Allow mid-session model switching with a persisted transcript marker

Picker stays live for the session; switches persist a model_switch notice (§17 revised).
Rebinds refused mid-turn; images become placeholders for non-vision targets at send time.

surfaces/gui/e2e/chat.spec.ts

@@ -21,9 +21,9 @@ test("send → user bubble → streamed echo reply renders", async ({ page }) =>
   // The message carried the composer's visible model (model-per-message contract): what the
   // user sees at send time is exactly what serves the turn.
   await expect(page.getByText("[model=anthropic:claude-opus-4-8]")).toBeVisible();
-  // …and having sent, the model is now FIXED for this session (§17/§22): the composer picker is
-  // gone and the fact reads in the topbar's facts subtitle instead.
-  await expect(page.locator(".dd").filter({ hasText: "Claude Opus" })).toHaveCount(0);
+  // …and the picker STAYS actionable after the first turn (§17 rev 2026-07-22 — mid-session
+  // switching shipped); the fact also reads in the topbar's facts subtitle.
+  await expect(page.locator(".dd").filter({ hasText: "Claude Opus" })).toBeVisible();
   await expect(page.getByTestId("session-subtitle")).toContainText("Claude Opus 4.8");
   // Composer cleared and re-armed for the next turn.
   await expect(box).toHaveValue("");

surfaces/gui/e2e/fixtures.ts

@@ -568,9 +568,11 @@ export async function mockApi(page: import("@playwright/test").Page) {
     send("ready");
     let pendingTool = "run_shell"; // which proposal the next approval decision resolves
     let epicTimer: ReturnType<typeof setInterval> | null = null; // the slow stream, stoppable via interrupt
+    let hadTurn = false; // a user_message landed — set_model is now a mid-session switch
     ws.onMessage((raw) => {
       const msg = JSON.parse(String(raw));
       if (msg.type === "user_message") {
+        hadTurn = true;
         send("turn_start", { input: msg.text });
         if (/run a tool/i.test(msg.text)) {
           pendingTool = "run_shell";
@@ -703,6 +705,14 @@ export async function mockApi(page: import("@playwright/test").Page) {
         }
         send("interrupted", {});
         send("turn_done");
+      } else if (msg.type === "set_model") {
+        // Mid-session switch: the server applies it and broadcasts the persisted marker.
+        // Like the real server, the FIRST bind (fresh session) is silent.
+        if (hadTurn)
+          send("model_changed", {
+            model: msg.model,
+            text: `Model switched to ${msg.model}`,
+          });
       } else if (msg.type === "retry") {
         // Like the real engine: re-runs with NO new user message (turn_start input is empty).
         send("turn_start", { input: "" });

surfaces/gui/e2e/model-switch.spec.ts

@@ -0,0 +1,33 @@
+// Model-layer roadmap item 3 (2026-07-22): the model picker stays actionable for the
+// session's whole life (supersedes the 2026-07-04 lock that hid it after the first turn).
+// A mid-session switch drops a persisted info marker into the transcript, and later
+// messages ride the new model.
+import { expect } from "@playwright/test";
+import { test } from "./fixtures";
+
+test("mid-session model switch shows the marker and later turns use the new model", async ({
+  page,
+}) => {
+  await page.goto("/");
+  await page.getByText("Draft the launch note").first().click();
+  const box = page.getByPlaceholder(/Ask the coworker/);
+  await box.fill("hello there");
+  await box.press("Enter");
+  await expect(page.getByText("Echo: hello there", { exact: false }).first()).toBeVisible();
+
+  // The picker is still in the composer after the first turn (the old lock hid it).
+  const picker = page.locator(".dd").filter({ hasText: "Claude Opus 4.8" });
+  await expect(picker).toBeVisible();
+  await picker.locator(".pill").click();
+  await page.locator(".dd-item").filter({ hasText: "GPT-5.5" }).click();
+
+  // The switch marker lands in the transcript…
+  await expect(page.getByText(/Model switched to gpt-5.5/).first()).toBeVisible();
+
+  // …and the next message carries the new model (the fixture echoes it back).
+  await box.fill("after the switch");
+  await box.press("Enter");
+  await expect(
+    page.getByText("Echo: after the switch [model=gpt-5.5]", { exact: false }).first(),
+  ).toBeVisible();
+});

surfaces/gui/e2e/session-shell.spec.ts

@@ -44,7 +44,8 @@ test("facts subtitle: absent on a fresh session, persona · model after the firs
   await expect(page.getByRole("button", { name: "About this persona" })).toHaveCount(0);
   await expect(page.locator(".dd").filter({ hasText: "Claude Opus 4.8" })).toBeVisible();
 
-  // First turn → the model chip leaves the composer; the facts move up to the subtitle.
+  // First turn → the facts move up to the subtitle; the picker STAYS in the composer
+  // (§17 rev 2026-07-22: mid-session model switching shipped, so it remains actionable).
   const box = page.getByPlaceholder(/Ask the coworker/);
   await box.fill("hello");
   await page.getByRole("button", { name: "Send" }).click();
@@ -52,7 +53,7 @@ test("facts subtitle: absent on a fresh session, persona · model after the firs
 
   const sub = page.getByTestId("session-subtitle");
   await expect(sub).toContainText("Coworker · Claude Opus 4.8");
-  await expect(page.locator(".dd").filter({ hasText: "Claude Opus 4.8" })).toHaveCount(0);
+  await expect(page.locator(".dd").filter({ hasText: "Claude Opus 4.8" })).toBeVisible();
 
