Show a Claude subagent's auto-mode report as markdown instead of a raw SubagentHandback card (#6248)

* Show a Claude subagent's handback report as its final message

Since Claude Code 2.1.271, a subagent in auto mode delivers its report
through a SubagentHandback tool call instead of its final text. Paseo
showed that call as a generic tool card with the report as escaped JSON.
Live sidechain frames and replayed history now map the call to the
subagent's final assistant message and hide its acknowledgement.

Closes #6241

* Keep a handback report's leading whitespace and hide its acknowledgement by call id

---------

Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>

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

@@ -712,4 +712,56 @@ describe("ClaudeAgentSession persisted subagent replay", () => {
     expect(timeline.length).toBeGreaterThan(0);
     expect(timeline[0]).toMatchObject({ id: TOOL_USE_ID });
   });
+
+  test("replays a SubagentHandback report as the subagent's final message", async () => {
+    const handbackId = "toolu_handback";
+    const report = "## Verdict\n\n- **Coherent**";
+    writeSession({
+      parentLines: [taskToolUse(), taskToolResult()],
+      meta: JSON.stringify({ toolUseId: TOOL_USE_ID }),
+      sidechainLines: [
+        JSON.stringify({
+          type: "assistant",
+          isSidechain: true,
+          agentId: AGENT_ID,
+          message: {
+            role: "assistant",
+            content: [
+              {
+                type: "tool_use",
+                id: handbackId,
+                name: "SubagentHandback",
+                input: { message: report },
+              },
+            ],
+          },
+        }),
+        JSON.stringify({
+          type: "user",
+          isSidechain: true,
+          agentId: AGENT_ID,
+          message: {
+            role: "user",
+            content: [
+              {
+                type: "tool_result",
+                tool_use_id: handbackId,
+                content: [
+                  {
+                    type: "text",
+                    text: '{"success":true,"message":"Report delivered to your caller."}',
+                  },
+                ],
+              },
+            ],
+          },
+        }),
+      ],
+    });
+
+    const items = (await replayDescriptors()).flatMap((event) =>
+      event.event.type === "timeline" ? [event.event.item] : [],
+    );
+    expect(items).toEqual([{ type: "assistant_message", text: report }]);
+  });
 });

packages/server/src/server/agent/providers/claude/agent.subagent-handback.real.e2e.test.ts

@@ -0,0 +1,56 @@
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import path from "node:path";
+
+import pino from "pino";
+import { expect, test } from "vitest";
+
+import type { AgentStreamEvent, AgentTimelineItem } from "../../agent-sdk-types.js";
+import { collectSessionTurnEvents } from "../test-utils/session-stream-adapter.js";
+import { ClaudeAgentClient } from "./agent.js";
+
+const ROOT_PROMPT = `Use Claude Code's native Agent tool exactly once, never Paseo tools, with this complete task:
+
+Do not use any tools. Reply with exactly this markdown report and nothing else:
+## HANDBACK_HEADING
+- **HANDBACK_BOLD** item
+
+Then reply exactly ROOT_DONE.`;
+
+function subagentItems(events: AgentStreamEvent[]): AgentTimelineItem[] {
+  return events.flatMap((event) =>
+    event.type === "provider_subagent" && event.event.type === "timeline" ? [event.event.item] : [],
+  );
+}
+
+function expectHandbackAsReport(items: AgentTimelineItem[]): void {
+  expect(items.filter((item) => item.type === "tool_call")).toEqual([]);
+  expect(
+    items.flatMap((item) => (item.type === "assistant_message" ? [item.text] : [])).join(""),
+  ).toContain("## HANDBACK_HEADING\n- **HANDBACK_BOLD** item");
+}
+
+test("an auto-mode Claude subagent's handback report shows as its final message", async () => {
+  const cwd = mkdtempSync(path.join(tmpdir(), "paseo-claude-subagent-handback-"));
+  const client = new ClaudeAgentClient({ logger: pino({ level: "warn" }) });
+  const session = await client.createSession({ provider: "claude", cwd, modeId: "auto" });
+  try {
+    const events = await collectSessionTurnEvents(session, ROOT_PROMPT);
+    expectHandbackAsReport(subagentItems(events));
+
+    const persistence = session.describePersistence();
+    expect(persistence).not.toBeNull();
+    await session.close();
+    const restored = await client.resumeSession(persistence!, { cwd });
+    try {
+      const replayed: AgentStreamEvent[] = [];
+      for await (const event of restored.streamHistory()) replayed.push(event);
+      expectHandbackAsReport(subagentItems(replayed));
+    } finally {
+      await restored.close();
+    }
+  } finally {
+    await session.close();
+    rmSync(cwd, { recursive: true, force: true });
+  }
+}, 240_000);

