fix app stream head replacement during init hydration (#663)

packages/app/src/contexts/session-stream-reducers.test.ts

@@ -910,6 +910,67 @@ describe("processAgentStreamEvents", () => {
       lastActivityAt: new Date(3000),
     });
   });
+
+  it("keeps a live Claude assistant paragraph contiguous when init tail hydration lands mid-stream", () => {
+    const seq186Text = "Now let me write the updated tool-call-detail-parser.ts with all the sub";
+    const seq187Text = "agent additions.";
+
+    const liveSeq186 = processAgentStreamEvent({
+      ...baseStreamInput,
+      event: makeTimelineEvent(seq186Text),
+      seq: 186,
+      epoch: "epoch-1",
+      currentTail: [],
+      currentHead: [],
+      currentCursor: undefined,
+      timestamp: new Date("2026-05-02T10:00:00.186Z"),
+    });
+
+    expect(getAssistantTexts(liveSeq186.head)).toEqual([seq186Text]);
+    expect(getAssistantTexts(liveSeq186.tail)).toEqual([]);
+
+    const initTailHydration = processTimelineResponse({
+      ...baseTimelineInput,
+      currentTail: liveSeq186.tail,
+      currentHead: liveSeq186.head,
+      currentCursor: liveSeq186.cursor ?? undefined,
+      isInitializing: true,
+      hasActiveInitDeferred: true,
+      initRequestDirection: "tail",
+      payload: {
+        agentId: "agent-1",
+        direction: "tail",
+        reset: false,
+        epoch: "epoch-1",
+        startCursor: { seq: 186 },
+        endCursor: { seq: 186 },
+        entries: [makeTimelineEntry(186, seq186Text)],
+        error: null,
+      },
+    });
+
+    expect(getAssistantTexts(initTailHydration.tail)).toEqual([]);
+    expect(getAssistantTexts(initTailHydration.head)).toEqual([seq186Text]);
+
+    const liveSeq187 = processAgentStreamEvent({
+      ...baseStreamInput,
+      event: makeTimelineEvent(seq187Text),
+      seq: 187,
+      epoch: "epoch-1",
+      currentTail: initTailHydration.tail,
+      currentHead: initTailHydration.head,
+      currentCursor: initTailHydration.cursor ?? undefined,
+      timestamp: new Date("2026-05-02T10:00:00.187Z"),
+    });
+
+    const finalAssistantItems = [...liveSeq187.tail, ...liveSeq187.head].filter(
+      (item): item is Extract<StreamItem, { kind: "assistant_message" }> =>
+        item.kind === "assistant_message",
+    );
+
+    expect(finalAssistantItems).toHaveLength(1);
+    expect(finalAssistantItems[0]?.text).toBe(`${seq186Text}${seq187Text}`);
+  });
 });
 
 describe("createAgentStreamReducerQueue", () => {

packages/app/src/contexts/session-stream-reducers.ts

@@ -99,16 +99,49 @@ interface TimelinePathResult {
   sideEffects: TimelineReducerSideEffect[];
 }
 
+function preserveReplacePathAssistantHead(params: {
+  tail: StreamItem[];
+  currentHead: StreamItem[];
+}): {
+  tail: StreamItem[];
+  head: StreamItem[];
+} {
+  const { tail, currentHead } = params;
+  const liveAssistant = currentHead.findLast(
+    (item): item is Extract<StreamItem, { kind: "assistant_message" }> =>
+      item.kind === "assistant_message",
+  );
+  if (!liveAssistant) {
+    return { tail, head: [] };
+  }
+  const tailAssistant = tail.at(-1);
+  if (!tailAssistant || tailAssistant.kind !== "assistant_message") {
+    return { tail, head: currentHead };
+  }
+  if (!liveAssistant.text.startsWith(tailAssistant.text)) {
+    return { tail, head: [] };
+  }
+  return {
+    tail: tail.slice(0, -1),
+    head: [{ ...liveAssistant, text: tailAssistant.text }],
+  };
+}
+
 function applyTimelineReplacePath(args: {
   timelineUnits: TimelineUnit[];
   payload: ProcessTimelineResponseInput["payload"];
   bootstrapPolicy: ReturnType<typeof deriveBootstrapTailTimelinePolicy>;
+  currentHead: StreamItem[];
   toHydratedEvents: (
     units: TimelineUnit[],
   ) => Array<{ event: AgentStreamEventPayload; timestamp: Date }>;
 }): TimelinePathResult {
-  const { timelineUnits, payload, bootstrapPolicy, toHydratedEvents } = args;
+  const { timelineUnits, payload, bootstrapPolicy, currentHead, toHydratedEvents } = args;
-  const tail = hydrateStreamState(toHydratedEvents(timelineUnits), { source: "canonical" });
+  const hydratedTail = hydrateStreamState(toHydratedEvents(timelineUnits), { source: "canonical" });
+  const { tail, head } = preserveReplacePathAssistantHead({
+    tail: hydratedTail,
+    currentHead,
+  });
   const cursor: TimelineCursor | null =
     payload.startCursor && payload.endCursor
       ? {
@@ -121,7 +154,7 @@ function applyTimelineReplacePath(args: {
   if (bootstrapPolicy.catchUpCursor) {
     sideEffects.push({ type: "catch_up", cursor: bootstrapPolicy.catchUpCursor });
   }
-  return { tail, head: [], cursor, cursorChanged: true, sideEffects };
+  return { tail, head, cursor, cursorChanged: true, sideEffects };
 }
 
 interface IncrementalAcceptResult {
@@ -307,6 +340,7 @@ export function processTimelineResponse(
         timelineUnits,
         payload,
         bootstrapPolicy,
+        currentHead,
         toHydratedEvents,
       })
     : applyTimelineIncrementalPath({