perf(server): use normal worker parallelism on Windows

Late child frames were discarded after the launching turn ended, and parent completion cleared accumulated child actions. Keep card updates attached to the launching turn and release child actions on task completion.

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

packages/server/src/server/agent/providers/claude/agent.background-subagent.test.ts

@@ -122,7 +122,7 @@ async function collectTimelineAcrossTurns(
           seq: rows.length + 1,
           timestamp: new Date(Date.UTC(2026, 9, 4, 0, 0, rows.length)).toISOString(),
           item: event.item,
-          ...(currentTurnId ? { turnId: currentTurnId } : {}),
+          ...((event.turnId ?? currentTurnId) ? { turnId: event.turnId ?? currentTurnId } : {}),
         });
       }
       if (event.type === "turn_completed" && ++completedTurns === turnCount) resolve();
@@ -469,122 +469,145 @@ describe("background Claude subagents", () => {
     });
   });
 
-  test("keeps one logged Task card when the child streams on after the parent's turn ends", async () => {
-    // A child can stream across the main session's turn boundary. Claude opens the next
-    // main-session turn with system/init; bare child frames do not open a turn.
-    queryFactory.mockImplementation(() =>
-      buildQueryMock([
-        { type: "system", subtype: "init", session_id: "bg-session", permissionMode: "default" },
-        {
-          type: "assistant",
-          message: {
-            model: "claude-haiku-4-5",
-            content: [
-              {
-                type: "tool_use",
-                id: "toolu_background",
-                name: "Agent",
-                input: {
-                  subagent_type: "general-purpose",
-                  description: "Count files here",
-                  prompt: "Run ls and reply with the number of entries.",
-                  run_in_background: true,
+  test.each(["before", "after"])(
+    "keeps one logged Task card when child tools start %s the parent finishes",
+    async (timing) => {
+      // A child can stream across the main session's turn boundary. Claude opens the next
+      // main-session turn with system/init; bare child frames do not open a turn.
+      queryFactory.mockImplementation(() =>
+        buildQueryMock([
+          { type: "system", subtype: "init", session_id: "bg-session", permissionMode: "default" },
+          {
+            type: "assistant",
+            message: {
+              model: "claude-haiku-4-5",
+              content: [
+                {
+                  type: "tool_use",
+                  id: "toolu_background",
+                  name: "Agent",
+                  input: {
+                    subagent_type: "general-purpose",
+                    description: "Count files here",
+                    prompt: "Run ls and reply with the number of entries.",
+                    run_in_background: true,
+                  },
                 },
-              },
-            ],
+              ],
+            },
           },
-        },
-        {
-          type: "system",
-          subtype: "task_started",
-          task_id: "bg-task",
-          tool_use_id: "toolu_background",
-          task_type: "local_agent",
-          subagent_type: "general-purpose",
+          {
+            type: "system",
+            subtype: "task_started",
+            task_id: "bg-task",
+            tool_use_id: "toolu_background",
+            task_type: "local_agent",
+            subagent_type: "general-purpose",
+            description: "Count files here",
+            prompt: "Run ls and reply with the number of entries.",
+          },
+          {
+            type: "user",
+            message: {
+              role: "user",
+              content: [
+                {
+                  type: "tool_result",
+                  tool_use_id: "toolu_background",
+                  content: "Async agent launched successfully.",
+                },
+              ],
+            },
+          },
+          ...(timing === "after"
+            ? [
+                { type: "result", subtype: "success", usage: {}, total_cost_usd: 0 },
+                {
+                  type: "system",
+                  subtype: "init",
+                  session_id: "bg-session",
+                  permissionMode: "default",
+                },
+              ]
+            : []),
+          {
+            type: "assistant",
+            parent_tool_use_id: "toolu_background",
+            message: {
+              model: "claude-haiku-4-5",
+              content: [
+                {
+                  type: "tool_use",
+                  id: "toolu_ls",
+                  name: "Bash",
+                  input: { command: "ls -1", description: "List files" },
+                },
+              ],
+            },
+          },
