fix(chat): model selection + multi-model follow-up correctness (onyx-dot-app#10075)

Author: nmgarza5

web/src/app/app/message/MultiModelResponseView.tsx

@@ -210,8 +210,10 @@ export default function MultiModelResponseView({
       const response = responses.find((r) => r.modelIndex === modelIndex);
       if (!response) return;
 
-      // Persist preferred response to backend + update local tree so the
-      // input bar unblocks (awaitingPreferredSelection clears).
+      // Persist preferred response + sync `latestChildNodeId`. Backend's
+      // `set_preferred_response` updates `latest_child_message_id`; if the
+      // frontend chain walk disagrees, the next follow-up fails with
+      // "not on the latest mainline".
       if (parentMessage?.messageId && response.messageId && currentSessionId) {
         setPreferredResponse(parentMessage.messageId, response.messageId).catch(
           (err) => console.error("Failed to persist preferred response:", err)
@@ -227,6 +229,7 @@ export default function MultiModelResponseView({
             updated.set(parentMessage.nodeId, {
               ...userMsg,
               preferredResponseId: response.messageId,
+              latestChildNodeId: response.nodeId,
             });
             updateSessionMessageTree(currentSessionId, updated);
           }

web/src/lib/hooks.ts

@@ -694,6 +694,25 @@ export function useLlmManager(
     prevAgentIdRef.current = liveAgent?.id;
   }, [liveAgent?.id]);
 
+  // Clear manual override when arriving at a *different* existing session
+  // from any previously-seen defined session. Tracks only the last
+  // *defined* session id so a round-trip through new-chat (A → undefined
+  // → B) still resets, while A → undefined (new-chat) preserves it.
+  const prevDefinedSessionIdRef = useRef<string | undefined>(undefined);
+  useEffect(() => {
+    const nextId = currentChatSession?.id;
+    if (
+      nextId !== undefined &&
+      prevDefinedSessionIdRef.current !== undefined &&
+      nextId !== prevDefinedSessionIdRef.current
+    ) {
+      setUserHasManuallyOverriddenLLM(false);
+    }
+    if (nextId !== undefined) {
+      prevDefinedSessionIdRef.current = nextId;
+    }
+  }, [currentChatSession?.id]);
+
   function getValidLlmDescriptor(
     modelName: string | null | undefined
   ): LlmDescriptor {
@@ -715,8 +734,9 @@ export function useLlmManager(
 
     if (llmProviders === undefined || llmProviders === null) {
       resolved = manualLlm;
-    } else if (userHasManuallyOverriddenLLM && !currentChatSession) {
-      // User has overridden in this session and switched to a new session
+    } else if (userHasManuallyOverriddenLLM) {
+      // Manual override wins over session's `current_alternate_model`.
+      // Cleared on cross-session navigation by the effect above.
       resolved = manualLlm;
     } else if (currentChatSession?.current_alternate_model) {
       resolved = getValidLlmDescriptorForProviders(
@@ -728,8 +748,6 @@ export function useLlmManager(
         liveAgent.llm_model_version_override,
         llmProviders
       );
-    } else if (userHasManuallyOverriddenLLM) {
-      resolved = manualLlm;
     } else if (user?.preferences?.default_model) {
       resolved = getValidLlmDescriptorForProviders(
         user.preferences.default_model,

web/src/refresh-components/popovers/ModelSelector.tsx

@@ -159,9 +159,12 @@ export default function ModelSelector({
                 );
 
                 if (!isMultiModel) {
+                  // Stable key — keying on model would unmount the pill
+                  // on change and leave Radix's anchorRef detached,
+                  // flashing the closing popover at (0,0).
                   return (
                     <OpenButton
-                      key={modelKey(model.provider, model.modelName)}
+                      key="single-model-pill"
                       icon={ProviderIcon}
                       onClick={(e: React.MouseEvent) =>
                         handlePillClick(index, e.currentTarget as HTMLElement)

web/src/refresh-pages/AppPage.tsx

@@ -425,16 +425,27 @@ export default function AppPage({ firstMessage }: ChatPageProps) {
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [multiModel.isMultiModelActive]);
 
-  // Sync single-model selection to llmManager so the submission path
-  // uses the correct provider/version (replaces the old LLMPopover sync).
+  // Sync single-model selection to llmManager so the submission path uses
+  // the correct provider/version. Guard against echoing derived state back
+  // — only call updateCurrentLlm when the selection actually differs from
+  // currentLlm, otherwise the initial [] → [currentLlmModel] sync would
+  // pin `userHasManuallyOverriddenLLM=true` with whatever was resolved
+  // first (often the default model before the session's alt_model loads).
   useEffect(() => {
     if (multiModel.selectedModels.length === 1) {
       const model = multiModel.selectedModels[0]!;
-      llmManager.updateCurrentLlm({
-        name: model.name,
-        provider: model.provider,
-        modelName: model.modelName,
-      });
+      const current = llmManager.currentLlm;
+      if (
+        model.provider !== current.provider ||
+        model.modelName !== current.modelName ||
+        model.name !== current.name
+      ) {
+        llmManager.updateCurrentLlm({
+          name: model.name,
+          provider: model.provider,
+          modelName: model.modelName,
+        });
+      }
     }
   }, [multiModel.selectedModels]);