refactor(app): unify assistant fork boundary resolution (#2091)

packages/app/src/agent-stream/turn-boundary.test.ts

@@ -1,9 +1,6 @@
 import { describe, expect, it } from "vitest";
 import type { StreamItem } from "@/types/stream";
-import {
-  resolveAssistantTurnBoundaryMessageId,
-  resolveAssistantTurnForkBoundary,
-} from "./turn-boundary";
+import { resolveAssistantTurnForkBoundary } from "./turn-boundary";
 
 function timestamp(seed: number): Date {
   return new Date(`2026-01-01T00:00:${seed.toString().padStart(2, "0")}.000Z`);
@@ -32,40 +29,6 @@ function assistantMessage(
   };
 }
 
-describe("resolveAssistantTurnBoundaryMessageId", () => {
-  it("uses the selected assistant message id", () => {
-    const selected = assistantMessage("assistant-1", 2, "msg-assistant-1");
-
-    expect(
-      resolveAssistantTurnBoundaryMessageId({
-        items: [userMessage("user-1", 1), selected],
-        startIndex: 1,
-      }),
-    ).toBe("msg-assistant-1");
-  });
-
-  it("does not borrow a boundary id from another assistant in the same turn", () => {
-    const first = assistantMessage("assistant-1", 2, "msg-assistant-1");
-    const selected = assistantMessage("assistant-2", 3);
-
-    expect(
-      resolveAssistantTurnBoundaryMessageId({
-        items: [userMessage("user-1", 1), first, selected],
-        startIndex: 2,
-      }),
-    ).toBeUndefined();
-  });
-
-  it("requires the selected item to be an assistant message", () => {
-    expect(
-      resolveAssistantTurnBoundaryMessageId({
-        items: [userMessage("user-1", 1), assistantMessage("assistant-1", 2, "msg-assistant-1")],
-        startIndex: 0,
-      }),
-    ).toBeUndefined();
-  });
-});
-
 describe("resolveAssistantTurnForkBoundary", () => {
   it("forks a failed assistant turn from its Paseo timeline cursor without a provider message id", () => {
     const failedTurn = {
@@ -117,6 +80,29 @@ describe("resolveAssistantTurnForkBoundary", () => {
     ).toEqual({ boundaryMessageId: "msg-assistant-1" });
   });
 
+  it("does not borrow a provider message id from another assistant in the same turn", () => {
+    const first = assistantMessage("assistant-1", 2, "msg-assistant-1");
+    const selected = assistantMessage("assistant-2", 3);
+
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [userMessage("user-1", 1), first, selected],
+        startIndex: 2,
+        supportsTimelineCursor: false,
+      }),
+    ).toBeUndefined();
+  });
+
+  it("requires the selected item to be an assistant message", () => {
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [userMessage("user-1", 1), assistantMessage("assistant-1", 2, "msg-assistant-1")],
+        startIndex: 0,
+        supportsTimelineCursor: false,
+      }),
+    ).toBeUndefined();
+  });
+
   it("does not offer an unavailable boundary", () => {
     expect(
       resolveAssistantTurnForkBoundary({

packages/app/src/agent-stream/turn-boundary.ts

@@ -4,18 +4,6 @@ export type AssistantTurnForkBoundary =
   | { boundaryCursor: TimelinePosition; boundaryMessageId?: string }
   | { boundaryCursor?: undefined; boundaryMessageId: string };
 
-export function resolveAssistantTurnBoundaryMessageId(input: {
-  items: readonly StreamItem[];
-  startIndex: number;
-}): string | undefined {
-  const item = input.items[input.startIndex];
-  if (item?.kind !== "assistant_message") {
-    return undefined;
-  }
-  // Forking without the selected assistant's durable message id would send the wrong slice.
-  return item.messageId || undefined;
-}
-
 export function resolveAssistantTurnForkBoundary(input: {
   items: readonly StreamItem[];
   startIndex: number;