+          ...(timing === "before"
+            ? [
+                { type: "result", subtype: "success", usage: {}, total_cost_usd: 0 },
+                {
+                  type: "system",
+                  subtype: "init",
+                  session_id: "bg-session",
+                  permissionMode: "default",
+                },
+              ]
+            : []),
+          {
+            type: "user",
+            parent_tool_use_id: "toolu_background",
+            message: {
+              role: "user",
+              content: [{ type: "tool_result", tool_use_id: "toolu_ls", content: "a\nb\nc" }],
+            },
+          },
+          {
+            type: "assistant",
+            parent_tool_use_id: "toolu_background",
+            message: { model: "claude-haiku-4-5", content: [{ type: "text", text: "3" }] },
+          },
+          {
+            type: "system",
+            subtype: "task_updated",
+            task_id: "bg-task",
+            patch: { status: "completed" },
+          },
+          {
+            type: "system",
+            subtype: "task_notification",
+            task_id: "bg-task",
+            tool_use_id: "toolu_background",
+            status: "completed",
+          },
+          { type: "result", subtype: "success", usage: {}, total_cost_usd: 0 },
+        ]),
+      );
+      const session = await new ClaudeAgentClient({
+        logger: createTestLogger(),
+        queryFactory,
+        resolveBinary: async () => "/test/claude/bin",
+      }).createSession({ provider: "claude", cwd: process.cwd() });
+      const rows = await collectTimelineAcrossTurns(session, "delegate work", 2);
+
+      const cards = projectTimelineRows({ rows, mode: "projected" })
+        .map((entry) => entry.item)
+        .filter((item) => item.type === "tool_call" && item.callId === "toolu_background");
+
+      expect(cards).toHaveLength(1);
+      expect(cards[0]).toMatchObject({
+        status: "completed",
+        detail: {
+          type: "sub_agent",
+          subAgentType: "general-purpose",
           description: "Count files here",
-          prompt: "Run ls and reply with the number of entries.",
+          log: "[Bash] ls -1",
         },
-        {
-          type: "user",
-          message: {
-            role: "user",
-            content: [
-              {
-                type: "tool_result",
-                tool_use_id: "toolu_background",
-                content: "Async agent launched successfully.",
-              },
-            ],
-          },
-        },
-        {
-          type: "assistant",
-          parent_tool_use_id: "toolu_background",
-          message: {
-            model: "claude-haiku-4-5",
-            content: [
-              {
-                type: "tool_use",
-                id: "toolu_ls",
-                name: "Bash",
-                input: { command: "ls -1", description: "List files" },
-              },
-            ],
-          },
-        },
-        { type: "result", subtype: "success", usage: {}, total_cost_usd: 0 },
-        { type: "system", subtype: "init", session_id: "bg-session", permissionMode: "default" },
-        {
-          type: "user",
-          parent_tool_use_id: "toolu_background",
-          message: {
-            role: "user",
-            content: [{ type: "tool_result", tool_use_id: "toolu_ls", content: "a\nb\nc" }],
-          },
-        },
-        {
-          type: "assistant",
-          parent_tool_use_id: "toolu_background",
-          message: { model: "claude-haiku-4-5", content: [{ type: "text", text: "3" }] },
-        },
-        {
-          type: "system",
-          subtype: "task_updated",
-          task_id: "bg-task",
-          patch: { status: "completed" },
-        },
-        {
-          type: "system",
-          subtype: "task_notification",
-          task_id: "bg-task",
-          tool_use_id: "toolu_background",
-          status: "completed",
-        },
-        { type: "result", subtype: "success", usage: {}, total_cost_usd: 0 },
-      ]),
-    );
-    const session = await new ClaudeAgentClient({
-      logger: createTestLogger(),
-      queryFactory,
-      resolveBinary: async () => "/test/claude/bin",
-    }).createSession({ provider: "claude", cwd: process.cwd() });
-    const rows = await collectTimelineAcrossTurns(session, "delegate work", 2);
-
-    const cards = projectTimelineRows({ rows, mode: "projected" })
-      .map((entry) => entry.item)
-      .filter((item) => item.type === "tool_call" && item.callId === "toolu_background");
-
-    expect(cards).toHaveLength(1);
-    expect(cards[0]).toMatchObject({
-      status: "completed",
-      detail: {
-        type: "sub_agent",
-        subAgentType: "general-purpose",
-        description: "Count files here",
-        log: "[Bash] ls -1",
-      },
-    });
-  });
+      });
+    },
+  );
 
