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/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