Fix Claude subagent narration leaking into chat (#1807)

* fix(claude): keep subagent text out of parent transcript

Sidechain tool activity is folded into the parent Task/Agent row, but sidechain assistant text was still going through the top-level text assembler. That made the chat show subagent narration without its matching tool rows.

* test(claude): simplify subagent sidechain assertion

packages/server/src/server/agent/providers/claude/agent.sub-agent-sidechain.test.ts

@@ -211,6 +211,20 @@ describe("ClaudeAgentSession sub-agent sidechain updates", () => {
           parent_tool_use_id: "task-call-1",
           elapsed_time_seconds: 1,
         },
+        {
+          type: "assistant",
+          parent_tool_use_id: "task-call-1",
+          message: {
+            id: "subagent-message-1",
+            role: "assistant",
+            content: [
+              {
+                type: "text",
+                text: "Sub-agent narration belongs inside the Task row, not the parent transcript.",
+              },
+            ],
+          },
+        },
         {
           type: "assistant",
           parent_tool_use_id: null,
@@ -295,6 +309,46 @@ describe("ClaudeAgentSession sub-agent sidechain updates", () => {
     expect(projectedTaskCalls).toHaveLength(1);
   });
 
+  test("keeps sidechain assistant text out of the parent transcript", async () => {
+    const session = await new ClaudeAgentClient({
+      logger,
+      queryFactory,
+      resolveBinary: async () => "/test/claude/bin",
+    }).createSession({
+      provider: "claude",
+      cwd: process.cwd(),
+    });
+
+    const events = await collectUntilTerminal(streamSession(session, "delegate work"));
+    await session.close();
+
+    const visibleAssistantText = events
+      .flatMap((event) =>
+        event.type === "timeline" && event.item.type === "assistant_message"
+          ? [event.item.text]
+          : [],
+      )
+      .join("");
+
+    expect(visibleAssistantText).not.toContain("Sub-agent narration");
+
+    const latestSubAgentUpdate = events
+      .filter(
+        (event): event is Extract<AgentStreamEvent, { type: "timeline" }> =>
+          event.type === "timeline" &&
+          event.item.type === "tool_call" &&
+          event.item.callId === "task-call-1" &&
+          event.item.detail.type === "sub_agent",
+      )
+      .map((event) => event.item)
+      .at(-1);
+
+    expect(latestSubAgentUpdate?.detail).toMatchObject({
+      type: "sub_agent",
+      log: expect.stringContaining("[Read] README.md"),
+    });
+  });
+
   test("tails sub-agent actions instead of dropping latest entries at cap", async () => {
     queryFactory.mockImplementation(() => buildQueryMock(buildTailScenarioEvents(205)));
 

packages/server/src/server/agent/providers/claude/agent.ts

@@ -1731,6 +1731,14 @@ function isClaudeSubagentToolName(name: string | undefined): boolean {
   return name === "Task" || name === "Agent";
 }
 
+function readClaudeParentToolUseId(message: SDKMessage): string | null {
+  if (!("parent_tool_use_id" in message)) {
+    return null;
+  }
+  const parentToolUseId = (message as { parent_tool_use_id?: unknown }).parent_tool_use_id;
+  return typeof parentToolUseId === "string" && parentToolUseId.length > 0 ? parentToolUseId : null;
+}
+
 class ClaudeContextUsageState {
   private contextWindowMaxTokens: number | undefined;
   private streamRequestInputTokens: number | undefined;
@@ -3495,20 +3503,22 @@ class ClaudeAgentSession implements AgentSession {
       suppressAssistantText: true,
       suppressReasoning: true,
     });
-    const assistantTimelineEvents = this.timelineAssembler
-      .consume({
-        message,
-        runId: turnId,
-        messageIdHint,
-      })
-      .map(
-        (item) =>
-          ({
-            type: "timeline",
-            item,
-            provider: "claude",
-          }) satisfies AgentStreamEvent,
-      );
+    const assistantTimelineEvents = readClaudeParentToolUseId(message)
+      ? []
+      : this.timelineAssembler
+          .consume({
+            message,
+            runId: turnId,
+            messageIdHint,
+          })
+          .map(
+            (item) =>
+              ({
+                type: "timeline",
+                item,
+                provider: "claude",
+              }) satisfies AgentStreamEvent,
+          );
 
     return [...messageEvents, ...assistantTimelineEvents];
   }
@@ -3582,10 +3592,7 @@ class ClaudeAgentSession implements AgentSession {
       suppressReasoning?: boolean;
     },
   ): AgentStreamEvent[] {
-    const parentToolUseId =
-      "parent_tool_use_id" in message
-        ? (message as { parent_tool_use_id: string | null }).parent_tool_use_id
-        : null;
+    const parentToolUseId = readClaudeParentToolUseId(message);
     if (parentToolUseId) {
       return this.sidechainTracker.handleMessage(message, parentToolUseId);
     }