chore: prototype opt-in RFD 2452 background shell bridge

2026-11-09 10:51:47 +00:00 · 2026-10-23 16:41:50 +02:00
27 changed files with 1001 additions and 1018 deletions
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "0.95.0"
+  ".": "0.94.0"
 }
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,27 +1,5 @@
 # Changelog

-## [0.95.0](https://github.com/agentclientprotocol/claude-agent-acp/compare/v0.94.0...v0.95.0) (2026-10-23)
-
-
-### Features
-
-* end a turn at its result for AIR clients with backgroundSubagents ([#1262](https://github.com/agentclientprotocol/claude-agent-acp/issues/1262)) ([9b3a5c8](https://github.com/agentclientprotocol/claude-agent-acp/commit/9b3a5c891c75e78b985adb31f4e4b59a16ad2ebc))
-* report read-only subagent configuration ([#1417](https://github.com/agentclientprotocol/claude-agent-acp/issues/1417)) ([6ab670b](https://github.com/agentclientprotocol/claude-agent-acp/commit/6ab670baaefb30aaac2606d8ee6524e08243e5b5))
-* report RFD delegations as linked session messages ([#1405](https://github.com/agentclientprotocol/claude-agent-acp/issues/1405)) ([59e292e](https://github.com/agentclientprotocol/claude-agent-acp/commit/59e292e29f773cc6f03c0999993cbe8a2d97a7e3))
-* serve experimental ACP v2 ([#1400](https://github.com/agentclientprotocol/claude-agent-acp/issues/1400)) ([4d378f9](https://github.com/agentclientprotocol/claude-agent-acp/commit/4d378f93f36d37ba5ef64af87b617706dd9c6e82))
-* show messages typed in other Remote Control clients ([#1371](https://github.com/agentclientprotocol/claude-agent-acp/issues/1371)) ([5b11f47](https://github.com/agentclientprotocol/claude-agent-acp/commit/5b11f470b1c05b114c876b3b6e5b4df9441b1087))
-* update to Claude Code 2.1.309 ([#1413](https://github.com/agentclientprotocol/claude-agent-acp/issues/1413)) ([a1521b6](https://github.com/agentclientprotocol/claude-agent-acp/commit/a1521b6196c58dbc5065f9c19fc9694273d3d435))
-
-
-### Bug Fixes
-
-* bump fast-uri from 3.1.8 to 3.1.9 ([#1410](https://github.com/agentclientprotocol/claude-agent-acp/issues/1410)) ([0fe22ae](https://github.com/agentclientprotocol/claude-agent-acp/commit/0fe22ae5a172b228a574665bb7136b900746041d))
-* bump postcss from 8.5.30 to 8.5.31 in the minor group ([#1403](https://github.com/agentclientprotocol/claude-agent-acp/issues/1403)) ([ecaa2c9](https://github.com/agentclientprotocol/claude-agent-acp/commit/ecaa2c935b9f2b63c1846ca17c6c3398390fb0ef))
-* bump the minor group across 1 directory with 2 updates ([#1416](https://github.com/agentclientprotocol/claude-agent-acp/issues/1416)) ([f7d6030](https://github.com/agentclientprotocol/claude-agent-acp/commit/f7d6030c04af23171058772480d55f6d2aee5298))
-* IJAI-3744 keep a session running while its background tasks are open ([#1399](https://github.com/agentclientprotocol/claude-agent-acp/issues/1399)) ([e9f986b](https://github.com/agentclientprotocol/claude-agent-acp/commit/e9f986b8a941f5c1789078a39e474f247bd0db60))
-* publish the session title as soon as the CLI has it ([#1406](https://github.com/agentclientprotocol/claude-agent-acp/issues/1406)) ([50cd76e](https://github.com/agentclientprotocol/claude-agent-acp/commit/50cd76e78dd9bb1ab0e4850a8d95431175ab5947))
-* publish turns that Claude Code starts after end_turn as an async task in v1 ([#1390](https://github.com/agentclientprotocol/claude-agent-acp/issues/1390)) ([a1d0eee](https://github.com/agentclientprotocol/claude-agent-acp/commit/a1d0eeeaad3d67a0765f4b89e3d81c1af6ca0dbb))
-
 ## [0.94.0](https://github.com/agentclientprotocol/claude-agent-acp/compare/v0.93.0...v0.94.0) (2026-10-19)


--- a/docs/background-work-experiment.md
+++ b/docs/background-work-experiment.md
@@ -0,0 +1,47 @@
+# Background work RFD experiment
+
+This branch prototypes [RFD 2452](https://github.com/agentclientprotocol/agent-client-protocol/pull/2452)
+at revision `33c473ba0723a0f6ffcfcbb54fd780c05df7b208`. It is an opt-in local
+experiment, not a released protocol implementation.
+
+Build with `npm ci --ignore-scripts` and `npm run build`. Launch with
+`ACP_BACKGROUND_WORK_EXPERIMENT=1 node dist/index.js`. Omit the environment
+variable to restore normal behavior. No package publication is needed.
+
+The message-stream bridge works on ACP v1 and v2. It reuses the installed
+native asynchronous-task adapter and its private negotiation internally. Known
+background shells keep the existing tool call `in_progress`; state snapshots
+reference that tool. `tool_call/cancel` is translated to the existing native
+task stop request. A successful response is acceptance only. A native terminal
+event finalizes the tool. ACP v1 represents cancellation as `failed` because its
+tool status enum has no `cancelled`; ACP v2 uses `cancelled`.
+
+Native replace-semantics task levels establish whether the list is complete.
+Before a native level arrives, after a malformed level, or while any native task
+cannot be mapped to an announced tool, `background` is `null`. This includes
+unmapped monitors, scheduled tasks and subagents. The prototype does not claim
+their full RFD support. Forks start with `background: []`. Reconnects remain
+unreported until native current state and referenced entities are available.
+
+This bridge does not upgrade the SDK schema. A client must accept the draft
+fields and raw `tool_call/cancel`, and must treat repeated `idle` snapshots
+without a stop reason as list changes rather than new completions. The private
+negotiation can also select the adapter's existing client-specific tool rendering;
+this is a bounded experiment, not general client capability negotiation.
+
+Suggested manual evaluation: start a bounded background shell, finish the
+foreground turn, cancel one shell, and separately allow a shell to finish while
+idle. Verify that the tool remains in progress and listed until native evidence
+ends it. Send foreground `session/cancel` while background work is listed, then
+close the session. Reconnect and fork must not invent live tools from history.
+Exercise pending permissions separately; deterministic protocol tests are not
+evidence of live native permission or session-close behavior.
+
+A tool already reported final before native background discovery is never revived.
+That task remains unreported by this bridge. Native subagents also remain unreported
+while associated, including an idle child whose own background state is unknown.
+
+Codex list disappearance proves the process ended but supplies no exit outcome.
+The existing native adapter reports this as stopped, represented here as cancelled
+(v2) or failed (v1); only an explicit native command completion supplies a reliable
+completed/failed outcome. This is a known limit of the experiment.
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,16 +1,16 @@
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+        "acorn": "^8.4.1",
+        "acorn-walk": "^8.1.1",
+        "arg": "^4.1.0",
+        "create-require": "^1.1.0",
+        "diff": "^4.0.1",
+        "make-error": "^1.1.1",
+        "v8-compile-cache-lib": "^3.0.1",
+        "yn": "3.1.1"
+      },
+      "bin": {
+        "ts-node": "dist/bin.js",
+        "ts-node-cwd": "dist/bin-cwd.js",
+        "ts-node-esm": "dist/bin-esm.js",
+        "ts-node-script": "dist/bin-script.js",
+        "ts-node-transpile-only": "dist/bin-transpile.js",
+        "ts-script": "dist/bin-script-deprecated.js"
+      },
+      "peerDependencies": {
+        "@swc/core": ">=1.2.50",
+        "@swc/wasm": ">=1.2.50",
+        "@types/node": "*",
+        "typescript": ">=2.7"
+      },
+      "peerDependenciesMeta": {
+        "@swc/core": {
+          "optional": true
+        },
+        "@swc/wasm": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/ts-node/node_modules/diff": {
+      "version": "4.0.4",
+      "resolved": "https://registry.npmjs.org/diff/-/diff-4.0.4.tgz",
+      "integrity": "sha512-X07nttJQkwkfKfvTPG/KSnE2OMdcUCao6+eXF3wmnIQRn2aPAHH3VxDbDOdegkd6JbPsXqShpvEOHfAT+nCNwQ==",
+      "dev": true,
+      "license": "BSD-3-Clause",
+      "engines": {
+        "node": ">=0.3.1"
+      }
+    },
    "node_modules/type-check": {
      "version": "0.4.0",
      "resolved": "https://registry.npmjs.org/type-check/-/type-check-0.4.0.tgz",
@@ -3644,6 +3773,13 @@
        "punycode": "^2.1.0"
      }
    },
+    "node_modules/v8-compile-cache-lib": {
+      "version": "3.0.1",
+      "resolved": "https://registry.npmjs.org/v8-compile-cache-lib/-/v8-compile-cache-lib-3.0.1.tgz",
+      "integrity": "sha512-wa7YjyUGfNZngI/vtK0UHAN+lgDCxBPCylVXGp0zu59Fz5aiGtNXaq3DhIov063MorB+VfufLh3JlF2KdTK3xg==",
+      "dev": true,
+      "license": "MIT"
+    },
    "node_modules/vary": {
      "version": "1.1.2",
      "resolved": "https://registry.npmjs.org/vary/-/vary-1.1.2.tgz",
@@ -3861,6 +3997,16 @@
      "license": "ISC",
      "peer": true
    },
+    "node_modules/yn": {
+      "version": "3.1.1",
+      "resolved": "https://registry.npmjs.org/yn/-/yn-3.1.1.tgz",
+      "integrity": "sha512-Ux4ygGWsu2c7isFWe8Yu1YluJmqVhxqK2cLXNQA5AcC3QfbGNpM7fu0Y8b/z16pXLnFxZYvWhd3fhBY9DLmC6Q==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=6"
+      }
+    },
    "node_modules/yocto-queue": {
      "version": "0.1.0",
      "resolved": "https://registry.npmjs.org/yocto-queue/-/yocto-queue-0.1.0.tgz",
--- a/package.json
+++ b/package.json
@@ -3,7 +3,7 @@
  "publishConfig": {
    "access": "public"
  },
-  "version": "0.95.0",
+  "version": "0.94.0",
  "description": "An ACP-compatible coding agent powered by the Claude Agent SDK (TypeScript)",
  "main": "dist/lib.js",
  "types": "dist/lib.d.ts",
@@ -65,24 +65,25 @@
    "node": ">=22"
  },
  "dependencies": {
-    "@agentclientprotocol/sdk": "2.0.1",
-    "@anthropic-ai/claude-agent-sdk": "0.3.309",
+    "@agentclientprotocol/sdk": "2.0.0",
+    "@anthropic-ai/claude-agent-sdk": "0.3.303",
    "diff": "9.0.0",
    "zod": "4.6.7"
  },
  "devDependencies": {
-    "@anthropic-ai/sdk": "1.1.0",
+    "@anthropic-ai/sdk": "0.135.0",
    "@eslint/js": "10.0.1",
    "@tsconfig/node22": "22.0.6",
    "@types/node": "26.6.5",
-    "@typescript-eslint/eslint-plugin": "8.72.0",
-    "@typescript-eslint/parser": "8.72.0",
+    "@typescript-eslint/eslint-plugin": "8.71.2",
+    "@typescript-eslint/parser": "8.71.2",
    "ajv": "8.20.0",
    "ajv-formats": "3.0.1",
    "eslint": "10.13.0",
    "eslint-config-prettier": "10.1.8",
    "globals": "17.13.0",
    "prettier": "3.9.10",
+    "ts-node": "10.9.2",
    "typescript": "6.0.3",
    "vitest": "5.0.5"
  }
--- a/src/acp-agent.ts
+++ b/src/acp-agent.ts
@@ -353,7 +353,6 @@ import {
  buildEffortConfigOption,
  EFFORT_CONFIG_ID,
  effortFlagSettings,
-  effortLevelName,
  mergeEffortSettings,
  settingsEffortForModel,
 } from "./session-effort.js";
@@ -4411,7 +4410,10 @@ export class ClaudeAcpAgent {
      } else {
        await session.query.stopTask(params.asyncTaskId);
      }
-      await asyncTasks.taskStopped(params.asyncTaskId);
+      // Acceptance is separate from the native terminal event in the RFD experiment.
+      if (process.env.ACP_BACKGROUND_WORK_EXPERIMENT !== "1") {
+        await asyncTasks.taskStopped(params.asyncTaskId);
+      }
      return { stopped: true };
    } catch (error) {
      asyncTasks.releaseStop(params.asyncTaskId);
@@ -5877,28 +5879,9 @@ export class ClaudeAcpAgent {
        // turn activation and settlement stay driven by echoes/results/idle.
        // (Raw-mode emission above still forwards these frames.)
        if ((message as { type: string }).type === "command_lifecycle") {
-          const frame = message as unknown as {
-            command_uuid: string;
-            state: string;
-            wakeup_source?: string;
-            scheduled_for?: number;
-          };
+          const frame = message as unknown as { command_uuid: string; state: string };
          switch (frame.state) {
            case "started": {
-              // CLIs 2.1.307+ (capability lifecycle_wakeup_source_v1) mark a
-              // command that a scheduler fire started. With no prompt of ours
-              // open, it starts an autonomous turn: name the fire as its cause,
-              // whether `session_state_changed` already opened the task or not.
-              const cause = scheduledFireCause(frame);
-              if (
-                cause &&
-                asyncTasks.enabled &&
-                !session.cancelled &&
-                !session.activeTurn &&
-                !firstUnsettledQueuedTurn()
-              ) {
-                await asyncTasks.autonomousTurnStarted(cause);
-              }
              // Remember dispatch on the live turn so a cancel() that orphans
              // it seeds the right state (see Turn.commandStarted)...
              const queued = findUnsettledTurn(frame.command_uuid);
@@ -6819,6 +6802,19 @@ export class ClaudeAcpAgent {
              case "control_request_progress":
                break;
              case "background_tasks_changed":
+                if (process.env.ACP_BACKGROUND_WORK_EXPERIMENT === "1") {
+                  await this.client.sessionUpdate({
+                    sessionId: params.sessionId,
+                    update: {
+                      sessionUpdate: "session_info_update",
+                      _meta: {
+                        backgroundWorkExperiment: {
+                          taskIds: message.tasks.map((task) => task.task_id),
+                        },
+                      },
+                    },
+                  });
+                }
                await asyncTasks.backgroundTasksChanged(message.tasks);
                // A level signal: the full live background-task set on every
                // membership change, with REPLACE semantics. Used only to
@@ -7649,11 +7645,6 @@ export class ClaudeAcpAgent {
            // it) can all be tagged with the same, replay-stable id.
            if (message.event.type === "message_start") {
              currentStreamMessageId = message.event.message.id || undefined;
-              await subagents.reportModel(
-                message.parent_tool_use_id,
-                message.event.message.model,
-                sendUpdate,
-              );
              // A new top-level message starts: clear any streamed-content
              // residue from a prior message that never reached its
              // consolidated reset — a cancelled turn breaks out before the
@@ -8020,14 +8011,6 @@ export class ClaudeAcpAgent {
              break;
            }