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

@@ -44,6 +44,7 @@ import {
 } from "./model-manifest.js";
 import { parsePartialJsonObject } from "./partial-json.js";
 import { ClaudeSidechainTracker } from "./sidechain-tracker.js";
+import { readClaudeSubagentHandback } from "./subagent-handback.js";
 import { ClaudeTaskState } from "./task-state.js";
 import {
   ClaudeTaskProtocolSource,
@@ -2084,6 +2085,7 @@ class ClaudeAgentSession implements AgentSession {
   private toolUseCache = new Map<string, ToolUseCacheEntry>();
   private toolUseIndexToId = new Map<number, string>();
   private toolUseInputBuffers = new Map<string, string>();
+  private subagentHandbackIds = new Set<string>();
   private pendingPermissions = new Map<string, PendingPermission>();
   private activeForegroundTurnId: string | null = null;
   private autonomousTurn: AutonomousTurnState | null = null;
@@ -5184,7 +5186,7 @@ class ClaudeAgentSession implements AgentSession {
     // User SDK entries can arrive as multiple text blocks, but Paseo treats them as one message.
     const userTextParts: string[] = [];
     for (const block of content) {
-      if (!isClaudeContentChunk(block)) {
+      if (!isClaudeContentChunk(block) || this.mapSubagentHandbackBlock(block, items)) {
         continue;
       }
       this.mapBlockToTimeline(block, {
@@ -5272,6 +5274,24 @@ class ClaudeAgentSession implements AgentSession {
     }
   }
 
+  // A subagent's handback call is its final message, and the call's result is only an
+  // acknowledgement, so neither becomes a tool call.
+  private mapSubagentHandbackBlock(block: ClaudeContentChunk, items: AgentTimelineItem[]): boolean {
+    if (typeof block.tool_use_id === "string" && this.subagentHandbackIds.has(block.tool_use_id)) {
+      this.subagentHandbackIds.delete(block.tool_use_id);
+      return true;
+    }
+    const handback = readClaudeSubagentHandback(block);
+    if (!handback) {
+      return false;
+    }
+    this.subagentHandbackIds.add(handback.callId);
+    if (handback.report) {
+      items.push({ type: "assistant_message", text: handback.report });
+    }
+    return true;
+  }
+
   private handleToolUseStart(block: ClaudeContentChunk, items: AgentTimelineItem[]): void {
     const entry = this.upsertToolUseEntry(block);
     if (!entry) {

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

@@ -35,4 +35,64 @@ describe("ClaudeSidechainTracker", () => {
       },
     });
   });
+
+  it("shows a SubagentHandback report as the subagent's final message", () => {
+    const tracker = new ClaudeSidechainTracker({ getToolInput: () => null });
+    const report = "    indented code\n\n## Verdict\n\n- **Coherent**";
+    const frames = [
+      {
+        type: "stream_event",
+        parent_tool_use_id: "task-1",
+        event: {
+          type: "content_block_start",
+          index: 0,
+          content_block: {
+            type: "tool_use",
+            id: "handback-1",
+            name: "SubagentHandback",
+            input: {},
+          },
+        },
+      },
+      {
+        type: "assistant",
+        parent_tool_use_id: "task-1",
+        message: {
+          id: "msg-1",
+          content: [
+            {
+              type: "tool_use",
+              id: "handback-1",
+              name: "SubagentHandback",
+              input: { message: report },
+            },
+          ],
+        },
+      },
+      {
+        type: "user",
+        parent_tool_use_id: "task-1",
+        message: {
+          content: [
+            {
+              type: "tool_result",
+              tool_use_id: "handback-1",
+              tool_name: "SubagentHandback",
+              content: [{ type: "text", text: '{"success":true}' }],
+            },
+          ],
+        },
+      },
+    ];
+
+    const childItems = frames
+      .flatMap((frame) => tracker.handleMessage(frame as unknown as SDKMessage, "task-1"))
+      .flatMap((event) =>
+        event.type === "provider_subagent" && event.event.type === "timeline"
+          ? [event.event.item]
+          : [],
+      );
+
+    expect(childItems).toEqual([{ type: "assistant_message", text: report, messageId: "msg-1" }]);
+  });
 });

