +490,9 @@ export function generateAgentEnv(roster: FleetRoster, agent: FleetAgent): string
   const workingDirectory = agent.workingDirectory ?? roster.defaults.workingDirectory;
   return [
     `MOSAIC_AGENT_NAME=${shellEnvValue(agent.name)}`,
+    // Per-agent class → start-agent-session.sh / launcher reads this to inject the
+    // matching persona contract for the agent's class (default `worker`).
+    `MOSAIC_AGENT_CLASS=${shellEnvValue(agent.className)}`,
     `MOSAIC_AGENT_RUNTIME=${shellEnvValue(agent.runtime)}`,
     // Per-agent model hint → start-agent-session.sh appends `--model <hint>` to
     // the `mosaic yolo` launch so workers run on the roster's model (e.g. pi on

packages/mosaic/src/commands/launch.ts

@@ -20,6 +20,7 @@ import { homedir } from 'node:os';
 import { join, dirname } from 'node:path';
 import type { Command } from 'commander';
 import { readFleetCommsBlock } from '../fleet/comms-onboarding.js';
+import { readPersonaContractBlock } from '../fleet/persona-contract.js';
 
 const MOSAIC_HOME = process.env['MOSAIC_HOME'] ?? join(homedir(), '.config', 'mosaic');
 
@@ -384,6 +385,16 @@ For required push/merge/issue-close/release actions, execute without routine con
   // Runtime-specific contract
   parts.push('\n\n# Runtime-Specific Contract\n\n' + readFileSync(runtimeFile, 'utf-8'));
 
+  // Persona contract (A3b): when this agent was spawned with a class
+  // (MOSAIC_AGENT_CLASS, exported into the pane env by A3a), inject its resolved
+  // role contract so its identity (mandate + boundaries) is resident from the
+  // first turn. Override-aware via the persona resolver: a user-customized
+  // persona in fleet/roles.local/ wins over the baseline (AC-NS-7 launch proof).
+  // Placed BEFORE fleet comms: identity first, then how-to-reach-peers. No-ops
+  // silently when the class is unset/unknown (mirrors the comms block).
+  const persona = readPersonaContractBlock(mosaicHome, process.env['MOSAIC_AGENT_CLASS']);
+  if (persona) parts.push('\n\n' + persona);
+
   // Fleet onboarding: when this is a spawned fleet agent (MOSAIC_AGENT_NAME set
   // and present in the roster), inject a comms cheat-sheet + peer roster so it
   // knows how to reach the orchestrator and its peers from its first turn.

packages/mosaic/src/fleet/persona-contract.spec.ts

@@ -0,0 +1,106 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { mkdtempSync, mkdirSync, writeFileSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { readPersonaContractBlock } from './persona-contract.js';
+
+/**
+ * Persona-contract launch injection (A3b). Asserts the override-aware resolver
+ * is wired so a customized persona in roles.local/ wins at launch (AC-NS-7), and
+ * that any miss (unset/empty/unknown class, missing file) no-ops silently —
+ * never throws — mirroring readFleetCommsBlock's tolerant contract.
+ */
+
+const BASELINE_CODER = `# Coder — fleet role definition
+
+The **coder** persona (\`class: coder\`, \`domain: engineering\`).
+
+## Mandate
+
+BASELINE-MANDATE: implement the assigned lane.
+`;
+
+const OVERRIDE_CODER = `# Coder — fleet role definition (override)
+
+The **coder** persona (\`class: coder\`).
+
+## Mandate
+
+OVERRIDE-MANDATE: implement the assigned lane, the user's way.
+`;
+
+function makeHome(): string {
+  const root = mkdtempSync(join(tmpdir(), 'mosaic-persona-'));
+  return join(root, 'mosaic-home');
+}
+
+function seedBaseline(home: string, klass: string, body: string): void {
+  const dir = join(home, 'fleet', 'roles');
+  mkdirSync(dir, { recursive: true });
+  writeFileSync(join(dir, `${klass}.md`), body);
+}
+
+function seedOverride(home: string, klass: string, body: string): void {
+  const dir = join(home, 'fleet', 'roles.local');
+  mkdirSync(dir, { recursive: true });
+  writeFileSync(join(dir, `${klass}.md`), body);
+}
+
+describe('readPersonaContractBlock — launch-time persona injection (A3b)', () => {
+  let home: string;
+
+  beforeEach(() => {
+    home = makeHome();
+  });
+
+  afterEach(() => {
+    // root is the parent of mosaic-home
+    rmSync(join(home, '..'), { recursive: true, force: true });
+  });
+
+  it('injects the baseline persona when the class has a fleet/roles/<class>.md', () => {
+    seedBaseline(home, 'coder', BASELINE_CODER);
+    const block = readPersonaContractBlock(home, 'coder');
+    expect(block).toContain('# Persona Contract (coder)');
+    expect(block).toContain('BASELINE-MANDATE');
+    expect(block).toContain('baseline `fleet/roles/` layer');
+  });
+
+  it('OVERRIDE WINS: roles.local/<class>.md content is injected over the baseline (AC-NS-7)', () => {
+    seedBaseline(home, 'coder', BASELINE_CODER);
+    seedOverride(home, 'coder', OVERRIDE_CODER);
+    const block = readPersonaContractBlock(home, 'coder');
+    expect(block).toContain('# Persona Contract (coder)');
+    expect(block).toContain('OVERRIDE-MANDATE'); // override body present
+    expect(block).not.toContain('BASELINE-MANDATE'); // baseline NOT used
+    expect(block).toContain('roles.local'); // layer note names the override layer
+  });
+
+  it('injects an override-only (user-added) persona with no baseline at all', () => {
+    seedOverride(home, 'reviewer', '# Reviewer\n\n(`class: reviewer`)\n\nCUSTOM-ROLE.\n');
+    const block = readPersonaContractBlock(home, 'reviewer');
+    expect(block).toContain('# Persona Contract (reviewer)');
+    expect(block).toContain('CUSTOM-ROLE');
+  });
+
+  it('no-ops (empty string) when the class is undefined', () => {
+    seedBaseline(home, 'coder', BASELINE_CODER);
+    expect(readPersonaContractBlock(home, undefined)).toBe('');
+  });
+
+  it('no-ops (empty string) when the class is empty/whitespace', () => {
+    seedBaseline(home, 'coder', BASELINE_CODER);
+    expect(readPersonaContractBlock(home, '')).toBe('');
+    expect(readPersonaContractBlock(home, '   ')).toBe('');
+  });
+
+  it('no-ops (empty string) for an unknown class with no role file', () => {
+    seedBaseline(home, 'coder', BASELINE_CODER);
+    expect(readPersonaContractBlock(home, 'nonexistent')).toBe('');
+  });
+
+  it('no-ops (empty string, no throw) when no roles directories exist at all', () => {
+    expect(() => readPersonaContractBlock(home, 'coder')).not.toThrow();
+    expect(readPersonaContractBlock(home, 'coder')).toBe('');
+  });
+});

