fix(codex): stream final answer partials (#88730)

extensions/codex/src/app-server/event-projector.test.ts

@@ -316,6 +316,29 @@ describe("CodexAppServerEventProjector", () => {
     expect(result.replayMetadata.replaySafe).toBe(true);
   });
 
+  it("streams final-answer assistant deltas into partial replies", async () => {
+    const { onPartialReply, projector } = await createProjectorWithAssistantHooks();
+
+    await projector.handleNotification(
+      forCurrentTurn("item/started", {
+        item: {
+          type: "agentMessage",
+          id: "msg-final",
+          phase: "final_answer",
+          text: "",
+        },
+      }),
+    );
+    await projector.handleNotification(agentMessageDelta("hel", "msg-final"));
+    await projector.handleNotification(agentMessageDelta("lo", "msg-final"));
+
+    expect(onPartialReply).toHaveBeenCalledTimes(2);
+    expect(onPartialReply.mock.calls.map((call) => call[0])).toEqual([
+      { text: "hel", delta: "hel" },
+      { text: "hello", delta: "lo" },
+    ]);
+  });
+
   it("suppresses mirrored user prompt when the inbound message was already persisted", async () => {
     const params = await createParams();
     const projector = await createProjector({

extensions/codex/src/app-server/event-projector.ts

@@ -447,6 +447,11 @@ export class CodexAppServerEventProjector {
     if (!delta) {
       return;
     }
+    this.rememberAssistantPhase(readItem(params.item));
+    const phase = readString(params, "phase");
+    if (phase) {
+      this.assistantPhaseByItem.set(itemId, phase);
+    }
     if (!this.assistantStarted) {
       this.assistantStarted = true;
       await this.params.onAssistantMessageStart?.();
@@ -456,10 +461,13 @@ export class CodexAppServerEventProjector {
     this.assistantTextByItem.set(itemId, text);
     if (this.isCommentaryAssistantItem(itemId)) {
       this.emitCommentaryProgress({ itemId, text });
+    } else if (this.shouldStreamAssistantPartial(itemId)) {
+      await this.params.onPartialReply?.({ text, delta });
     }
     // Codex app-server can emit multiple agentMessage items per turn, including
     // intermediate coordination/progress prose. Keep those deltas internal until
-    // turn completion chooses the last assistant item as the user-visible reply.
+    // their phase identifies terminal answer text or turn completion chooses the
+    // last assistant item as the user-visible reply.
   }
 
   private async handleReasoningDelta(
@@ -970,6 +978,10 @@ export class CodexAppServerEventProjector {
     return this.assistantPhaseByItem.get(itemId) === "commentary";
   }
 
+  private shouldStreamAssistantPartial(itemId: string): boolean {
+    return this.assistantPhaseByItem.get(itemId) === "final_answer";
+  }
+
   private emitCommentaryProgress(params: { itemId: string; text: string }): void {
     const progressText = params.text.replace(/\s+/g, " ").trim();
     if (