-            if (message.type === "assistant") {
-              await subagents.reportModel(
-                message.parent_tool_use_id,
-                message.message.model,
-                sendUpdate,
-              );
-            }
-
            // Snapshot the latest top-level assistant usage and model so the
            // next `result` can emit a usage_update tied to the right context
            // window. Subagent messages are excluded to keep the snapshot
@@ -8629,7 +8612,13 @@ export class ClaudeAcpAgent {
    session.cancelled = true;
    // The interrupt below makes Claude Code withdraw the requests that are open
    // now (see `withPendingUserInput`).
-    for (const input of session.openUserInputs ?? []) input.cancelled = true;
+    for (const input of session.openUserInputs ?? []) {
+      if (
+        process.env.ACP_BACKGROUND_WORK_EXPERIMENT !== "1" ||
+        blocksForeground(session, input.agentID)
+      )
+        input.cancelled = true;
+    }
    // Before anything awaits, so a model request that the consumer reads in
    // the meantime is already judged against these prompts.
    session.cancelledPrompts ??= new Set();
@@ -10153,69 +10142,6 @@ export class ClaudeAcpAgent {
    };
  }

-  private subagentConfigurationHook(sessionId: string): HookCallback {
-    return async (input) => {
-      const session = this.sessions[sessionId];
-      if (!session || !input.agent_id) return { continue: true };
-      const options: SessionConfigOption[] = [];
-      if (input.permission_mode) {
-        const mode = session.modes.availableModes.find((mode) => mode.id === input.permission_mode);
-        options.push(
-          SessionModeManager.configOption({
-            currentModeId: input.permission_mode,
-            availableModes: [mode ?? { id: input.permission_mode, name: input.permission_mode }],
-          }),
-        );
-      }
-      if (input.hook_event_name === "PostModelSwitch" && input.to_model) {
-        options.push({
-          id: MODEL_CONFIG_ID,
-          name: "Model",
-          category: "model",
-          type: "select",
-          currentValue: input.to_model,
-          options: [{ value: input.to_model, name: input.to_model }],
-        });
-      }
-      if (input.effort?.level) {
-        options.push({
-          id: EFFORT_CONFIG_ID,
-          name: "Effort",
-          description: "Current reasoning effort",
-          category: "thought_level",
-          type: "select",
-          currentValue: input.effort.level,
-          options: [{ value: input.effort.level, name: effortLevelName(input.effort.level) }],
-        });
-      }
-      if (input.agent_type) {
-        const agent = session.agents.find((agent) => agent.name === input.agent_type);
-        options.push({
-          id: AGENT_CONFIG_ID,
-          name: "Agent",
-          description: "Subagent type",
-          type: "select",
-          currentValue: input.agent_type,
-          options: [
-            {
-              value: input.agent_type,
-              name: input.agent_type,
-              ...(agent?.description ? { description: agent.description } : {}),
-            },
-          ],
-        });
-      }
-      await session.nativeSubagentRuntime
-        ?.reportConfiguration(input.agent_id, options)
-        .catch((error) =>
-          this.logger.error(
-            `Session ${sessionId}: failed to report subagent configuration: ${error}`,
-          ),
-        );
-      return { continue: true };
-    };
-  }
-
  /**
   * The PreToolUse hook of a v2 connection, for Edit and Write: the tool call
   * shows the exact patch of the change before the tool runs.
@@ -11478,10 +11404,6 @@ export class ClaudeAcpAgent {
      `[session/query] sessionId=${sessionId} resume=${creationOpts?.resume ?? "none"} apiType=${resolvedProvider?.apiType ?? "native"} baseUrl=${resolvedProvider?.baseUrl ?? "native"}`,
    );