   test.each([
     {

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

@@ -2250,7 +2250,7 @@ class ClaudeAgentSession implements AgentSession {
   private readonly sidechainTracker = new ClaudeSidechainTracker({
     getToolInput: (toolUseId) =>
       this.toolUseCache.get(toolUseId)?.input ??
-      this.settledSubagentToolCallInCurrentTurn(toolUseId)?.input ??
+      this.settledSubagentToolCalls.get(toolUseId)?.input ??
       null,
     // Releases that predate the task protocol announce nothing, so the tracker keeps deriving
     // identity and status from frames for them. Detecting the capability beats comparing version
@@ -2258,13 +2258,11 @@ class ClaudeAgentSession implements AgentSession {
     isDescriptorOwnedElsewhere: () => this.taskProtocolSource.isActive,
     // A background child streams its frames after Claude has already answered the call, so its
     // card is rebuilt from the settled call: a running one would replace the settled, labeled card.
-    // A card belongs to the turn that answered the call, so frames from a later turn leave it be.
+    // Late card updates retain the turn that answered the call.
     needsSyntheticParentToolCard: (toolUseId) =>
-      (this.toolUseCache.has(toolUseId) ||
-        this.settledSubagentToolCallInCurrentTurn(toolUseId) !== undefined) &&
+      (this.toolUseCache.has(toolUseId) || this.settledSubagentToolCalls.has(toolUseId)) &&
       this.taskProtocolSource.needsSyntheticParentToolCard(toolUseId),
-    getSettledParentToolCall: (toolUseId) =>
-      this.settledSubagentToolCallInCurrentTurn(toolUseId)?.item,
+    getSettledParentToolCall: (toolUseId) => this.settledSubagentToolCalls.get(toolUseId)?.item,
   });
   private persistedHistory: PersistedTimelineEntry[] = [];
   private persistedProviderSubagentEvents: Extract<
@@ -4490,7 +4488,12 @@ class ClaudeAgentSession implements AgentSession {
       ),
     ).map((event): AgentStreamEvent => ({ type: "provider_subagent", provider: "claude", event }));
     const routedId = canonicalSubagentId ?? parentToolUseId;
-    return [...runtimeEvents, ...this.sidechainTracker.handleMessage(message, routedId)];
+    const settled = this.settledSubagentToolCalls.get(routedId);
+    const sidechainEvents = this.sidechainTracker.handleMessage(message, routedId).map((event) => {
+      if (event.type === "timeline" && settled?.turnId) event.turnId = settled.turnId;
+      return event;
+    });
+    return [...runtimeEvents, ...sidechainEvents];
   }
 
   /**
@@ -5145,7 +5148,8 @@ class ClaudeAgentSession implements AgentSession {
   }
 
   private notifySubscribers(event: AgentStreamEvent): void {
-    const turnId = this.activeForegroundTurnId ?? this.autonomousTurn?.id;
+    const turnId =
+      getAgentStreamEventTurnId(event) ?? this.activeForegroundTurnId ?? this.autonomousTurn?.id;
     const tagged = turnId ? { ...event, turnId } : event;
     this.logger.trace(
       {
@@ -5621,17 +5625,12 @@ class ClaudeAgentSession implements AgentSession {
     });
   }
 