packages/mosaic/src/fleet/persona-contract.ts

@@ -0,0 +1,63 @@
+/**
+ * Persona-contract injection at launch (North Star A3b).
+ *
+ * A spawned fleet agent should boot already knowing WHO it is: its class's role
+ * contract (mandate + boundaries). The companion goal A3a exports the agent's
+ * resolved class into the pane env as `MOSAIC_AGENT_CLASS`; here we read that
+ * class at launch (composeContract → system prompt) and inject the resolved
+ * persona contract so the identity is resident from the agent's first turn.
+ *
+ * OVERRIDE-AWARE: resolution goes through fleet-personas' resolver, so a
+ * user-customized persona in the PRESERVE-protected `fleet/roles.local/` layer
+ * WINS over the baseline `fleet/roles/` of the same class. That is the
+ * launch-time proof of AC-NS-7 — a customized persona actually reaches the model
+ * when the agent boots, not just in `mosaic fleet persona show`.
+ *
+ * Tolerant by contract (mirrors readFleetCommsBlock): an empty/missing class, an
+ * unknown class, or a missing role file all yield '' so the launcher no-ops
+ * silently. This MUST never throw during launch.
+ *
+ * Standalone module (no fleet.ts import) to keep launch.ts's prompt path free of
+ * the heavy fleet command module; it depends only on the lightweight persona
+ * resolver.
+ */
+
+import {
+  resolvePersonaSync,
+  defaultRolesDir,
+  defaultOverrideDir,
+} from '../commands/fleet-personas.js';
+
+/**
+ * Resolve `klass`'s persona contract (override-aware) and render it as a
+ * clearly-delimited launch block. Returns '' on any miss (falsy class, unknown
+ * class, missing/unreadable file) so composeContract can push it unconditionally
+ * and have it no-op silently. Never throws.
+ */
+export function readPersonaContractBlock(mosaicHome: string, klass: string | undefined): string {
+  if (!klass || !klass.trim()) return '';
+  let resolved: ReturnType<typeof resolvePersonaSync>;
+  try {
+    resolved = resolvePersonaSync(klass.trim(), {
+      rolesDir: defaultRolesDir(mosaicHome),
+      overrideDir: defaultOverrideDir(mosaicHome),
+    });
+  } catch {
+    // Best-effort onboarding: a resolver hiccup must not abort the launch.
+    return '';
+  }
+  if (!resolved) return '';
+
+  const layerNote =
+    resolved.layer === 'override'
+      ? '_(resolved from the `fleet/roles.local/` override layer — wins over baseline)_'
+      : '_(resolved from the baseline `fleet/roles/` layer)_';
+
+  return `# Persona Contract (${resolved.klass})
+
+${layerNote}
+
+You are operating as the **${resolved.klass}** persona. The role contract below is your identity — its mandate and boundaries govern what you own and what you must not do for this assignment.
+
+${resolved.content.trim()}`;
+}