-    const subagentConfigHook =
-      subagentForm(this.clientCapabilities) === "rfd"
-        ? this.subagentConfigurationHook(sessionId)
-        : undefined;
    const options: Options = {
      systemPrompt,
      settingSources: ["user", "project", "local"],
@@ -11496,7 +11418,10 @@ export class ClaudeAcpAgent {
      // `stopBackgroundWorkflows` stops instead. Background commands survive an
      // interrupt either way. Before `userProvidedOptions`, so a client's own
      // `perTaskStopAffordance` wins.
-      ...(subagentForm(this.clientCapabilities) === "rfd" ? { perTaskStopAffordance: true } : {}),
+      ...(subagentForm(this.clientCapabilities) === "rfd" ||
+      process.env.ACP_BACKGROUND_WORK_EXPERIMENT === "1"
+        ? { perTaskStopAffordance: true }
+        : {}),
      // One predicted follow-up prompt per turn, published as
      // `session_info_update._meta.promptSuggestions`. It rides the turn's own
      // prompt cache, and the user can turn it off in settings.json
@@ -11550,33 +11475,14 @@ export class ClaudeAcpAgent {
      tools,
      hooks: {
        ...userProvidedOptions?.hooks,
-        ...(subagentConfigHook
-          ? {
-              SubagentStart: [
-                ...(userProvidedOptions?.hooks?.SubagentStart ?? []),
-                { hooks: [subagentConfigHook] },
-              ],
-              SubagentStop: [
-                ...(userProvidedOptions?.hooks?.SubagentStop ?? []),
-                { hooks: [subagentConfigHook] },
-              ],
-              PostToolUseFailure: [
-                ...(userProvidedOptions?.hooks?.PostToolUseFailure ?? []),
-                { hooks: [subagentConfigHook] },
-              ],
-            }
-          : {}),
        // On v2, an Edit or Write shows its exact patch before it runs, also
        // when no permission request shows it (`fileChangePreviewHook`): a v2
        // diff cannot hold the input's snippet.
-        ...(this.v2 || subagentConfigHook
+        ...(this.v2
          ? {
              PreToolUse: [
                ...(userProvidedOptions?.hooks?.PreToolUse || []),
-                ...(subagentConfigHook ? [{ hooks: [subagentConfigHook] }] : []),
-                ...(this.v2
-                  ? [{ matcher: "Edit|Write", hooks: [this.fileChangePreviewHook(sessionId)] }]
-                  : []),
+                { matcher: "Edit|Write", hooks: [this.fileChangePreviewHook(sessionId)] },
              ],
            }
          : {}),
@@ -11584,7 +11490,6 @@ export class ClaudeAcpAgent {
          ...(userProvidedOptions?.hooks?.PostToolUse || []),
          {
            hooks: [
-              ...(subagentConfigHook ? [subagentConfigHook] : []),
              createPostToolUseHook({
                onEnterPlanMode: async () => {
                  await this.sessionModes.publishCurrent(sessionId, "plan");
@@ -11608,11 +11513,9 @@ export class ClaudeAcpAgent {
          ...(userProvidedOptions?.hooks?.PostModelSwitch || []),
          {
            hooks: [
-              ...(subagentConfigHook ? [subagentConfigHook] : []),
              async (input) => {
                if (
                  input.hook_event_name === "PostModelSwitch" &&
-                  !input.agent_id &&
                  input.source !== "sdk" &&
                  input.source !== "resume"
                ) {
@@ -14095,29 +13998,3 @@ function getMatchingModelUsage(modelUsage: Record<string, ModelUsage>, currentMo
    return { key: bestKey, usage: modelUsage[bestKey] };
  }
 }
-
-/**
- * What a `command_lifecycle` "started" frame says started the command: a
- * ScheduleWakeup (dynamic `/loop`) wakeup, with the time it was set for in
- * epoch milliseconds, or another scheduled task such as a cron job. Undefined
- * for any other command, and for a value this adapter does not know. The time
- * is given in UTC because the agent does not know the client's time zone.
- */
-function scheduledFireCause(frame: {
-  wakeup_source?: string;
-  scheduled_for?: number;
-}): string | undefined {
-  switch (frame.wakeup_source) {
-    case "loop_wakeup": {
-      const scheduledFor =
-        typeof frame.scheduled_for === "number" ? new Date(frame.scheduled_for) : undefined;
-      return scheduledFor && !Number.isNaN(scheduledFor.getTime())
-        ? `A scheduled wakeup fired (set for ${scheduledFor.toISOString()}).`
-        : "A scheduled wakeup fired.";
-    }
-    case "schedule_wakeup":
-      return "A scheduled task fired.";
-    default:
-      return undefined;
-  }
-}
--- a/src/async-tasks.ts
+++ b/src/async-tasks.ts
@@ -458,25 +458,18 @@ export class AsyncTaskRuntime {
   * example to answer a task notification. ACP v1 has no session state, and
   * the client already got the stop reason of its last prompt, so this task is
   * the only sign that the agent works. The tool calls of the turn are in the
-   * transcript already, so the task is not shown there.
-   *
-   * [cause] names what started the turn when Claude Code says so, for example
-   * "A scheduled wakeup fired." It replaces the description of a task that is
-   * already open; without it, an open task is left as it is.
+   * transcript already, so the task is not shown there. Does nothing while
+   * such a task is open.
   */
-  async autonomousTurnStarted(cause?: string): Promise<void> {
-    if (!this.enabled) return;
-    if (this.autonomousTurn) {
-      if (cause) await this.publishProgress(this.autonomousTurn, { description: cause });
-      return;
-    }
+  async autonomousTurnStarted(): Promise<void> {
+    if (!this.enabled || this.autonomousTurn) return;
    const task = this.task(`autonomous-turn-${randomUUID()}`);
    task.autonomousTurn = true;
    task.startedObserved = true;
    task.name = "Claude is working";
    task.taskType = "turn";
    task.description =
-      cause ?? this.lastNotificationSummary ?? "Claude Code started a turn without a prompt.";
+      this.lastNotificationSummary ?? "Claude Code started a turn without a prompt.";
    task.showInTranscript = false;
    this.lastNotificationSummary = undefined;
    this.autonomousTurn = task;
--- a/src/background-work-experiment.ts
+++ b/src/background-work-experiment.ts
@@ -0,0 +1,362 @@
+/** Opt-in wire prototype for RFD 2452 over the installed native task adapters. */
+type Json = Record<string, unknown>;
+type Task = {
+  id: string;
+  toolCallId?: string;
+  canStop: boolean;
+  final: boolean;
+  supported: boolean;
+};
+type Session = {
+  state: string;
+  known: boolean;
+  level?: Set<string>;
+  tasks: Map<string, Task>;
+  tools: Set<string>;
+  finalTools: Set<string>;
+  children: Set<string>;
+};
+function object(value: unknown): Json {
+  return value !== null && typeof value === "object" && !Array.isArray(value)
+    ? (value as Json)
+    : {};
+}
+
+export class BackgroundWorkExperiment {
+  private sessions = new Map<string, Session>();
+  private requests = new Map<unknown, { method: string; sessionId?: string }>();
+  private cancellations = new Set<unknown>();
+  private protocol = 1;
+  private session(id: string): Session {
+    let session = this.sessions.get(id);
+    if (!session) {
+      session = {
+        state: "idle",
+        known: false,
+        tasks: new Map(),
+        tools: new Set(),
+        finalTools: new Set(),
+        children: new Set(),
+      };
+      this.sessions.set(id, session);
+    }
+    return session;
+  }
+  private notification(sessionId: string, update: Json): Json {
+    return {
+      jsonrpc: "2.0",
+      method: "session/update",
+      params: { sessionId, update },
+    };
+  }
+  snapshot(sessionId: string): Json {
+    const session = this.session(sessionId);
+    const live = [...session.tasks.values()].filter(
+      (task) => !task.final && session.level?.has(task.id),
+    );
+    const complete =
+      session.known &&
+      session.children.size === 0 &&
+      [...(session.level ?? [])].every((id) => {
+        const task = session.tasks.get(id);
+        return (
+          task &&
+          task.supported &&
+          !task.final &&
+          task.toolCallId !== undefined &&
+          session.tools.has(task.toolCallId) &&
+          !session.finalTools.has(task.toolCallId)
+        );
+      });
+    return this.notification(sessionId, {
+      sessionUpdate: "state_update",
+      state: session.state,
+      background: complete
+        ? live.map((task) => ({
+            type: "tool_call",
+            toolCallId: task.toolCallId,
+          }))
+        : null,
+    });
+  }
+  inbound(message: Json): { forward?: Json; send: Json[] } {
+    const params = object(message["params"]);
+    const sessionId = typeof params["sessionId"] === "string" ? params["sessionId"] : undefined;
+    const send: Json[] = [];
+    if (message["method"] === "initialize") {
+      this.protocol = typeof params["protocolVersion"] === "number" ? params["protocolVersion"] : 1;
+      const capabilities = object(params["clientCapabilities"]),
+        meta = object(capabilities["_meta"]);
+      const jetbrains = object(meta["jetbrains"]),
+        air = object(jetbrains["air"]);
+      const old = Array.isArray(air["capabilities"]) ? air["capabilities"] : [];
+      // The native task adapters already implement these private capabilities.
+      message = {
+        ...message,
+        params: {
+          ...params,
+          clientCapabilities: {
+            ...capabilities,
+            _meta: {
+              ...meta,
+              jetbrains: {
+                ...jetbrains,
+                air: {
+                  ...air,
+                  version: 1,
+                  capabilities: [...new Set([...old, "asyncTasks", "backgroundSubagents"])],
+                },
+              },
+            },
+          },
+        },
+      };
+    }
+    if (["session/resume", "session/load"].includes(String(message["method"])) && sessionId) {
+      const session = this.session(sessionId);
+      session.known = false;
+      delete session.level;
+      session.tools.clear();
+    }
+    if (message["method"] === "tool_call/cancel") {
+      const session = sessionId ? this.session(sessionId) : undefined;
+      const task =
+        session &&
+        [...session.tasks.values()].find(
+          (task) =>
+            !task.final &&
+            task.supported &&
+            task.canStop &&
+            task.toolCallId === params["toolCallId"] &&
+            task.toolCallId !== undefined &&
+            session.tools.has(task.toolCallId) &&
+            !session.finalTools.has(task.toolCallId),
+        );
+      if (!task)
+        return {
+          send: [
+            {
+              jsonrpc: "2.0",
+              id: message["id"],
+              error: {
+                code: -32602,
+                message: "Tool call is not cancellable background work",
+              },
+            },
+          ],
+        };
+      this.cancellations.add(message["id"]);
+      return {
+        forward: {
+          ...message,
+          method: "_session/async_task/stop",
+          params: { sessionId, asyncTaskId: task.id },
+        },
+        send,
+      };
+    }
+    if (message["id"] !== undefined && typeof message["method"] === "string")
+      this.requests.set(message["id"], {
+        method: message["method"],
+        sessionId,
+      });
+    if (message["method"] === "session/prompt" && sessionId && this.protocol < 2) {
+      this.session(sessionId).state = "running";
+      send.push(this.snapshot(sessionId));
+    }
+    return { forward: message, send };
+  }
+  outbound(message: Json): Json[] {
+    if (!message["method"] && this.cancellations.delete(message["id"])) {
+      if (message["error"]) return [message];
+      return object(message["result"])["stopped"] === true
+        ? [{ ...message, result: {} }]
+        : [
+            {
+              jsonrpc: "2.0",
+              id: message["id"],
+              error: {
+                code: -32602,
+                message: "Native cancellation was not accepted",
+              },
+            },
+          ];
+    }
+    const request = !message["method"] ? this.requests.get(message["id"]) : undefined;
+    if (request) {
+      this.requests.delete(message["id"]);
+      const result = object(message["result"]);
+      const id = typeof result["sessionId"] === "string" ? result["sessionId"] : request.sessionId;
+      if (id && !message["error"]) {
+        if (request.method === "session/new" || request.method === "session/fork") {
+          if (request.method === "session/fork") {
+            this.session(id).known = true;
+            this.session(id).level = new Set();
+          }
+          return [message, this.snapshot(id)];
+        }
+        if (request.method === "session/resume" || request.method === "session/load")
+          return [this.snapshot(id), message];
+        if (request.method === "session/close") this.sessions.delete(id);
+      }
+      if (id && request.method === "session/prompt" && this.protocol < 2) {
+        this.session(id).state = "idle";
+        return [this.snapshot(id), message];
+      }
+    }
+    const params = object(message["params"]),
+      update = object(params["update"]);
+    const sessionId = typeof params["sessionId"] === "string" ? params["sessionId"] : undefined;
+    if (message["method"] !== "session/update" || !sessionId) return [message];
+    const session = this.session(sessionId);
+    const marker = object(object(update["_meta"])["backgroundWorkExperiment"]);
+    if (update["sessionUpdate"] === "session_info_update" && "taskIds" in marker) {
+      if (
+        !Array.isArray(marker["taskIds"]) ||
+        !marker["taskIds"].every((id) => typeof id === "string" && id.length > 0)
+      ) {
+        session.known = false;
+      } else {
+        session.level = new Set(marker["taskIds"] as string[]);
+        session.known = true;
+      }
+      return [this.snapshot(sessionId)];
+    }
+    if (update["sessionUpdate"] === "state_update") {
+      if (typeof update["state"] === "string") session.state = update["state"];
+      return [
+        {
+          ...message,
+          params: {
+            ...params,
+            update: {
+              ...update,
+              background: object(object(this.snapshot(sessionId)["params"])["update"])[
+                "background"
+              ],
+            },
+          },
+        },
+      ];
+    }
+    if (
+      ["subagent_update", "_subagent_update", "subagent_spawned", "subagent_state_update"].includes(
+        String(update["sessionUpdate"]),
+      )
+    ) {
+      const id = String(update["sessionId"] ?? update["subagentSessionId"] ?? "unknown");
+      const state =
+        typeof update["state"] === "string" ? update["state"] : object(update["state"])["state"];
+      if (["completed", "failed", "cancelled"].includes(String(state))) session.children.delete(id);
+      else session.children.add(id);
+      return [message, this.snapshot(sessionId)];
+    }
+    if (update["sessionUpdate"] === "tool_call" || update["sessionUpdate"] === "tool_call_update") {
+      const id = typeof update["toolCallId"] === "string" ? update["toolCallId"] : "";
+      if (id && (this.protocol >= 2 || update["sessionUpdate"] === "tool_call"))
+        session.tools.add(id);
+      const task = [...session.tasks.values()].find(
+        (task) =>
+          !task.final && task.supported && task.toolCallId === id && !session.finalTools.has(id),
+      );
+      if (!task && ["completed", "failed", "cancelled"].includes(String(update["status"])))
+        session.finalTools.add(id);
+      return task
+        ? [
+            {
+              ...message,
+              params: {
+                ...params,
+                update: {
+                  ...update,
+                  status: "in_progress",
+                  capabilities: task.canStop ? { cancel: {} } : {},
+                },
+              },
+            },
+          ]
+        : [message];
+    }
+    if (
+      typeof update["sessionUpdate"] !== "string" ||
+      !update["sessionUpdate"].replace(/^_/, "").startsWith("async_task_")
+    )
+      return [message];
+    const id = typeof update["asyncTaskId"] === "string" ? update["asyncTaskId"] : undefined;
+    if (!id) {
+      session.known = false;
+      return [message, this.snapshot(sessionId)];
+    }
+    let task = session.tasks.get(id);
+    if (!task) {
+      task = { id, canStop: false, final: false, supported: false };
+      session.tasks.set(id, task);
+    }
+    if (String(update["sessionUpdate"]).replace(/^_/, "") === "async_task_spawned")
+      task.supported = update["taskType"] === "shell";
+    if (typeof update["toolCallId"] === "string") task.toolCallId = update["toolCallId"];
+    if (update["canStop"] === true) task.canStop = true;
+    const terminal = ["completed", "failed", "stopped"].includes(String(update["state"]));
+    if (!terminal && session.level && !session.level.has(id)) session.known = false;
+    const wasFinal = task.final;
+    if (terminal) {
+      task.final = true;
+      session.level?.delete(id);
+    }
+    if (wasFinal) return [this.snapshot(sessionId)];
+    if (
+      !task.supported ||
+      !task.toolCallId ||
+      !session.tools.has(task.toolCallId) ||
+      session.finalTools.has(task.toolCallId)
+    )
+      return [message, this.snapshot(sessionId)];
+    const updates: Json[] = [];
+    if (!wasFinal)
+      updates.push(
+        this.notification(sessionId, {
+          sessionUpdate: "tool_call_update",
+          toolCallId: task.toolCallId,
+          status: terminal
+            ? update["state"] === "stopped"
+              ? this.protocol < 2
+                ? "failed"
+                : "cancelled"
+              : update["state"]
+            : "in_progress",
+          capabilities: terminal ? null : task.canStop ? { cancel: {} } : {},
+        }),
+      );
+    if (terminal) session.finalTools.add(task.toolCallId);
+    return [...updates, this.snapshot(sessionId)];
+  }
+}
+
+/** Draft messages are added at the stream boundary, outside the installed SDK schema. */
+export function backgroundWorkStream<T>(stream: {
+  readable: ReadableStream<T>;
+  writable: WritableStream<T>;
+}): typeof stream {
+  if (process.env["ACP_BACKGROUND_WORK_EXPERIMENT"] !== "1") return stream;
+  const experiment = new BackgroundWorkExperiment(),
+    writer = stream.writable.getWriter();
+  const send = async (messages: Json[]) => {
+    for (const message of messages) await writer.write(message as T);
+  };
+  return {
+    readable: stream.readable.pipeThrough(
+      new TransformStream<T, T>({
+        async transform(message, controller) {
+          const result = experiment.inbound(object(message));
+          await send(result["send"]);
+          if (result["forward"]) controller.enqueue(result["forward"] as T);
+        },
+      }),
+    ),
+    writable: new WritableStream<T>({
+      write: (message) => send(experiment.outbound(object(message))),
+      close: () => writer.close(),
+      abort: (reason) => writer.abort(reason),
+    }),
+  };
+}
--- a/src/native-subagents.ts
+++ b/src/native-subagents.ts
@@ -1,8 +1,4 @@
-import {
-  RequestError,
-  type SessionConfigOption,
-  type SessionNotification,
-} from "@agentclientprotocol/sdk/unstable";
+import { RequestError, type SessionNotification } from "@agentclientprotocol/sdk/unstable";
 import type { AcpSessionNotification, SubagentForm, SubagentState } from "./acp-subagents.js";
 import { AIR_SUBAGENT_KEY, airExtensionMeta } from "./air-extension.js";
 import { isFinalToolCallStatus } from "./tool-calls/renderer.js";
@@ -68,12 +64,6 @@ export type NativeSubagentSession = {
 type Publish = (notification: AcpSessionNotification) => Promise<void>;
 type Logger = { log(message: string): void };