-  private settledSubagentToolCallInCurrentTurn(subagentId: string) {
-    const settled = this.settledSubagentToolCalls.get(subagentId);
-    const currentTurnId = this.activeForegroundTurnId ?? this.autonomousTurn?.id ?? null;
-    return settled && settled.turnId === currentTurnId ? settled : undefined;
-  }
-
   /** A finished child streams no more frames, so its settled call has nothing left to update. */
   private releaseFinishedSubagentToolCalls(observations: SubagentObservation[]): void {
     for (const observation of observations) {
       if (observation.kind === "status" && observation.status !== "running") {
         this.settledSubagentToolCalls.delete(observation.id);
+        this.sidechainTracker.delete(observation.id);
       }
     }
   }

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

@@ -4,6 +4,40 @@ import type { SDKMessage } from "@anthropic-ai/claude-agent-sdk";
 import { ClaudeSidechainTracker } from "./sidechain-tracker.js";
 
 describe("ClaudeSidechainTracker", () => {
+  it("retains a background child's actions across the parent finishing", () => {
+    const tracker = new ClaudeSidechainTracker({
+      getToolInput: () => ({ subagent_type: "general-purpose", description: "List files" }),
+      isDescriptorOwnedElsewhere: () => true,
+    });
+    const action = (id: string, command: string) =>
+      ({
+        type: "assistant",
+        parent_tool_use_id: "task-1",
+        message: { content: [{ type: "tool_use", id, name: "Bash", input: { command } }] },
+      }) as unknown as SDKMessage;
+    tracker.handleMessage(action("ls", "ls -1"), "task-1");
+    expect(tracker.finishAll("completed")).toEqual([]);
+    const events = tracker.handleMessage(action("pwd", "pwd"), "task-1");
+    expect(events).toContainEqual(
+      expect.objectContaining({
+        type: "timeline",
+        item: expect.objectContaining({
+          detail: expect.objectContaining({ log: "[Bash] ls -1\n[Bash] pwd" }),
+        }),
+      }),
+    );
+    tracker.delete("task-1");
+    const next = tracker.handleMessage(action("next", "ls"), "task-1");
+    expect(next).toContainEqual(
+      expect.objectContaining({
+        type: "timeline",
+        item: expect.objectContaining({
+          detail: expect.objectContaining({ log: "[Bash] ls" }),
+        }),
+      }),
+    );
+  });
+
   it("uses Claude's native agent name for the provider subagent title", () => {
     const tracker = new ClaudeSidechainTracker({
       getToolInput: () => ({

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

@@ -199,7 +199,7 @@ export class ClaudeSidechainTracker {
   finishAll(status: "completed" | "failed" | "canceled"): AgentStreamEvent[] {
     const events: AgentStreamEvent[] = [];
     if (this.isDescriptorOwnedElsewhere()) {
-      this.activeSidechains.clear();
+      // The task protocol owns child completion; the parent finishing does not end its children.
       return events;
     }
     for (const [id, state] of this.activeSidechains) {

packages/server/src/server/daemon-e2e/background-subagent-action-log.real.e2e.test.ts

