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.