-type ChildConfiguration = {
-  options: Map<string, SessionConfigOption>;
-  published?: string;
-  publishing: Promise<void>;
-};
-
 /** What the SDK says about the end of a task, beyond its status. */
 export type TaskEnd = {
  /** The SDK's text for a failure. */
@@ -147,7 +137,6 @@ export class NativeSubagentRuntime {
  private readonly parentByToolUse: Map<string, string>;
  private readonly identityByToolUse = new Map<string, SubagentIdentity>();
  private readonly controlByToolUse = new Map<string, AcpSessionNotification>();
-  private readonly configBySession = new Map<string, ChildConfiguration>();
  private readonly childByParentToolUse = new Map<string, NativeSubagent>();
  /**
   * The child session of each tool call that went to a child session. A later
@@ -227,95 +216,6 @@ export class NativeSubagentRuntime {
    return routed;
  }

-  /** Report the model of a child response, not the parent's model or a launch-time alias. */
-  async reportModel(
-    parentToolUseId: string | null,
-    model: string | undefined,
-    deliver: Publish,
-  ): Promise<void> {
-    if (this.form !== "rfd" || !parentToolUseId || !model || model === "<synthetic>") {
-      return;
-    }
-    await deliver({
-      sessionId: this.rootSessionId,
-      update: {
-        sessionUpdate: "config_option_update",
-        configOptions: [
-          {
-            id: "model",
-            name: "Model",
-            category: "model",
-            type: "select",
-            currentValue: model,
-            options: [{ value: model, name: model }],
-          },
-        ],
-        _meta: { claudeCode: { parentToolUseId } },
-      },
-    });
-  }
-
-  /** Hook observations can precede the task event that establishes a child's parent. */
-  async reportConfiguration(
-    agentId: string,
-    options: SessionConfigOption[],
-    meta?: SessionNotification["update"]["_meta"],
-  ): Promise<void> {
-    if (this.form !== "rfd" || agentId === this.rootSessionId || options.length === 0) return;
-    let config = this.configBySession.get(agentId);
-    if (!config) {
-      const pending = [...this.configBySession.keys()].filter(
-        (id) => !this.children.has(id),
-      ).length;
-      if (!this.children.has(agentId) && pending >= MAX_PENDING_PARENTS) {
-        this.logger.log(
-          `Session ${this.rootSessionId}: dropping configuration for unannounced subagent ${agentId}; pending buffer limit reached`,
-        );
-        return;
-      }
-      config = { options: new Map(), publishing: Promise.resolve() };
-      this.configBySession.set(agentId, config);
-    }
-    for (const option of options) config.options.set(option.id, option);
-    const child = this.children.get(agentId);
-    if (child?.announced) await this.publishConfiguration(child, meta);
-  }
-
-  private async publishConfiguration(
-    child: NativeSubagent,
-    meta?: SessionNotification["update"]["_meta"],
-  ): Promise<void> {
-    const config = this.configBySession.get(child.sessionId);
-    if (!config) return;
-    // SDK hooks can run concurrently with streamed responses. Serialize snapshots
-    // so a delayed publication cannot replace newer settings with older ones.
-    const publishing = config.publishing.then(async () => {
-      if (this.configBySession.get(child.sessionId) !== config) return;
-      const priority = ["mode", "model", "thought_level", "model_config"];
-      const rank = (option: SessionConfigOption) => {
-        const index = priority.indexOf(option.category ?? "");
-        return index < 0 ? priority.length : index;
-      };
-      const configOptions = [...config.options.values()].sort(
-        (left, right) => rank(left) - rank(right),
-      );
-      const snapshot = JSON.stringify(configOptions);
-      if (config.published === snapshot) return;
-      await this.publish({
-        sessionId: child.sessionId,
-        update: {
-          sessionUpdate: "config_option_update",
-          configOptions,
-          ...(meta ? { _meta: meta } : {}),
-        },
-      });
-      config.published = snapshot;
-    });
-    // The caller receives the error; later observations must still be able to retry.
-    config.publishing = publishing.catch(() => {});
-    await publishing;
-  }
-
  private async routeUpdate(
    notification: AcpSessionNotification,
    deliver: Publish,
@@ -416,14 +316,6 @@ export class NativeSubagentRuntime {
        ? this.childByParentToolUse.get(claudeMeta.parentToolUseId)
        : undefined);
    if (this.form === "rfd" && announcedChild?.announced) {
-      if (update.sessionUpdate === "config_option_update") {
-        await this.reportConfiguration(
-          announcedChild.sessionId,
-          update.configOptions,
-          update._meta,
-        );
-        return null;
-      }
      const handback = await this.routeRfdHandback(notification, announcedChild);
      if (handback !== undefined) return handback;
    }
@@ -703,7 +595,6 @@ export class NativeSubagentRuntime {
      this.pendingCount = 0;
      this.identityByToolUse.clear();
      this.controlByToolUse.clear();
-      this.configBySession.clear();
      this.parentByToolUse.clear();
      this.forgetRestores();
    }
@@ -1017,7 +908,6 @@ export class NativeSubagentRuntime {
   */
  private withdraw(taskId: string, child: NativeSubagent): void {
    this.children.delete(taskId);
-    this.configBySession.delete(child.sessionId);
    if (child.parentToolUseId) {
      this.takePending(child.parentToolUseId);
      this.cleanupControl(child.parentToolUseId);
@@ -1042,7 +932,6 @@ export class NativeSubagentRuntime {
  /** Announces `child` in the client's form. A runtime without one never does. */
  private async announce(child: NativeSubagent): Promise<void> {
    if (this.form) await announceNativeSubagent(child, this.publish, this.form);
-    await this.publishConfiguration(child);
  }

  /** The RFD form: publishes the work state of `child` on its parent. */
@@ -1268,7 +1157,6 @@ export class NativeSubagentRuntime {
    this.parentByToolUse.clear();
    this.identityByToolUse.clear();
    this.controlByToolUse.clear();
-    this.configBySession.clear();
    this.childByParentToolUse.clear();
    this.taskFinishPromises.clear();
    this.senderByToolUse.clear();
--- a/src/serve.ts
+++ b/src/serve.ts
@@ -1,3 +1,4 @@
+import { backgroundWorkStream } from "./background-work-experiment.js";
 import * as v2 from "@agentclientprotocol/sdk/experimental/v2";
 import { v1AgentApp, type ClaudeAcpAgent, type Logger } from "./acp-agent.js";
 import { nodeToWebReadable, nodeToWebWritable } from "./utils.js";
@@ -37,7 +38,7 @@ export function serveAcp(
    agent = created;
  };
  const { closed } = acpProtocolRouter(logger, onAgent, options.protocols).connect(
-    v2.ndJsonStream(output, input),
+    backgroundWorkStream(v2.ndJsonStream(output, input)),
  );
  return {
    closed,
--- a/src/session-effort.ts
+++ b/src/session-effort.ts
@@ -79,13 +79,6 @@ export function mergeEffortSettings(base: Settings, override: Settings | undefin
  };
 }

-export function effortLevelName(level: string): string {
-  return level
-    .split(/[_-]/)
-    .map((part) => (part ? part.charAt(0).toUpperCase() + part.slice(1) : part))
-    .join(" ");
-}
-
 export function buildEffortConfigOption(
  modelInfos: ModelInfo[],
  currentModelId: string,
@@ -105,7 +98,10 @@ export function buildEffortConfigOption(
    ...(useRecommendedValue ? [] : [{ value: "default", name: "Default" }]),
    ...supportedLevels.map((level) => ({
      value: level,
-      name: effortLevelName(level),
+      name: level
+        .split(/[_-]/)
+        .map((part) => (part ? part.charAt(0).toUpperCase() + part.slice(1) : part))
+        .join(" "),
    })),
  ];
  const includes = (level: string) =>
--- a/src/subagent-history.ts
+++ b/src/subagent-history.ts
@@ -292,13 +292,9 @@ function isForkDirectiveMessage(message: { type?: unknown; message?: unknown }):
 * The transcript of a `subagent_type: "fork"` child without the context it
 * inherited. A fork transcript starts with a copy of the parent's launching
 * assistant message (its text, thinking and every tool_use, including the
- * Agent call itself), then a user message with a placeholder tool_result for
- * each of those tool_uses and the text
- * `<fork-boilerplate>…</fork-boilerplate>\nYour directive: <prompt>`. Since
- * Claude Code 2.1.307 the fork's own Agent call gets "Fork started — that fork
- * is you." and the other tool_uses get "This is the parent's call, not
- * yours. …"; before that, every one got "Fork started — processing in
- * background". Only the directive text is matched here. A fork
+ * Agent call itself), then a user message with a "Fork started — processing
+ * in background" tool_result for each of those tool_uses and the text
+ * `<fork-boilerplate>…</fork-boilerplate>\nYour directive: <prompt>`. A fork
 * launched from inside a subagent also gets a copy of that subagent's history
 * before them. None of it is the child's own work: replayed as is, the
 * parent's text and tool calls would show again in the child.
--- a/src/tests/acp-agent.test.ts
+++ b/src/tests/acp-agent.test.ts
@@ -2269,57 +2269,6 @@ describe("usage Markdown", () => {
    expect(formatted).not.toContain("lowest");
  });

