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/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 {