packages/server/src/server/agent/providers/claude/sidechain-tracker.ts

@@ -6,6 +6,10 @@ import {
   mapClaudeRunningToolCall,
 } from "./tool-call-mapper.js";
 import { buildToolCallDisplayModel } from "@getpaseo/protocol/tool-call-display";
+import {
+  isClaudeSubagentHandbackToolName,
+  readClaudeSubagentHandback,
+} from "./subagent-handback.js";
 
 import type { AgentMetadata, AgentStreamEvent, AgentTimelineItem } from "../../agent-sdk-types.js";
 
@@ -91,7 +95,7 @@ export class ClaudeSidechainTracker {
     const contextUpdated = this.updateSubAgentContextFromTaskInput(state, parentToolUseId);
     const actionCandidates = this.extractSubAgentActionCandidates(message);
     const childTimelineItems = [
-      ...this.extractSubAgentTimelineItems(message),
+      ...this.extractSubAgentTimelineItems(message, state),
       ...this.extractSubAgentToolResults(message, state),
     ];
     let actionUpdated = false;
@@ -233,7 +237,10 @@ export class ClaudeSidechainTracker {
     this.activeSidechains.clear();
   }
 
-  private extractSubAgentTimelineItems(message: SDKMessage): AgentTimelineItem[] {
+  private extractSubAgentTimelineItems(
+    message: SDKMessage,
+    state: SubAgentActivityState,
+  ): AgentTimelineItem[] {
     if (message.type !== "assistant" || !Array.isArray(message.message?.content)) {
       return [];
     }
@@ -253,6 +260,18 @@ export class ClaudeSidechainTracker {
       } else if (block.type === "thinking") {
         const text = readTrimmedString(block.thinking);
         if (text) items.push({ type: "reasoning", text });
+      } else {
+        const handback = readClaudeSubagentHandback(block);
+        if (!handback) continue;
+        // The handback's result is only an acknowledgement, never a tool card.
+        state.completedActionKeys.add(handback.callId);
+        if (handback.report) {
+          items.push({
+            type: "assistant_message",
+            text: handback.report,
+            ...(messageId ? { messageId } : {}),
+          });
+        }
       }
     }
     return items;
@@ -391,6 +410,12 @@ export class ClaudeSidechainTracker {
   }
 
   private extractSubAgentActionCandidates(message: SDKMessage): SubAgentActionCandidate[] {
+    return this.extractToolActionCandidates(message).filter(
+      (action) => !isClaudeSubagentHandbackToolName(action.toolName),
+    );
+  }
+
+  private extractToolActionCandidates(message: SDKMessage): SubAgentActionCandidate[] {
     if (message.type === "assistant") {
       return this.extractAssistantMessageActions(message);
     }

packages/server/src/server/agent/providers/claude/subagent-handback.ts

@@ -0,0 +1,27 @@
+/**
+ * Since Claude Code 2.1.271, a subagent in auto mode delivers its report to its caller through a
+ * `SubagentHandback` tool call instead of its final text. The report is the subagent's final
+ * message, so Paseo shows it as one and hides the call and its acknowledgement.
+ */
+const SUBAGENT_HANDBACK_TOOL_NAME = "SubagentHandback";
+
+export interface ClaudeSubagentHandback {
+  callId: string;
+  /** Null while the call is still streaming its input. */
+  report: string | null;
+}
+
+export function isClaudeSubagentHandbackToolName(name: unknown): boolean {
+  return name === SUBAGENT_HANDBACK_TOOL_NAME;
+}
+
+export function readClaudeSubagentHandback(block: {
+  type: string;
+  [key: string]: unknown;
+}): ClaudeSubagentHandback | null {
+  if (block.type !== "tool_use" || !isClaudeSubagentHandbackToolName(block.name)) return null;
+  if (typeof block.id !== "string" || block.id.length === 0) return null;
+  const input = block.input as { message?: unknown } | null | undefined;
+  const report = typeof input?.message === "string" ? input.message : "";
+  return { callId: block.id, report: report.trim().length > 0 ? report : null };
+}