-  it("renders each window's recommendations in order, escaped, before its MCP table", () => {
-    const formatted = formatUsageResponse({
-      ...usageResponse,
-      behaviors: {
-        ...usageResponse.behaviors,
-        day: {
-          ...usageResponse.behaviors.day,
-          recommendations: [
-            { key: "mcp_server", pct: 41, title: "Disable my_server", detail: "/mcp" },
-            { key: "effort", pct: 12, title: "Lower effort | max" },
-          ],
-          fix: { keys: ["mcp_server"], messages: ["/mcp disable my_server"] },
-        },
-        week: {
-          ...usageResponse.behaviors.week,
-          mcp_servers: [],
-          recommendations: [{ key: "agent", pct: 5, title: "Trim the reviewer agent" }],
-        },
-      },
-    });
-    const day = formatted.slice(
-      formatted.indexOf("**Last 24h**"),
-      formatted.indexOf("**Last 7d**"),
-    );
-    expect(day).toContain(
-      "- **41%** Disable my\\_server · /mcp\n- **12%** Lower effort \\| max\n\n| MCP server |",
-    );
-    expect(formatted).not.toContain("undefined");
-    expect(formatted).not.toContain("/mcp disable");
-    expect(
-      formatted.endsWith(
-        "**Last 7d** · 43 requests · 5 sessions\n\n- **5%** Trim the reviewer agent",
-      ),
-    ).toBe(true);
-  });
-
-  it("falls back to the CLI text for a malformed recommendation", () => {
-    expect(
-      parseUsageResponse({
-        ...usageResponse,
-        behaviors: {
-          ...usageResponse.behaviors,
-          day: {
-            ...usageResponse.behaviors.day,
-            recommendations: [{ key: "effort", pct: "high", title: "Lower effort" }],
-          },
-        },
-      }),
-    ).toBeNull();
-  });
-
  it("recognizes only exact local usage commands", () => {
    expect(isUsageCommandText(" /usage ")).toBe(true);
    expect(isUsageCommandText("/cost")).toBe(false);
@@ -3888,8 +3837,7 @@ describe("subagent transcript replay", () => {
    it("replays a fork child without the launching message and boilerplate it inherited", async () => {
      // A `subagent_type: "fork"` transcript starts with a copy of the
      // launching message (same API message id) and a directive message with
-      // a placeholder result for each of its tool_uses, worded as Claude Code
-      // 2.1.307+ writes them.
+      // a "Fork started" result for each of its tool_uses.
      const launching = [
        { type: "text", text: "I'll fork a worker." },
        toolUse("toolu_fork", "Agent", {
@@ -3902,15 +3850,7 @@ describe("subagent transcript replay", () => {
      const forkStarted = (id: string) => ({
        type: "tool_result",
        tool_use_id: id,
-        content: [
-          {
-            type: "text",
-            text:
-              id === "toolu_fork"
-                ? "Fork started — that fork is you."
-                : "This is the parent's call, not yours. It may not have run yet, and its result goes to the parent.",
-          },
-        ],
+        content: [{ type: "text", text: "Fork started — processing in background" }],
      });
      const trace = await replayRfd(
        [
@@ -7237,64 +7177,6 @@ describe("subagent permission attribution (issue #851)", () => {
      session_id: "test-session",
    });

-    it("reports the child's observed model as a singleton config without enabling changes", async () => {
-      const response = (model: string) => ({
-        type: "assistant",
-        parent_tool_use_id: "toolu_parent",
-        uuid: randomUUID(),
-        session_id: "test-session",
-        message: {
-          id: randomUUID(),
-          role: "assistant",
-          model,
-          content: [{ type: "text", text: "Working" }],
-          usage: { input_tokens: 1, output_tokens: 1 },
-        },
-      });
-      const first = response("child-model");
-      const { agent, updates } = await runAgent([
-        { ...taskStarted("agent-42", "toolu_parent"), subagent_type: "Explore" },
-        {
-          type: "stream_event",
-          parent_tool_use_id: "toolu_parent",
-          uuid: randomUUID(),
-          session_id: "test-session",
-          event: { type: "message_start", message: first.message },
-        },
-        first,
-        response("fallback-model"),
-        taskUpdated({ status: "completed" }),
-      ]);
-      const configs = updates.filter(
-        ({ update }) => update.sessionUpdate === "config_option_update",
-      );
-      expect(configs).toHaveLength(2);
-      for (const [index, model] of ["child-model", "fallback-model"].entries()) {
-        expect(configs[index]).toMatchObject({
-          sessionId: "agent-42",
-          update: {
-            configOptions: [
-              {
-                id: "model",
-                currentValue: model,
-                options: [{ value: model, name: model }],
-              },
-            ],
-          },
-        });
-      }
-      expect(
-        updates.findIndex(({ update }) => update.sessionUpdate === "subagent_update"),
-      ).toBeLessThan(updates.indexOf(configs[0]!));
-      await expect(
-        agent.setSessionConfigOption({
-          sessionId: "agent-42",
-          configId: "model",
-          value: "fallback-model",
-        }),
-      ).rejects.toThrow("Session not found");
-    });
-
    it("announces the child as running, then sends it the prompt as a message", async () => {
      const updates = await run([
        { ...taskStarted("agent-42", "toolu_parent"), subagent_type: "Explore", prompt: "Go" },
@@ -24281,93 +24163,6 @@ describe("deferred settlement for live background subagents (issues #864/#866)",
      ]);
    });

