⚡ perf: Append Saved Message Ids Instead of Rebuilding the Conversation Array (#15141)

* ⚡ perf: Append Saved Message Ids Instead of Rebuilding the Conversation Array

Every saveConvo read every message id in the conversation (sorted) and
wrote the array back onto the document — twice per chat turn, O(n) in
conversation length, from a write path. The turn's savers know exactly
which message they just wrote, so they now pass it as
metadata.appendMessageIds and saveConvo $addToSet-s it, skipping the
read and the full-array rewrite. Every save without the option — titles,
archive, fork, import, threads — still rebuilds from the database, which
remains the heal point for the drift that message deletion has always
left behind (deletes never ran saveConvo).

The array's consumers read presence or length, or use it as an
optimistic cache placeholder, so incremental maintenance is
behaviorally identical; on traced turns the array stays exactly equal
to the messages collection.

Per-turn queries: 15 -> 13 (two Message.find gone), and the growing
array payload no longer crosses the wire twice per turn.

* 🎯 fix: Brand the Lineage-Only Resolved Conversation Instead of Guessing by Shape

The resolved-conversation files fast path treated an absent files
property as unresolved so the lineage-only partial from a bound
agent-event continuation could not silently hide a conversation's
uploads. But MongoDB never stores an empty files array, so nearly every
real conversation also lacks the property and the fast path never fired
— a follow-up turn on an upload-free conversation still paid the
getConvoFiles round trip.

The synthesized partial is the one object that cannot speak for the
database, so it now carries an explicit symbol brand
(PARTIAL_RESOLVED_CONVERSATION, non-serializing and invisible to key
iteration), and a stored document without files means what it means:
no files. Traced follow-up turns drop from 14 queries to 13.

* 🧪 test: Expect the Appended Message Id in the Route's saveConvo Metadata

messages-get.spec.js pins the exact metadata POST /api/messages passes to
saveConvo; the route now forwards the saved message's _id as
appendMessageIds, which is the behavior the append path depends on.

api/app/clients/BaseClient.js

@@ -1315,6 +1315,7 @@ class BaseClient {
       unsetFields,
       noUpsert: req?._agentEventBindingParentConversationId != null,
       createdAtOnInsert: shouldSetCreatedAtOnInsert ? validCreatedAtOnInsert : undefined,
+      ...(savedMessage?._id != null ? { appendMessageIds: [savedMessage._id] } : {}),
     });
 
     return { message: savedMessage, conversation };

api/app/clients/specs/BaseClient.test.js

@@ -1871,6 +1871,36 @@ describe('BaseClient', () => {
       );
     });
 
+    test('saveMessageToDatabase appends the saved message id instead of rebuilding the array', async () => {
+      const savedId = new (require('mongoose').Types.ObjectId)();
+      saveMessage.mockResolvedValueOnce({ _id: savedId, messageId: 'saved-1' });
+      saveConvo.mockResolvedValueOnce({ conversationId });
+
+      await TestClient.saveMessageToDatabase(
+        { messageId: 'saved-1', conversationId, text: 'hi' },
+        TestClient.getSaveOptions(),
+      );
+
+      expect(saveConvo).toHaveBeenCalledWith(
+        expect.anything(),
+        expect.anything(),
+        expect.objectContaining({ appendMessageIds: [savedId] }),
+      );
+    });
+
+    test('saveMessageToDatabase rebuilds the array when the saved message has no _id', async () => {
+      saveMessage.mockResolvedValueOnce({ messageId: 'saved-2' });
+      saveConvo.mockResolvedValueOnce({ conversationId });
+
+      await TestClient.saveMessageToDatabase(
+        { messageId: 'saved-2', conversationId, text: 'hi' },
+        TestClient.getSaveOptions(),
+      );
+
+      const metadata = saveConvo.mock.calls[saveConvo.mock.calls.length - 1][2];
+      expect(metadata).not.toHaveProperty('appendMessageIds');
+    });
+
     test('saveMessageToDatabase returns early when this.options is null (client disposed)', async () => {
       const savedOptions = TestClient.options;
       TestClient.options = null;

api/server/routes/__tests__/messages-get.spec.js

@@ -443,7 +443,10 @@ describe('message route conversation ownership filters', () => {
         model: savedMessage.model,
         iconURL: savedMessage.iconURL,
       },
-      { context: 'POST /api/messages/:conversationId' },
+      {
+        context: 'POST /api/messages/:conversationId',
+        appendMessageIds: [savedMessage._id],
+      },
     );
   });
 

api/server/routes/messages.js

@@ -504,6 +504,7 @@ router.post('/:conversationId', storedMessageMutationMiddleware, async (req, res
     };
     await db.saveConvo(reqCtx, conversationUpdate, {
       context: 'POST /api/messages/:conversationId',
+      ...(savedMessage._id != null ? { appendMessageIds: [savedMessage._id] } : {}),
     });
     res.status(201).json(savedMessage);
   } catch (error) {