@@ -0,0 +1,105 @@
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+import { fileURLToPath } from "node:url";
+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 { ClaudeAgentClient } from "../agent/providers/claude/agent.js";
+import { createTestPaseoDaemon } from "../test-utils/paseo-daemon.js";
+import { DaemonClient } from "../test-utils/daemon-client.js";
+
+test("background cards retain child actions after the parent finishes (#6253)", async () => {
+  const cwd = mkdtempSync(path.join(tmpdir(), "paseo-background-card-"));
+  const logger = pino({ level: "silent" });
+  const daemon = await createTestPaseoDaemon({
+    logger,
+    agentClients: { claude: new ClaudeAgentClient({ logger }) },
+  });
+  const client = new DaemonClient({ url: `ws://127.0.0.1:${daemon.port}/ws` });
+  try {
+    await client.connect();
+    await client.fetchAgents({ subscribe: {} });
+    const agent = await client.createAgent({
+      provider: "claude",
+      model: "haiku",
+      modeId: "bypassPermissions",
+      cwd,
+      title: "Background action log reproduction",
+    });
+    await client.sendMessage(
+      agent.id,
+      'In one message, call Agent twice with run_in_background true: first with subagent_type "general-purpose", description "List files", prompt "Run ls -1, then run sleep 20, then reply with the number of entries."; second with subagent_type "general-purpose", description "Print cwd", prompt "Run pwd, then run sleep 20, then reply with the path." After launching both, reply "launched" and end your turn immediately without waiting.',
+    );
+    await client.waitForFinish(agent.id, 180_000);
+    const atFinish = await client.fetchAgentTimeline(agent.id, {
+      direction: "tail",
+      limit: 0,
+      projection: "projected",
+    });
+    await expect
+      .poll(
+        async () => {
+          const { subagents } = await client.listProviderSubagents(agent.id);
+          return subagents.length === 2 && subagents.every((child) => child.status === "completed");
+        },
+        { timeout: 180_000, interval: 1000 },
+      )
+      .toBe(true);
+    const { subagents } = await client.listProviderSubagents(agent.id);
+    const children = await Promise.all(
+      subagents.map(async (child) => ({
+        description: child.description,
+        timeline: await client.fetchProviderSubagentTimeline(agent.id, child.id, {
+          direction: "tail",
+          limit: 0,
+        }),
+      })),
+    );
+    const final = await client.fetchAgentTimeline(agent.id, {
+      direction: "tail",
+      limit: 0,
+      projection: "projected",
+    });
+    const { stdout } = await promisify(execFile)(
+      process.execPath,
+      [
+        "--conditions=source",
+        "--import",
+        "tsx",
+        fileURLToPath(new URL("../../../../cli/src/index.ts", import.meta.url)),
+        "--host",
+        `127.0.0.1:${daemon.port}`,
+        "logs",
+        agent.id,
+      ],
+      { env: { ...process.env, PASEO_HOME: daemon.paseoHome }, timeout: 30_000 },
+    );
+    expect(stdout).toContain("[Bash] ls -1");
+    expect(stdout).toContain("[Bash] pwd");
+    expect(
+      atFinish.entries.filter(
+        (row) => row.item.type === "tool_call" && row.item.detail?.type === "sub_agent",
+      ),
+    ).toHaveLength(2);
+    const cards = final.entries
+      .map((row) => row.item)
+      .filter((item) => item.type === "tool_call" && item.detail?.type === "sub_agent");
+    expect(cards).toHaveLength(2);
+    for (const child of children) {
+      expect(
+        child.timeline.rows.some(
+          (row) => row.item.type === "tool_call" && row.item.name === "Bash",
+        ),
+      ).toBe(true);
+    }
+    for (const card of cards) {
+      expect(card.detail?.type === "sub_agent" ? card.detail.log : "").toContain("[Bash]");
+    }
+  } finally {
+    await client.close();
+    await daemon.close();
+    rmSync(cwd, { recursive: true, force: true });
+  }
+}, 420_000);

packages/server/vitest.config.ts

@@ -19,9 +19,6 @@ export default defineConfig({
     setupFiles: [path.resolve(__dirname, "./src/test-utils/vitest-setup.ts")],
     pool: "forks",
     fileParallelism: false,
-    // Even two concurrent Git-heavy suites can exhaust Windows test deadlines.
-    // Run files serially so their subprocesses do not compete across suites.
-    maxWorkers: process.platform === "win32" ? 1 : undefined,
     exclude: ["**/node_modules/**", "**/dist/**", "**/.claude/**", "**/.dev/**"],
   },
 });