-    describe("started by a scheduler fire", () => {
-      const lifecycleStarted = (fire: { wakeup_source?: string; scheduled_for?: number }) =>
-        ({
-          type: "command_lifecycle",
-          command_uuid: "scheduler-fire",
-          state: "started",
-          ...fire,
-          uuid: randomUUID(),
-          session_id: "test-session",
-        }) as any;
-
-      const autonomousTurn = async (fireFrames: (afterRunning: boolean) => any[]) => {
-        const { agent, asyncTaskUpdates } = await airAgent();
-        let wake = () => {};
-        const woken = new Promise<void>((resolve) => (wake = resolve));
-        injectGeneratorSession(agent, (input) => {
-          async function* messageGenerator() {
-            const iter = input[Symbol.asyncIterator]();
-            yield userEcho((await iter.next()).value);
-            yield running();
-            yield resultMessage();
-            yield idle();
-            await woken;
-            yield* fireFrames(false);
-            yield running();
-            yield* fireFrames(true);
-            yield assistantText("woke up");
-            yield resultMessage();
-            yield idle();
-          }
-          return messageGenerator();
-        });
-        await agent.prompt({ sessionId: "test-session", prompt: [{ type: "text", text: "go" }] });
-        wake();
-        await agent.sessions["test-session"]?.consumer;
-        return asyncTaskUpdates()
-          .filter((update) => update.sessionUpdate !== "async_task_state_update")
-          .map(({ sessionUpdate, description }) => ({ sessionUpdate, description }));
-      };
-
-      it("names a ScheduleWakeup fire that arrives before the running state", async () => {
-        const updates = await autonomousTurn((afterRunning) =>
-          afterRunning
-            ? []
-            : [
-                lifecycleStarted({
-                  wakeup_source: "loop_wakeup",
-                  scheduled_for: 1_790_000_000_000,
-                }),
-              ],
-        );
-        expect(updates).toEqual([
-          {
-            sessionUpdate: "async_task_spawned",
-            description: "A scheduled wakeup fired (set for 2026-09-21T14:13:20.000Z).",
-          },
-        ]);
-      });
-
-      it("renames the open task for a scheduled task fire after the running state", async () => {
-        const updates = await autonomousTurn((afterRunning) =>
-          afterRunning ? [lifecycleStarted({ wakeup_source: "schedule_wakeup" })] : [],
-        );
-        expect(updates).toEqual([
-          {
-            sessionUpdate: "async_task_spawned",
-            description: "Claude Code started a turn without a prompt.",
-          },
-          { sessionUpdate: "async_task_progress", description: "A scheduled task fired." },
-        ]);
-      });
-
-      it("leaves the task as before for a started frame without a known wakeup source", async () => {
-        const updates = await autonomousTurn((afterRunning) =>
-          afterRunning
-            ? [lifecycleStarted({}), lifecycleStarted({ wakeup_source: "future_source" })]
-            : [],
-        );
-        expect(updates).toEqual([
-          {
-            sessionUpdate: "async_task_spawned",
-            description: "Claude Code started a turn without a prompt.",
-          },
-        ]);
-      });
-    });
-
    it("ends the turn task when a prompt is folded into the turn", async () => {
      const { agent, updates, asyncTaskUpdates } = await airAgent();
      let wake = () => {};
--- a/src/tests/acp-scenarios/snapshots/v2/subagent-agent-async.jsonl
+++ b/src/tests/acp-scenarios/snapshots/v2/subagent-agent-async.jsonl
@@ -9,7 +9,6 @@
 {"kind":"notification","method":"session/update","payload":{"sessionId":"a2","update":{"sessionUpdate":"state_update","state":"running"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"<session>","update":{"content":[{"text":"Review the diff","type":"text"}],"messageId":"toolu_agent","recipientMessageId":"toolu_agent","recipientSessionId":"a2","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"a2","update":{"content":[{"text":"Review the diff","type":"text"}],"messageId":"toolu_agent","recipientSessionId":"a2","senderMessageId":"toolu_agent","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
-{"kind":"notification","method":"session/update","payload":{"sessionId":"a2","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_agent"}},"configOptions":[{"category":"model","configId":"model","currentValue":"claude-sonnet-4-6","name":"Model","options":[{"name":"claude-sonnet-4-6","value":"claude-sonnet-4-6"}],"type":"select"}],"sessionUpdate":"config_option_update"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"a2","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_agent"}},"content":{"text":"Reviewi","type":"text"},"messageId":"msg_async_sub","sessionUpdate":"agent_message_chunk"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"a2","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_agent"}},"content":{"text":"ng now.","type":"text"},"messageId":"msg_async_sub","sessionUpdate":"agent_message_chunk"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"<session>","update":{"sessionId":"a2","sessionUpdate":"subagent_update","state":{"state":"idle","stopReason":"end_turn"}}}}
--- a/src/tests/acp-scenarios/snapshots/v2/subagent-late-child-update.jsonl
+++ b/src/tests/acp-scenarios/snapshots/v2/subagent-late-child-update.jsonl
@@ -9,7 +9,6 @@
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_late","update":{"sessionUpdate":"state_update","state":"running"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"<session>","update":{"content":[{"text":"Check logs","type":"text"}],"messageId":"toolu_late","recipientMessageId":"toolu_late","recipientSessionId":"agent_late","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_late","update":{"content":[{"text":"Check logs","type":"text"}],"messageId":"toolu_late","recipientSessionId":"agent_late","senderMessageId":"toolu_late","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
-{"kind":"notification","method":"session/update","payload":{"sessionId":"agent_late","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_late"}},"configOptions":[{"category":"model","configId":"model","currentValue":"claude-sonnet-4-6","name":"Model","options":[{"name":"claude-sonnet-4-6","value":"claude-sonnet-4-6"}],"type":"select"}],"sessionUpdate":"config_option_update"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_late","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_late"}},"content":[],"kind":"read","locations":[],"name":"Read","rawInput":{},"sessionUpdate":"tool_call_update","status":"pending","title":"Read File","toolCallId":"toolu_late_read"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_late","update":{"locations":[{"line":1,"path":"/var/log/app.log"}],"rawInput":{"file_path":"/var/log/app.log"},"sessionUpdate":"tool_call_update","title":"Read /var/log/app.log","toolCallId":"toolu_late_read"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"<session>","update":{"content":[{"text":"The logs are clean.","type":"text"}],"messageId":"toolu_late:reply","recipientSessionId":"<session>","senderSessionId":"agent_late","sessionUpdate":"session_message"}}}
--- a/src/tests/acp-scenarios/snapshots/v2/subagent-native-sessions.jsonl
+++ b/src/tests/acp-scenarios/snapshots/v2/subagent-native-sessions.jsonl
@@ -9,7 +9,6 @@
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_n","update":{"sessionUpdate":"state_update","state":"running"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"<session>","update":{"content":[{"text":"Make the build pass","type":"text"}],"messageId":"toolu_native","recipientMessageId":"toolu_native","recipientSessionId":"agent_n","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_n","update":{"content":[{"text":"Make the build pass","type":"text"}],"messageId":"toolu_native","recipientSessionId":"agent_n","senderMessageId":"toolu_native","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
-{"kind":"notification","method":"session/update","payload":{"sessionId":"agent_n","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_native"}},"configOptions":[{"category":"model","configId":"model","currentValue":"claude-sonnet-4-6","name":"Model","options":[{"name":"claude-sonnet-4-6","value":"claude-sonnet-4-6"}],"type":"select"}],"sessionUpdate":"config_option_update"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_n","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_native"}},"content":{"text":"Running t","type":"text"},"messageId":"msg_native_sub_1","sessionUpdate":"agent_message_chunk"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_n","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_native"}},"content":{"text":"he build.","type":"text"},"messageId":"msg_native_sub_1","sessionUpdate":"agent_message_chunk"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_n","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_native"}},"content":[],"kind":"search","name":"Grep","rawInput":{},"sessionUpdate":"tool_call_update","status":"pending","title":"grep","toolCallId":"toolu_native_grep"}}}
--- a/src/tests/acp-scenarios/snapshots/v2/subagent-nested.jsonl
+++ b/src/tests/acp-scenarios/snapshots/v2/subagent-nested.jsonl
@@ -9,12 +9,10 @@
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_outer","update":{"sessionUpdate":"state_update","state":"running"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"<session>","update":{"content":[{"text":"Plan it","type":"text"}],"messageId":"toolu_outer","recipientMessageId":"toolu_outer","recipientSessionId":"agent_outer","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_outer","update":{"content":[{"text":"Plan it","type":"text"}],"messageId":"toolu_outer","recipientSessionId":"agent_outer","senderMessageId":"toolu_outer","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
-{"kind":"notification","method":"session/update","payload":{"sessionId":"agent_outer","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_outer"}},"configOptions":[{"category":"model","configId":"model","currentValue":"claude-sonnet-4-6","name":"Model","options":[{"name":"claude-sonnet-4-6","value":"claude-sonnet-4-6"}],"type":"select"}],"sessionUpdate":"config_option_update"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_outer","update":{"capabilities":{"cancel":{}},"description":"Read the spec","sessionId":"agent_inner","sessionUpdate":"subagent_update","state":{"state":"running"},"title":"Explore"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_inner","update":{"sessionUpdate":"state_update","state":"running"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_outer","update":{"content":[{"text":"Read spec.md","type":"text"}],"messageId":"toolu_inner","recipientMessageId":"toolu_inner","recipientSessionId":"agent_inner","senderSessionId":"agent_outer","sessionUpdate":"session_message"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_inner","update":{"content":[{"text":"Read spec.md","type":"text"}],"messageId":"toolu_inner","recipientSessionId":"agent_inner","senderMessageId":"toolu_inner","senderSessionId":"agent_outer","sessionUpdate":"session_message"}}}
-{"kind":"notification","method":"session/update","payload":{"sessionId":"agent_inner","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_inner"}},"configOptions":[{"category":"model","configId":"model","currentValue":"claude-sonnet-4-6","name":"Model","options":[{"name":"claude-sonnet-4-6","value":"claude-sonnet-4-6"}],"type":"select"}],"sessionUpdate":"config_option_update"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_inner","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_inner"}},"content":[],"kind":"read","locations":[],"name":"Read","rawInput":{},"sessionUpdate":"tool_call_update","status":"pending","title":"Read File","toolCallId":"toolu_grandchild_read"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_inner","update":{"locations":[{"line":1,"path":"/spec.md"}],"rawInput":{"file_path":"/spec.md"},"sessionUpdate":"tool_call_update","title":"Read /spec.md","toolCallId":"toolu_grandchild_read"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_inner","update":{"content":[{"content":{"text":"```\n1\tspec\n```","type":"text"},"type":"content"}],"rawOutput":"1\tspec","sessionUpdate":"tool_call_update","status":"completed","toolCallId":"toolu_grandchild_read"}}}
--- a/src/tests/acp-scenarios/snapshots/v2/subagent-task-legacy.jsonl
+++ b/src/tests/acp-scenarios/snapshots/v2/subagent-task-legacy.jsonl
@@ -9,7 +9,6 @@
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_1","update":{"sessionUpdate":"state_update","state":"running"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"<session>","update":{"content":[{"text":"Find the parser","type":"text"}],"messageId":"toolu_task","recipientMessageId":"toolu_task","recipientSessionId":"agent_1","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_1","update":{"content":[{"text":"Find the parser","type":"text"}],"messageId":"toolu_task","recipientSessionId":"agent_1","senderMessageId":"toolu_task","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
-{"kind":"notification","method":"session/update","payload":{"sessionId":"agent_1","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_task"}},"configOptions":[{"category":"model","configId":"model","currentValue":"claude-sonnet-4-6","name":"Model","options":[{"name":"claude-sonnet-4-6","value":"claude-sonnet-4-6"}],"type":"select"}],"sessionUpdate":"config_option_update"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_1","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_task"}},"content":{"text":"Looking for","type":"text"},"messageId":"msg_sub_1","sessionUpdate":"agent_message_chunk"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_1","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_task"}},"content":{"text":" the parser.","type":"text"},"messageId":"msg_sub_1","sessionUpdate":"agent_message_chunk"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_1","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_task"}},"content":[],"kind":"read","locations":[],"name":"Read","rawInput":{},"sessionUpdate":"tool_call_update","status":"pending","title":"Read File","toolCallId":"toolu_sub_read"}}}
--- a/src/tests/acp-scenarios/snapshots/v2/subagent-transcript-extension.jsonl
+++ b/src/tests/acp-scenarios/snapshots/v2/subagent-transcript-extension.jsonl
@@ -9,7 +9,6 @@
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_t","update":{"sessionUpdate":"state_update","state":"running"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"<session>","update":{"content":[{"text":"Summarize the repo","type":"text"}],"messageId":"toolu_transcript","recipientMessageId":"toolu_transcript","recipientSessionId":"agent_t","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_t","update":{"content":[{"text":"Summarize the repo","type":"text"}],"messageId":"toolu_transcript","recipientSessionId":"agent_t","senderMessageId":"toolu_transcript","senderSessionId":"<session>","sessionUpdate":"session_message"}}}
-{"kind":"notification","method":"session/update","payload":{"sessionId":"agent_t","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_transcript"}},"configOptions":[{"category":"model","configId":"model","currentValue":"claude-sonnet-4-6","name":"Model","options":[{"name":"claude-sonnet-4-6","value":"claude-sonnet-4-6"}],"type":"select"}],"sessionUpdate":"config_option_update"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_t","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_transcript"}},"content":{"text":"Look at t","type":"text"},"messageId":"msg_transcript_sub","sessionUpdate":"agent_thought_chunk"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_t","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_transcript"}},"content":{"text":"he layout.","type":"text"},"messageId":"msg_transcript_sub","sessionUpdate":"agent_thought_chunk"}}}
 {"kind":"notification","method":"session/update","payload":{"sessionId":"agent_t","update":{"_meta":{"claudeCode":{"parentToolUseId":"toolu_transcript"}},"content":{"text":"The repo has ","type":"text"},"messageId":"msg_transcript_sub","sessionUpdate":"agent_message_chunk"}}}
--- a/src/tests/background-work-experiment.test.ts
+++ b/src/tests/background-work-experiment.test.ts
@@ -0,0 +1,244 @@
+import { describe, expect, it } from "vitest";
+import { BackgroundWorkExperiment, backgroundWorkStream } from "../background-work-experiment.js";
+
+const update = (value: Record<string, unknown>, sessionId = "s") => ({
+  method: "session/update",
+  params: { sessionId, update: value },
+});
+const level = (taskIds: unknown) =>
+  update({
+    sessionUpdate: "session_info_update",
+    _meta: { backgroundWorkExperiment: { taskIds } },
+  });
+const expected = (background: unknown, state = "idle") => ({
+  params: {
+    sessionId: "s",
+    update: { sessionUpdate: "state_update", state, background },
+  },
+});
+
+describe.each([1, 2])("background work experiment v%s", (version) => {
+  function fixture() {
+    const bridge = new BackgroundWorkExperiment();
+    bridge.inbound({
+      id: 1,
+      method: "initialize",
+      params: { protocolVersion: version },
+    });
+    bridge.inbound({ id: 2, method: "session/new", params: {} });
+    bridge.outbound({ id: 2, result: { sessionId: "s" } });
+    return bridge;
+  }
+  function spawn(bridge: BackgroundWorkExperiment) {
+    bridge.outbound(
+      update({
+        sessionUpdate: version === 1 ? "tool_call" : "tool_call_update",
+        toolCallId: "tool",
+        status: "in_progress",
+      }),
+    );
+    bridge.outbound(level(["native"]));
+    return bridge.outbound(
+      update({
+        sessionUpdate: version === 1 ? "async_task_spawned" : "_async_task_spawned",
+        asyncTaskId: "native",
+        taskType: "shell",
+        toolCallId: "tool",
+        canStop: true,
+      }),
+    );
+  }
+  it("does not infer empty from a silent backend or unmapped monitor", () => {
+    const bridge = fixture();
+    expect(bridge.snapshot("s")).toMatchObject(expected(null));
+    expect(bridge.outbound(level(["monitor"]))).toMatchObject([expected(null)]);
+    expect(bridge.outbound(level([]))).toMatchObject([expected([])]);
+    expect(bridge.outbound(level([null]))).toMatchObject([expected(null)]);
+    expect(bridge.outbound(level("damaged"))).toMatchObject([expected(null)]);
+  });
+  it("references announced tools and keeps them running after foreground completion", () => {
+    const bridge = fixture();
+    expect(spawn(bridge)).toMatchObject([
+      {
+        params: {
+          update: {
+            sessionUpdate: "tool_call_update",
+            toolCallId: "tool",
+            status: "in_progress",
+            capabilities: { cancel: {} },
+          },
+        },
+      },
+      expected([{ type: "tool_call", toolCallId: "tool" }]),
+    ]);
+    expect(
+      bridge.outbound(
+        update({
+          sessionUpdate: "tool_call_update",
+          toolCallId: "tool",
+          status: "completed",
+        }),
+      ),
+    ).toMatchObject([{ params: { update: { status: "in_progress" } } }]);
+    bridge.outbound(
+      update({
+        sessionUpdate: "state_update",
+        state: "idle",
+        stopReason: "end_turn",
+      }),
+    );
+    expect(bridge.outbound(level(["native"]))).toMatchObject([
+      expected([{ type: "tool_call", toolCallId: "tool" }]),
+    ]);
+    expect(bridge.outbound(level(["native"]))[0]).not.toHaveProperty("params.update.stopReason");
+    bridge.inbound({
+      method: "session/cancel",
+      params: { sessionId: "s" },
+    });
+    expect(bridge.snapshot("s")).toMatchObject(
+      expected([{ type: "tool_call", toolCallId: "tool" }]),
+    );
+  });
+  it("acceptance does not finalize, and completion wins a cancellation race", () => {
+    const bridge = fixture();
+    spawn(bridge);
+    expect(
+      bridge.inbound({
+        id: 3,
+        method: "tool_call/cancel",
+        params: { sessionId: "s", toolCallId: "tool" },
+      }),
+    ).toMatchObject({
+      forward: {
+        method: "_session/async_task/stop",
+        params: { asyncTaskId: "native" },
+      },
+    });
+    expect(bridge.outbound({ id: 3, result: { stopped: true } })).toMatchObject([{ result: {} }]);
+    expect(bridge.snapshot("s")).toMatchObject(
+      expected([{ type: "tool_call", toolCallId: "tool" }]),
+    );
+    expect(
+      bridge.outbound(
+        update({
+          sessionUpdate: "async_task_state_update",
+          asyncTaskId: "native",
+          state: "completed",
+        }),
+      ),
+    ).toMatchObject([
+      { params: { update: { status: "completed", capabilities: null } } },
+      expected([]),
+    ]);
+    expect(
+      bridge.outbound(
+        update({
+          sessionUpdate: "async_task_state_update",
+          asyncTaskId: "native",
+          state: "stopped",
+        }),
+      ),
+    ).toMatchObject([expected([])]);
+    expect(
+      bridge.inbound({
+        id: 4,
+        method: "tool_call/cancel",
+        params: { sessionId: "s", toolCallId: "tool" },
+      }),
+    ).toMatchObject({ send: [{ error: { code: -32602 } }] });
+  });
+  it("does not revive a tool finalized before late native discovery", () => {
+    const bridge = fixture();
+    bridge.outbound(
+      update({
+        sessionUpdate: version === 1 ? "tool_call" : "tool_call_update",
+        toolCallId: "late",
+        status: "completed",
+      }),
+    );
+    bridge.outbound(level(["native"]));
+    const messages = bridge.outbound(
+      update({
+        sessionUpdate: "async_task_spawned",
+        asyncTaskId: "native",
+        taskType: "shell",
+        toolCallId: "late",
+        canStop: true,
+      }),
+    );
+    expect(messages).toContainEqual(expect.objectContaining(expected(null)));
+    expect(messages).not.toContainEqual(
+      expect.objectContaining({
+        params: expect.objectContaining({
+          update: expect.objectContaining({ status: "in_progress" }),
+        }),
+      }),
+    );
+    expect(
+      bridge.inbound({
+        id: 30,
+        method: "tool_call/cancel",
+        params: { sessionId: "s", toolCallId: "late" },
+      }),
+    ).toMatchObject({ send: [{ error: { code: -32602 } }] });
+  });
+  it("does not claim scheduled workflows even when they name an announced tool", () => {
+    const bridge = fixture();
+    bridge.outbound(
+      update({
+        sessionUpdate: version === 1 ? "tool_call" : "tool_call_update",
+        toolCallId: "scheduled",
+        status: "pending",
+      }),
+    );
+    bridge.outbound(level(["workflow"]));
+    const messages = bridge.outbound(
+      update({
+        sessionUpdate: "async_task_spawned",
+        asyncTaskId: "workflow",
+        taskType: "workflow",
+        toolCallId: "scheduled",
+        canStop: true,
+      }),
+    );
+    expect(messages.at(-1)).toMatchObject(expected(null));
+    expect(
+      bridge.inbound({
+        id: 31,
+        method: "tool_call/cancel",
+        params: { sessionId: "s", toolCallId: "scheduled" },
+      }),
+    ).toMatchObject({ send: [{ error: { code: -32602 } }] });
+    expect(
+      bridge.outbound(
+        update({ sessionUpdate: "tool_call_update", toolCallId: "scheduled", status: "pending" }),
+      )[0],
+    ).toMatchObject({ params: { update: { status: "pending" } } });
+  });
+  it("does not fabricate referenced tools during resume and starts forks empty", () => {
+    const bridge = fixture();
+    bridge.inbound({
+      id: 8,
+      method: "session/resume",
+      params: { sessionId: "restored" },
+    });
+    expect(bridge.outbound({ id: 8, result: {} })[0]).toMatchObject({
+      params: { sessionId: "restored", update: { background: null } },
+    });
+    bridge.inbound({
+      id: 9,
+      method: "session/fork",
+      params: { sessionId: "s" },
+    });
+    expect(bridge.outbound({ id: 9, result: { sessionId: "fork" } })[1]).toMatchObject({
+      params: { sessionId: "fork", update: { background: [] } },
+    });
+  });
+  it("leaves the stream untouched when disabled", () => {
+    const stream = {
+      readable: new ReadableStream(),
+      writable: new WritableStream(),
+    };
+    expect(backgroundWorkStream(stream)).toBe(stream);
+  });
+});
--- a/src/tests/create-session-options.test.ts
+++ b/src/tests/create-session-options.test.ts
@@ -7,9 +7,7 @@ import {
 import type { ClientCapabilities } from "@agentclientprotocol/sdk/unstable";
 import { getSessionMessages, type Options } from "@anthropic-ai/claude-agent-sdk";
 import type { AcpClient, ClaudeAcpAgent as ClaudeAcpAgentType } from "../acp-agent.js";
-import type { AcpSessionNotification } from "../acp-subagents.js";
 import { ALLOW_BYPASS } from "../permissions/modes.js";
-import { NativeSubagentRuntime } from "../native-subagents.js";
 import * as fs from "node:fs";
 import * as os from "node:os";
 import * as path from "node:path";
@@ -187,122 +185,6 @@ describe("createSession options merging", () => {
    ]);
  });

