a chat can change model mid-conversation

the selector locked on `hasStarted`, so the tier was fixed for the life of a conversation. nothing
justified it — the commit that added the flag says only "default model" — and the hazard it resembles is
narrower than what it blocked: crossing HARNESS hands an opencode `ses_…` id to `claude --resume`, which
reads as a dead agent rather than a wrong dispatch. changing TIER was always safe.

so tier is free, harness is pinned, and the server refuses a crossing rather than relying on the UI to.

the claude side does it the way the CLI's /model does: `Query.setModel()`, a control request that needs
streaming input mode, which is the only mode this file uses. nothing is torn down and nothing is
re-resumed — cwd, permission mode and transcript all continue and only the next response changes. an
earlier plan here was to kill the session and re-create it with the stored resume id; that would have
worked and was unnecessary.

opencode already switched per turn (serve-runner POSTs the model to the live session), so only the UI was
in its way.

FIXES A SILENT MISMATCH while it is here. model reached the SDK once, in createSession's query() options,
and every later turn was pushed onto that same call — so a changed model was dropped while session:init,
the cost fold and the durable log all reported the new tier. reachable today by leaving a chat idle past
the 30-minute GC: the UI said sonnet, opus answered, nothing logged it. the fold now tracks what the
harness actually did rather than what the client asked for, and a refused switch reverts the picker.

the switch is a durable `model:switched` event and a centred marker in the transcript, because the useful
question is the BOUNDARY — which half of this conversation is opus. per-message model was already stamped
server-side and shown nowhere, and claude-sessions.ts reads the FIRST model in a transcript, so a switch
was invisible in history.

tests: turn-stream proves the marker is durable and lands after the previous model's last words; the
selector tests pull in opposite directions, so unlocking everything fails one and restoring the blanket
lock fails the other. verified by re-breaking both.

not done: opencode's `steer` path (a send mid-turn) returns before the model POST, so a switch there
applies on the next full turn. session.model also stays stale for opencode tier changes — harness routing
is unaffected.

src/servers/sidecar/claude/claude-manager.ts

@@ -220,6 +220,16 @@ type PersistentSession = {
   interrupted: boolean;
   /** What we passed as `--resume`, so a failure can name it and decide whether it was to blame. */
   resumeId?: string;
+  /**
+   * The model this session is currently answering as, `harness/tier`.
+   *
+   * Recorded because `model` reaches the SDK exactly once, in `createSession`'s `query()` options, while
+   * every later turn is pushed onto that same call — so without this there is nothing to compare a turn's
+   * requested model against, and a change was silently dropped. Officer's UI, its cost folding and its
+   * durable log all took the client's word for it, so a switch produced a session that reported one tier
+   * and answered as another.
+   */
+  model?: string;
   /** The tail of the child's stderr, so its exit can carry a reason instead of just a number. */
   stderrTail: string;
   /** Whether the child ever produced a single message. False plus a non-zero exit means it never started. */
@@ -468,6 +478,7 @@ function createSession(params: ClaudeSpawnStreamingParams, onEvent: (event: Chat
   const resumeId = getClaudeSession(sessionKey, params.userId) ?? params.resumeSessionId;
   const subModel = params.model?.split('/')[1];
   session.resumeId = resumeId;
+  session.model = params.model;
 
   /**
    * What we are about to run, before we run it.
@@ -662,12 +673,48 @@ export async function spawnClaudeStreaming(
   }
   if (session) {
     session.emit = onEvent; // adopt the latest emitter (equivalent across turns; keeps events flowing)
+    await applyModelChange(session, params.model);
   } else {
     session = createSession(params, onEvent);
   }
   session.pushTurn(params.prompt, params.images);
 }
 
+/**
+ * Move a LIVE session onto a different model, the way the CLI's `/model` does.
+ *
+ * `model` is a `query()` option and options are read once, so before this a changed model was dropped on
+ * the floor — the session kept answering as whatever it started as while Officer's UI, its cost folding and
+ * its durable log all reported the new tier. The mismatch was invisible: no log line, no error, and the one
+ * way to trigger it was to leave a chat idle past the 30-minute GC.
+ *
+ * `setModel` is a control request, so it needs streaming input mode — which is the only mode this file uses.
+ * That is why nothing has to be torn down and re-resumed: the conversation, its cwd, its permission mode and
+ * its transcript all continue, and only the next response comes from a different model.
+ *
+ * A refusal is reported and swallowed. The turn the user just typed is worth more than the tier they picked,
+ * so a `setModel` the harness rejects must not lose the prompt — it answers as the old model and says so.
+ */
+async function applyModelChange(session: PersistentSession, model: string | undefined): Promise<void> {
+  if (!model || model === session.model) return;
+
+  const previous = session.model;
+  const subModel = model.split('/')[1];
+  if (!subModel) return;
+
+  try {
+    await session.query.setModel(subModel);
+    session.model = model;
+    console.log(`[claude:model:${session.sessionKey}] ${previous ?? 'default'} -> ${model}`);
+    session.emit({ type: 'model:switched', from: previous ?? null, to: model, ok: true });
+  } catch (err) {
+    // Leave `session.model` alone, so the next turn tries again rather than believing a switch that failed.
+    const reason = err instanceof Error ? err.message : String(err);
+    console.error(`[claude:model:${session.sessionKey}] refused ${previous ?? 'default'} -> ${model}: ${reason}`);
+    session.emit({ type: 'model:switched', from: previous ?? null, to: model, ok: false, error: reason });
+  }
+}
+
 /** Interrupt the current turn but KEEP the session alive (the "stop" button). */
 export async function interruptClaudeSession(sessionKey: string, userId: number): Promise<boolean> {
   const session = ownedSession(sessionKey, userId);