fix: persist /model selection for default conversations (#1239)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/App.tsx

@@ -3269,8 +3269,12 @@ export default function App({
     const syncConversationModel = async () => {
       // "default" is a virtual sentinel for the agent's primary message history,
       // not a real conversation object — skip the API call.
+      // If the user just switched models via /model, honour the local override
+      // until the next agent state refresh brings back the updated model.
       if (conversationId === "default") {
-        applyAgentModelLocally();
+        if (!hasConversationModelOverrideRef.current) {
+          applyAgentModelLocally();
+        }
         return;
       }
 
@@ -11014,8 +11018,10 @@ ${SYSTEM_REMINDER_CLOSE}
             phase: "running",
           });
 
-          // "default" is a virtual sentinel, not a real conversation object —
-          // skip the API call and fall through with undefined model_settings.
+          // Persist model change to the backend.
+          // For real conversations, update the conversation-scoped override.
+          // For "default" (virtual sentinel with no real conversation object),
+          // update the agent itself so the model sticks across messages.
           let conversationModelSettings:
             | AgentState["model_settings"]
             | null
@@ -11034,6 +11040,14 @@ ${SYSTEM_REMINDER_CLOSE}
                 model_settings?: AgentState["model_settings"] | null;
               }
             ).model_settings;
+          } else {
+            const { updateAgentLLMConfig } = await import("../agent/modify");
+            const updatedAgent = await updateAgentLLMConfig(
+              agentId,
+              modelHandle,
+              model.updateArgs,
+            );
+            conversationModelSettings = updatedAgent.model_settings;
           }
 
           // The API may not echo reasoning_effort back, so populate it from

src/tests/agent/model-preset-refresh.wiring.test.ts

@@ -71,7 +71,7 @@ describe("model preset refresh wiring", () => {
     expect(updateSegment).not.toContain("client.agents.update(");
   });
 
-  test("/model handler updates conversation model (not agent model)", () => {
+  test("/model handler updates conversation model and falls back to agent for default", () => {
     const path = fileURLToPath(new URL("../../cli/App.tsx", import.meta.url));
     const source = readFileSync(path, "utf-8");
 
@@ -86,7 +86,10 @@ describe("model preset refresh wiring", () => {
 
     expect(segment).toContain("updateConversationLLMConfig(");
     expect(segment).toContain("conversationIdRef.current");
-    expect(segment).not.toContain("updateAgentLLMConfig(");
+    // For the "default" virtual conversation (no real conversation object),
+    // the handler falls back to updating the agent directly.
+    expect(segment).toContain("updateAgentLLMConfig(");
+    expect(segment).toContain('conversationIdRef.current !== "default"');
   });
 
   test("App defines helper to carry over active conversation model", () => {