-  it.each([false, true])("reports child settings through registered hooks (v2: %s)", async (v2) => {
-    const updates: AcpSessionNotification[] = [];
-    const client = createMockClient();
-    client.sessionUpdate = async (notification) => {
-      updates.push(notification);
-    };
-    const observedAgent = new ClaudeAcpAgent(client, undefined, { v2 });
-    await observedAgent.initialize({ protocolVersion: 1, clientCapabilities: { subagents: {} } });
-    const { sessionId } = await observedAgent.newSession({ cwd: process.cwd(), mcpServers: [] });
-    const session = observedAgent.sessions[sessionId]!;
-    const parentOptions = structuredClone(session.configOptions);
-    const runtime = (session.nativeSubagentRuntime = new NativeSubagentRuntime(
-      "rfd",
-      sessionId,
-      session,
-      (notification) => client.sessionUpdate(notification),
-      { log: () => {} },
-    ));
-    const invoke = async (
-      event: "PreToolUse" | "PostToolUse" | "PostModelSwitch" | "SubagentStart" | "SubagentStop",
-      fields: Record<string, unknown>,
-    ) => {
-      const matcher = capturedOptions!.hooks![event]!.find((matcher) => !matcher.matcher);
-      expect(matcher).toBeDefined();
-      for (const hook of matcher!.hooks) {
-        await expect(
-          hook(
-            {
-              hook_event_name: event,
-              session_id: sessionId,
-              cwd: process.cwd(),
-              transcript_path: "/tmp/transcript",
-              ...fields,
-            } as any,
-            undefined,
-            { signal: new AbortController().signal },
-          ),
-        ).resolves.toMatchObject({ continue: true });
-      }
-    };
-    // A main-thread --agent persona has agent_type too, but no agent_id.
-    await invoke("PreToolUse", {
-      agent_type: "reviewer",
-      permission_mode: "plan",
-      effort: { level: "max" },
-    });
-    expect(updates).toEqual([]);
-    await invoke("SubagentStart", {
-      agent_id: "child",
-      agent_type: "Explore",
-      permission_mode: "acceptEdits",
-    });
-    expect(updates).toEqual([]);
-    await runtime.taskStarted({ taskId: "child", subagentType: "Explore" }, async () => {});
-    const configs = () =>
-      updates.flatMap(({ update }) =>
-        update.sessionUpdate === "config_option_update" ? [update.configOptions] : [],
-      );
-    expect(
-      configs()
-        .at(-1)
-        ?.map((option) => [option.id, option.currentValue]),
-    ).toEqual([
-      ["mode", "acceptEdits"],
-      ["agent", "Explore"],
-    ]);
-    await invoke("PreToolUse", {
-      agent_id: "child",
-      agent_type: "Explore",
-      effort: { level: "high" },
-    });
-    expect(
-      configs()
-        .at(-1)
-        ?.find((option) => option.id === "effort"),
-    ).toMatchObject({
-      category: "thought_level",
-      currentValue: "high",
-      options: [{ value: "high", name: "High" }],
-    });
-    await invoke("PostToolUse", {
-      agent_id: "child",
-      tool_name: "EnterPlanMode",
-      permission_mode: "plan",
-    });
-    await invoke("PostModelSwitch", {
-      agent_id: "child",
-      to_model: "child-model",
-      source: "auto",
-      effort: { level: "low" },
-    });
-    await invoke("SubagentStop", { agent_id: "child", agent_type: "Explore" });
-    expect(
-      configs()
-        .at(-1)
-        ?.map((option) => [option.id, option.currentValue]),
-    ).toEqual([
-      ["mode", "plan"],
-      ["model", "child-model"],
-      ["effort", "low"],
-      ["agent", "Explore"],
-    ]);
-    expect(
-      configs()
-        .at(-1)
-        ?.every((option) => option.type === "select" && option.options.length === 1),
-    ).toBe(true);
-    await new Promise((resolve) => setImmediate(resolve));
-    expect(session.configOptions).toEqual(parentOptions);
-    expect(
-      updates
-        .filter(({ update }) => update.sessionUpdate === "config_option_update")
-        .every(({ sessionId: target }) => target === "child"),
-    ).toBe(true);
-  });
-
  it("merges user-provided disallowedTools with ACP internal list", async () => {
    await agent.newSession({
      cwd: process.cwd(),
--- a/src/tests/native-subagents.test.ts
+++ b/src/tests/native-subagents.test.ts
@@ -842,221 +842,6 @@ describe("NativeSubagentRuntime in the RFD form", () => {
      update.sessionUpdate === "subagent_update" && update.state ? [update.state] : [],
    );