   // The subtitle is the session's fixed facts — clicking it opens the coworker (persona) page,
   // replacing the old topbar sliders button.

surfaces/gui/src/App.tsx

@@ -638,6 +638,12 @@ export function App() {
           if (d.status === "max_iterations_exceeded")
             setItems((p) => [...p, { kind: "notice", tone: "warn", text: "Stopped: max iterations reached." }]);
           break;
+        case "model_changed":
+          // Mid-session switch (server-applied): update the header fact and drop the
+          // persisted marker into the live transcript (replay renders it from history).
+          if (d.model) setModel(d.model);
+          setItems((p) => [...p, { kind: "notice", tone: "info", text: d.text || "Model switched" }]);
+          break;
         case "interrupted":
           flushPartialStream();
           setItems((p) => [...p, { kind: "notice", tone: "warn", text: "Interrupted." }]);
@@ -812,6 +818,7 @@ export function App() {
     sessionRef.current?.setMode(m);
   };
   const changeModel = (m: string) => {
+    if (running) return; // the server refuses mid-turn rebinds — don't let the header lie
     setModel(m);
     sessionRef.current?.setModel(m);
   };
@@ -1451,7 +1458,6 @@ export function App() {
               model={model}
               models={models}
               modelLabels={modelLabels}
-              modelLocked={items.length > 0}
               running={running}
               connected={connected}
               modelReady={modelReady}

surfaces/gui/src/components/Composer.tsx

@@ -49,7 +49,6 @@ interface Props {
   // The model is FIXED once the session has history (§17): the picker renders ONLY on a fresh
   // session; after the first turn the fact lives in the topbar subtitle (§22) — no
   // interactive-then-disabled control.
-  modelLocked?: boolean;
   running: boolean;
   connected: boolean;
   // False when the default model's provider has no key — the composer shows a "connect a model"
@@ -461,8 +460,9 @@ export function Composer(props: Props) {
 
           <span className="ml-auto" />
 
-          {/* model — a quiet chip on a FRESH session only; once the session has history the
-              fact moves up to the topbar subtitle (§17 expressed spatially). */}
+          {/* model — a quiet chip, now for the session's whole life (§17 rev 2026-07-22:
+              mid-session switching shipped, so the picker stays actionable; the topbar
+              subtitle still states the current model). */}
           {!dictation?.recording && (needsModel ? (
             <button
               className="pill model-warn chip"
@@ -473,20 +473,17 @@ export function Composer(props: Props) {
               <span className="pill-label">No model</span>
               <span className="model-warn-ico" aria-hidden>⚠</span>
             </button>
+          ) : modelsLoaded ? (
+            <Dropdown value={props.model} options={modelOptions} onChange={props.onModelChange} align="right" />
           ) : (
-            !props.modelLocked &&
-              (modelsLoaded ? (
-                <Dropdown value={props.model} options={modelOptions} onChange={props.onModelChange} align="right" />
-              ) : (
-                <button
-                  className="pill chip text-faint cursor-default"
-                  disabled
-                  data-testid="models-loading"
-                  title="Fetching the model list from the server"
-                >
-                  <span className="pill-label">Loading models…</span>
-                </button>
-              ))
+            <button
+              className="pill chip text-faint cursor-default"
+              disabled
+              data-testid="models-loading"
+              title="Fetching the model list from the server"
+            >
+              <span className="pill-label">Loading models…</span>
+            </button>
           ))}
 
           {/* mic — immediately before send (owner call, DMG #28 walkthrough) */}

surfaces/gui/src/itemsFromMessages.test.ts

@@ -68,3 +68,17 @@ describe("itemsFromMessages notices", () => {
     ]);
   });
 });
+
+describe("itemsFromMessages model switch", () => {
+  it("replays the persisted model_switch marker as an info notice", () => {
+    const items = itemsFromMessages([
+      { role: "user", content: "hi" },
+      { role: "notice", kind: "model_switch", text: "Model switched to Kimi K2.6 · Moonshot" },
+    ] as any);
+    expect(items[1]).toEqual({
+      kind: "notice",
+      tone: "info",
+      text: "Model switched to Kimi K2.6 · Moonshot",
+    });
+  });
+});

surfaces/gui/src/itemsFromMessages.ts

@@ -59,13 +59,15 @@ export function itemsFromMessages(messages: ConversationMessage[]): Item[] {
         });
       }
     } else if (m.role === "notice") {
-      // Persisted turn-ending marker (engine `_append_notice`): error/interrupted survive
+      // Persisted markers (engine `_append_notice`): error/interrupted/model-switch survive
       // reload exactly like the live view rendered them. An error notice is retriable —
       // the Transcript only offers the button when it's the transcript tail.
       items.push(
         m.kind === "interrupted"
           ? { kind: "notice", tone: "warn", text: "Interrupted." }
-          : { kind: "notice", tone: "warn", text: "Error: " + (m.text || "unknown"), retriable: true },
+          : m.kind === "model_switch"
+            ? { kind: "notice", tone: "info", text: m.text || "Model switched" }
+            : { kind: "notice", tone: "warn", text: "Error: " + (m.text || "unknown"), retriable: true },
       );
     }
     // system messages are omitted; tool-result messages are folded into the tool row above

surfaces/gui/src/types.ts

@@ -15,6 +15,7 @@ export type EventType =
   | "turn_end"
   | "error"
   | "interrupted"
+  | "model_changed"
   | "turn_done";
 
 export interface WsEvent {