fix(chat): tighten fork boundary handling

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

@@ -83,4 +83,47 @@ describe("resolveAssistantTurnForkBoundary", () => {
       boundaryCursor: { epoch: "timeline-1", seq: 42 },
     });
   });
+
+  it("includes the provider message id with a supported timeline cursor", () => {
+    const selected = {
+      ...assistantMessage("assistant-1", 2, "msg-assistant-1"),
+      timelineCursor: { epoch: "timeline-1", seq: 42 },
+    };
+
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [selected],
+        startIndex: 0,
+        supportsTimelineCursor: true,
+      }),
+    ).toEqual({
+      boundaryCursor: { epoch: "timeline-1", seq: 42 },
+      boundaryMessageId: "msg-assistant-1",
+    });
+  });
+
+  it("falls back to the provider message id when timeline cursors are unsupported", () => {
+    const selected = {
+      ...assistantMessage("assistant-1", 2, "msg-assistant-1"),
+      timelineCursor: { epoch: "timeline-1", seq: 42 },
+    };
+
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [selected],
+        startIndex: 0,
+        supportsTimelineCursor: false,
+      }),
+    ).toEqual({ boundaryMessageId: "msg-assistant-1" });
+  });
+
+  it("does not offer an unavailable boundary", () => {
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [assistantMessage("assistant-1", 2)],
+        startIndex: 0,
+        supportsTimelineCursor: false,
+      }),
+    ).toBeUndefined();
+  });
 });

packages/server/src/server/agent/activity-curator.test.ts

@@ -398,8 +398,10 @@ second line'`,
 
   it("selects a synthetic assistant error by its timeline cursor", () => {
     const result = buildAgentForkContextAttachment({
-      timelineEpoch: "timeline-1",
-      boundaryCursor: { epoch: "timeline-1", seq: 2 },
+      cursorBoundary: {
+        timelineEpoch: "timeline-1",
+        cursor: { epoch: "timeline-1", seq: 2 },
+      },
       rows: [
         row(1, { type: "user_message", text: "Try the task", messageId: "user-1" }),
         row(2, { type: "assistant_message", text: "[System Error] provider failed" }),
@@ -417,6 +419,18 @@ second line'`,
     expect(result.attachment.text).not.toContain("This belongs to a later turn.");
   });
 
+  it("rejects a cursor from a previous timeline epoch", () => {
+    expect(() =>
+      buildAgentForkContextAttachment({
+        cursorBoundary: {
+          timelineEpoch: "timeline-2",
+          cursor: { epoch: "timeline-1", seq: 2 },
+        },
+        rows: [row(2, { type: "assistant_message", text: "Stale result." })],
+      }),
+    ).toThrow("Selected timeline position is no longer available.");
+  });
+
   it("rejects missing assistant boundaries instead of silently using the wrong context", () => {
     expect(() =>
       buildAgentForkContextAttachment({

packages/server/src/server/agent/activity-curator.ts

@@ -216,17 +216,21 @@ export function curateAgentActivity(
     : "No activity to display.";
 }
 
+interface ForkCursorBoundary {
+  timelineEpoch: string;
+  cursor: { epoch: string; seq: number };
+}
+
 function selectForkContextRows(input: {
   rows: readonly AgentTimelineRow[];
-  timelineEpoch?: string;
-  boundaryCursor?: { epoch: string; seq: number } | null;
+  cursorBoundary?: ForkCursorBoundary | null;
   boundaryMessageId?: string | null;
 }): {
   items: AgentTimelineItem[];
   boundaryCursor: { epoch: string; seq: number } | null;
   boundaryMessageId: string | null;
 } {
-  const boundaryCursor = input.boundaryCursor ?? null;
+  const boundaryCursor = input.cursorBoundary?.cursor ?? null;
   const boundaryMessageId = input.boundaryMessageId?.trim() || null;
   if (!boundaryCursor && !boundaryMessageId) {
     const projected = projectTimelineRows({ rows: input.rows, mode: "projected" });
@@ -237,7 +241,10 @@ function selectForkContextRows(input: {
     };
   }
 
-  if (boundaryCursor && boundaryCursor.epoch !== input.timelineEpoch) {
+  if (
+    input.cursorBoundary &&
+    input.cursorBoundary.cursor.epoch !== input.cursorBoundary.timelineEpoch
+  ) {
     throw new Error("Selected timeline position is no longer available.");
   }
   const boundaryIndex = boundaryCursor
@@ -286,8 +293,7 @@ function buildForkContextText(input: {
 
 export function buildAgentForkContextAttachment(input: {
   rows: readonly AgentTimelineRow[];
-  timelineEpoch?: string;
-  boundaryCursor?: { epoch: string; seq: number } | null;
+  cursorBoundary?: ForkCursorBoundary | null;
   boundaryMessageId?: string | null;
   agentTitle?: string | null;
   cwd?: string | null;
@@ -299,8 +305,7 @@ export function buildAgentForkContextAttachment(input: {
 } {
   const selected = selectForkContextRows({
     rows: input.rows,
-    timelineEpoch: input.timelineEpoch,
-    boundaryCursor: input.boundaryCursor,
+    cursorBoundary: input.cursorBoundary,
     boundaryMessageId: input.boundaryMessageId,
   });
   const entries = curateProjectedActivityEntries(selected.items, {

packages/server/src/server/session.ts

@@ -5480,8 +5480,9 @@ export class Session {
       });
       const forkContext = buildAgentForkContextAttachment({
         rows: timeline.rows,
-        timelineEpoch: timeline.epoch,
-        boundaryCursor: msg.boundaryCursor,
+        cursorBoundary: msg.boundaryCursor
+          ? { timelineEpoch: timeline.epoch, cursor: msg.boundaryCursor }
+          : null,
         boundaryMessageId: msg.boundaryMessageId,
         agentTitle: agentPayload.title,
         cwd: snapshot.cwd,