-  it("reports only the observed child model and refreshes it on a model change", async () => {
-    const { runtime, published } = await runningWorker({ model: "inherit" });
-    const deliver = async (notification: AcpSessionNotification) => {
-      const routed = await runtime.route(notification, deliver);
-      if (routed) published.push(routed);
-    };
-    await runtime.reportModel(null, "parent-model", deliver);
-    await runtime.reportModel("launch-1", "<synthetic>", deliver);
-    expect(published.some(({ update }) => update.sessionUpdate === "config_option_update")).toBe(
-      false,
-    );
-
-    await runtime.reportModel("launch-1", "child-model", deliver);
-    await runtime.reportModel("launch-1", "child-model", deliver);
-    await runtime.finishTask("worker-1", "completed", deliver, "launch-1");
-    await runtime.taskStarted(
-      { taskId: "worker-1", toolUseId: "resume-1", prompt: "Continue" },
-      deliver,
-    );
-    await runtime.reportModel("resume-1", "fallback-model", deliver);
-
-    const configs = published.filter(
-      ({ update }) => update.sessionUpdate === "config_option_update",
-    );
-    expect(configs).toHaveLength(2);
-    for (const [index, model] of ["child-model", "fallback-model"].entries()) {
-      expect(configs[index]).toMatchObject({
-        sessionId: "worker-1",
-        update: {
-          sessionUpdate: "config_option_update",
-          configOptions: [
-            {
-              id: "model",
-              category: "model",
-              type: "select",
-              currentValue: model,
-              options: [{ value: model, name: model }],
-            },
-          ],
-        },
-      });
-    }
-  });
-
-  it("buffers model observations until the child is announced", async () => {
-    const published: AcpSessionNotification[] = [];
-    const runtime = new NativeSubagentRuntime(
-      "rfd",
-      "root",
-      {},
-      async (notification) => {
-        published.push(notification);
-      },
-      { log: () => {} },
-    );
-    const deliver = async (notification: AcpSessionNotification) => {
-      const routed = await runtime.route(notification, deliver);
-      if (routed) published.push(routed);
-    };
-    await runtime.reportModel("launch-1", "child-model", deliver);
-    expect(published).toEqual([]);
-    await runtime.taskStarted(
-      { taskId: "worker-1", toolUseId: "launch-1", subagentType: "Explore" },
-      deliver,
-    );
-    expect(published).toEqual([]);
-    await deliver(launch("launch-1", {}));
-    expect(published.map(({ update }) => update.sessionUpdate)).toEqual([
-      "subagent_update",
-      "config_option_update",
-    ]);
-    expect(published[1]?.sessionId).toBe("worker-1");
-  });
-
-  const effortOption = (level: string) => ({
-    id: "effort",
-    name: "Effort",
-    category: "thought_level",
-    type: "select" as const,
-    currentValue: level,
-    options: [{ value: level, name: level }],
-  });
-
-  it("merges hook observations with model snapshots without inheriting other sessions", async () => {
-    const { runtime, published } = await runningWorker();
-    const deliver = async (notification: AcpSessionNotification) => {
-      const routed = await runtime.route(notification, deliver);
-      if (routed) published.push(routed);
-    };
-    await runtime.reportConfiguration("worker-1", [
-      effortOption("high"),
-      {
-        id: "mode",
-        name: "Mode",
-        category: "mode",
-        type: "select",
-        currentValue: "plan",
-        options: [{ value: "plan", name: "Plan" }],
-      },
-    ]);
-    await runtime.reportModel("launch-1", "child-model", deliver);
-    await runtime.reportConfiguration("worker-1", [effortOption("low")]);
-    await runtime.reportModel("launch-1", "fallback-model", deliver);
-    const configs = published.flatMap(({ update }) =>
-      update.sessionUpdate === "config_option_update" ? [update.configOptions] : [],
-    );
-    expect(configs).toHaveLength(4);
-    expect(configs.at(-1)?.map((option) => [option.id, option.currentValue])).toEqual([
-      ["mode", "plan"],
-      ["model", "fallback-model"],
-      ["effort", "low"],
-    ]);
-    expect(
-      configs.at(-1)?.every((option) => option.type === "select" && option.options.length === 1),
-    ).toBe(true);
-    await runtime.reportConfiguration("root", [effortOption("max")]);
-    expect(
-      published.filter(({ update }) => update.sessionUpdate === "config_option_update"),
-    ).toHaveLength(4);
-  });
-
-  it("keeps hook observations pending until truthful parentage is announced", async () => {
-    const published: AcpSessionNotification[] = [];
-    const runtime = new NativeSubagentRuntime(
-      "rfd",
-      "root",
-      {},
-      async (notification) => {
-        published.push(notification);
-      },
-      { log: () => {} },
-    );
-    await runtime.reportConfiguration("worker-1", [effortOption("high")]);
-    expect(published).toEqual([]);
-    await runtime.taskStarted(
-      { taskId: "worker-1", toolUseId: "launch-1", subagentType: "Explore" },
-      async () => {},
-    );
-    expect(published).toEqual([]);
-    await runtime.route(launch("launch-1", {}), async () => {});
-    expect(published.map(({ sessionId, update }) => [sessionId, update.sessionUpdate])).toEqual([
-      ["root", "subagent_update"],
-      ["worker-1", "config_option_update"],
-    ]);
-  });
-
-  it("serializes concurrent config publications and retries a failed snapshot", async () => {
-    const started = deferred();
-    const release = deferred();
-    const published: AcpSessionNotification[] = [];
-    const value = child({ announced: true });
-    let fail = false;
-    const runtime = new NativeSubagentRuntime(
-      "rfd",
-      "root",
-      sessionWith(value),
-      async (notification) => {
-        if (fail) throw new Error("transport failed");
-        published.push(notification);
-        if (published.length === 1) {
-          started.resolve();
-          await release.promise;
-        }
-      },
-      { log: () => {} },
-    );
-    const first = runtime.reportConfiguration(value.sessionId, [effortOption("high")]);
-    await started.promise;
-    const second = runtime.reportConfiguration(value.sessionId, [effortOption("low")]);
-    release.resolve();
-    await Promise.all([first, second]);
-    expect(
-      published.flatMap(({ update }) =>
-        update.sessionUpdate === "config_option_update"
-          ? update.configOptions.map((option) => option.currentValue)
-          : [],
-      ),
-    ).toEqual(["high", "low"]);
-    fail = true;
-    await expect(
-      runtime.reportConfiguration(value.sessionId, [effortOption("max")]),
-    ).rejects.toThrow("transport failed");
-    fail = false;
-    await runtime.reportConfiguration(value.sessionId, [effortOption("max")]);
-    expect(published).toHaveLength(3);
-  });
-
-  it("forgets unannounced configuration when the runtime is cleared", async () => {
-    const published: AcpSessionNotification[] = [];
-    const runtime = new NativeSubagentRuntime(
-      "rfd",
-      "root",
-      {},
-      async (notification) => {
-        published.push(notification);
-      },
-      { log: () => {} },
-    );
-    await runtime.reportConfiguration("worker-1", [effortOption("high")]);
-    runtime.clear();
-    await runtime.taskStarted({ taskId: "worker-1", subagentType: "Explore" }, async () => {});
-    expect(published.map(({ update }) => update.sessionUpdate)).toEqual(["subagent_update"]);
-  });
-
-  it.each(["air", undefined] as const)(
-    "does not report child config in the %s form",
-    async (form) => {
-      const deliver = vi.fn(async () => {});
-      const runtime = new NativeSubagentRuntime(form, "root", {}, deliver, { log: () => {} });
-      await runtime.reportModel("launch-1", "child-model", deliver);
-      await runtime.reportConfiguration("worker-1", [effortOption("high")]);
-      expect(deliver).not.toHaveBeenCalled();
-    },
-  );
-
  it("hides the Agent tool call, which the parent's session messages replace", async () => {
    const { runtime, control } = await runningWorker();

--- a/src/tests/subagent-history.test.ts
+++ b/src/tests/subagent-history.test.ts
@@ -41,8 +41,7 @@ const launch = (uuid: string, launchId: string) => ({
 });
 /**
 * The directive message that Claude Code writes after the copy of the launching
- * message in a `subagent_type: "fork"` child's transcript, with the tool_result
- * text of Claude Code before 2.1.307.
+ * message in a `subagent_type: "fork"` child's transcript.
 */
 const forkDirective = (uuid: string, launchIds: string[], prompt: string) => ({
  ...base,
--- a/src/tools.ts
+++ b/src/tools.ts
@@ -431,7 +431,7 @@ export const createPostToolUseHook =
  async (input: any, toolUseID: string | undefined): Promise<{ continue: boolean }> => {
    if (input.hook_event_name === "PostToolUse") {
      // Handle EnterPlanMode tool - notify client of mode change after successful execution
-      if (input.tool_name === "EnterPlanMode" && !input.agent_id && options?.onEnterPlanMode) {
+      if (input.tool_name === "EnterPlanMode" && options?.onEnterPlanMode) {
        await options.onEnterPlanMode();
      }

--- a/src/usage-markdown.ts
+++ b/src/usage-markdown.ts
@@ -8,18 +8,10 @@ const usageWindowSchema = z
  .nullable()
  .optional();
 const contributionSchema = z.object({ name: z.string(), pct: percentSchema });
-// Claude Code 2.1.305+ suggests one change per notable source, largest share first.
-const recommendationSchema = z.object({
-  key: z.string(),
-  pct: percentSchema,
-  title: z.string(),
-  detail: z.string().optional(),
-});
 const behaviorPeriodSchema = z.object({
  request_count: countSchema,
  session_count: countSchema,
  mcp_servers: z.array(contributionSchema),
-  recommendations: z.array(recommendationSchema).optional(),
 });
 const modelUsageSchema = z.object({
  inputTokens: countSchema,
@@ -141,23 +133,12 @@ function appendContributions(
    request_count: number;
    session_count: number;
    mcp_servers: { name: string; pct: number }[];
-    recommendations?: { pct: number; title: string; detail?: string }[];
  },
 ): void {
  lines.push(
    "",
    `**${label}** · ${period.request_count} requests · ${period.session_count} sessions`,
  );
-  // Rendered in the order Claude Code sends them, as its own `/usage` does. The
-  // window's `fix` is left out: its messages must be queued at priority "later"
-  // with `client_composed` set, which an ACP prompt cannot express.
-  if (period.recommendations?.length) {
-    lines.push("");
-    for (const recommendation of period.recommendations) {
-      const detail = recommendation.detail ? ` · ${escapeMarkdown(recommendation.detail)}` : "";
-      lines.push(`- **${recommendation.pct}%** ${escapeMarkdown(recommendation.title)}${detail}`);
-    }
-  }
  if (period.mcp_servers.length === 0) return;
  lines.push("", "| MCP server | Usage |", "|:--|--:|");
  for (const server of [...period.mcp_servers].sort((a, b) => b.pct - a.pct).slice(0, 3)) {
--- a/src/v2/agent.ts
+++ b/src/v2/agent.ts
@@ -14,6 +14,7 @@ import type {
  RequestPermissionResponse,
 } from "@agentclientprotocol/sdk/unstable";
 import * as v2 from "@agentclientprotocol/sdk/experimental/v2/unstable";
+import { z } from "zod";
 import { ClaudeAcpAgent, type AcpClient, type Logger } from "../acp-agent.js";
 import type { AcpSessionNotification } from "../acp-subagents.js";
 import type { AcpPermissionRequest } from "../permissions/presentation.js";
@@ -129,7 +130,16 @@ export function v2AgentApp(
        void client.send({ sessionId: params.sessionId, update });
      }),
    )
-    .onNotification(v2.methods.agent.session.cancel, ({ params }) => agent.cancel(params));
+    .onNotification(v2.methods.agent.session.cancel, ({ params }) => agent.cancel(params))
+    .onRequest(
+      "_session/async_task/stop",
+      z.object({ sessionId: z.string(), asyncTaskId: z.string() }),
+      ({ params }) => {
+        if (process.env.ACP_BACKGROUND_WORK_EXPERIMENT !== "1")
+          throw new Error("Background work experiment is disabled");
+        return agent.stopAsyncTask(params);
+      },
+    );
 }

 /** The `state_update` that reports `state` as the current foreground state. */
--- a/src/v2/session-update.ts
+++ b/src/v2/session-update.ts
@@ -76,6 +76,15 @@ export function v2SessionUpdate(
      // `configOptions` of a `session/set_config_option` response.
      return undefined;
    default:
+      if (
+        process.env.ACP_BACKGROUND_WORK_EXPERIMENT === "1" &&
+        update.sessionUpdate.startsWith("async_task_")
+      ) {
+        return {
+          ...update,
+          sessionUpdate: `_${update.sessionUpdate}`,
+        } as unknown as v2.SessionUpdate;
+      }
      throw new Error(
        `The ACP v2 surface does not translate ${update.sessionUpdate} session updates yet`,
      );