From 89073d4c5c1b8330634e034fa2fdb80334745e85 Mon Sep 17 00:00:00 2001
From: "paseo-bot[bot]" <266920839+paseo-bot[bot]@users.noreply.github.com>
Date: Wed, 23 Sep 2026 19:45:11 +0200
Subject: [PATCH] Keep a Claude rewind anchor off subagent messages (#5289)

Task subagent frames stream on the same query as the conversation, and
rememberTranscriptProgress recorded any assistant frame as the current turn's
rewind anchor. When a turn's last assistant frame came from a subagent, the
anchor held a uuid that lives on the subagent's sidechain rather than on the
session's own chain, so forkSession rejected it with "Message <uuid> not found
in session" and every later rewind in that conversation failed.

Skip frames that carry a parent_tool_use_id when recording anchors. The
persisted history path already skips sidechain entries; the live stream now
applies the same rule.

Closes #5284

Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>
---
 .../claude/agent.rewind-anchors.test.ts       | 40 ++++++++++++++++++-
 .../server/agent/providers/claude/agent.ts    |  7 ++++
 2 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/packages/server/src/server/agent/providers/claude/agent.rewind-anchors.test.ts b/packages/server/src/server/agent/providers/claude/agent.rewind-anchors.test.ts
index 15a2b8eba9..1b1932e9cd 100644
--- a/packages/server/src/server/agent/providers/claude/agent.rewind-anchors.test.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.rewind-anchors.test.ts
@@ -9,7 +9,7 @@ import type { AgentSession } from "../../agent-sdk-types.js";
 const SESSION_ID = "session-1";
 
 /** Claude Code replies to a turn either with an assistant message or with nothing at all. */
-type TurnOutcome = { assistantMessageId: string } | "no-response";
+type TurnOutcome = { assistantMessageId: string; subagentMessageId?: string } | "no-response";
 
 function initMessage(): Record<string, unknown> {
   return {
@@ -40,6 +40,21 @@ function assistantReply(uuid: string): Record<string, unknown> {
   };
 }
 
+/**
+ * A message from a Task subagent. The CLI streams these on the same query as the main
+ * conversation, and their uuids live on the subagent's sidechain, not on the session's
+ * own message chain, so `forkSession` cannot resolve one.
+ */
+function subagentReply(uuid: string): Record<string, unknown> {
+  return {
+    type: "assistant",
+    uuid,
+    session_id: SESSION_ID,
+    parent_tool_use_id: "toolu_subagent_1",
+    message: { id: uuid, role: "assistant", content: [{ type: "text", text: "subagent work" }] },
+  };
+}
+
 function successResult(): Record<string, unknown> {
   return {
     type: "result",
@@ -110,6 +125,9 @@ function createConversation(outcomes: TurnOutcome[]): Conversation {
           continue;
         }
         enqueue(assistantReply(outcome.assistantMessageId));
+        if (outcome.subagentMessageId) {
+          enqueue(subagentReply(outcome.subagentMessageId));
+        }
         enqueue(successResult());
       }
       closedRef.value = true;
@@ -229,3 +247,23 @@ describe("Claude rewind across a turn that produced no response", () => {
     expect(rewindSdk.recordedForks).toEqual([{ upToMessageId: "assistant-1" }]);
   });
 });
+
+describe("Claude rewind after a turn whose last assistant message came from a subagent", () => {
+  test("forks at the turn's own reply, not at the subagent message", async () => {
+    const conversation = createConversation([
+      { assistantMessageId: "assistant-1", subagentMessageId: "subagent-1" },
+      { assistantMessageId: "assistant-2" },
+    ]);
+    const rewindSdk = new FakeClaudeSdk();
+    const session = await createSession(conversation, rewindSdk);
+
+    try {
+      await runTurns(session, 2);
+      await session.revertConversation?.({ messageId: conversation.userMessageIds[1] });
+    } finally {
+      await session.close();
+    }
+
+    expect(rewindSdk.recordedForks).toEqual([{ upToMessageId: "assistant-1" }]);
+  });
+});
diff --git a/packages/server/src/server/agent/providers/claude/agent.ts b/packages/server/src/server/agent/providers/claude/agent.ts
index 30fdebd974..b4b516fe1e 100644
--- a/packages/server/src/server/agent/providers/claude/agent.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.ts
@@ -3054,6 +3054,13 @@ class ClaudeAgentSession implements AgentSession {
     if (!messageId) {
       return;
     }
+    // Subagent frames ride the same stream as the conversation, but their uuids live on the
+    // subagent's sidechain, so forkSession cannot resolve one. Anchoring a turn on one leaves
+    // rewind throwing "Message <uuid> not found in session". The persisted history path already
+    // skips sidechain entries; the live stream has to skip them too.
+    if (readClaudeParentToolUseId(message)) {
+      return;
+    }
     if (
       message.type === "user" &&
       !isSyntheticUserEntry(message) &&