Fix spurious session restarts on runtime-mode-set and remove dead fallback chain in plan implementation

cursoragent · cursoragent · commit 14070d6c4586 · 2026-03-24T19:07:10.000Z
- Remove fallback to toProviderModelOptions(thread.modelSelection) in the
  thread.runtime-mode-set handler. Before any turn was sent, the fallback would
  produce defined modelOptions while the cache had none, causing
  sameModelOptions(undefined, derivedOptions) to return false and triggering an
  unnecessary session restart for Claude threads.

- Simplify nextThreadModelSelection in ChatView to use selectedModelSelection
  directly, removing unreachable ?? fallbacks (selectedModelSelection is always
  a non-null ModelSelection from useMemo).
diff --git a/apps/server/src/orchestration/Layers/ProviderCommandReactor.ts b/apps/server/src/orchestration/Layers/ProviderCommandReactor.ts
@@ -705,9 +705,7 @@ const make = Effect.gen(function* () {
             return;
           }
           const cachedProviderOptions = threadProviderOptions.get(event.payload.threadId);
-          const cachedModelOptions =
-            threadModelOptions.get(event.payload.threadId) ??
-            toProviderModelOptions(thread.modelSelection);
+          const cachedModelOptions = threadModelOptions.get(event.payload.threadId);
           yield* ensureSessionForThread(event.payload.threadId, event.occurredAt, {
             ...(cachedProviderOptions !== undefined
               ? { providerOptions: cachedProviderOptions }
diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
@@ -3035,12 +3035,7 @@ export default function ChatView({ threadId }: ChatViewProps) {
       text: implementationPrompt,
     });
     const nextThreadTitle = truncateTitle(buildPlanImplementationThreadTitle(planMarkdown));
-    const nextThreadModelSelection: ModelSelection = selectedModelSelection ??
-      activeThread.modelSelection ??
-      activeProject.defaultModelSelection ?? {
-        provider: "codex",
-        model: DEFAULT_MODEL_BY_PROVIDER.codex,
-      };
+    const nextThreadModelSelection: ModelSelection = selectedModelSelection;
 
     sendInFlightRef.current = true;
     beginSendPhase("sending-turn");
