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aeb69e7ed1
A client on the subagents RFD can now stop one subagent without
cancelling the turn. Each running child is announced with `capabilities:
{ cancel: {} }`, and `session/cancel` with the child's session id stops
that child's current work.
## What Claude Code does
From direct SDK probes of `Query.stopTask(taskId)` and runs through the
adapter (Claude Code 2.1.293 and 2.1.296):
| Case | What Claude Code sends |
|---|---|
| Foreground subagent running a command | At once: `task_updated`
`killed` and `task_notification` `stopped` for the subagent, `stopped`
for its command's task, the command's interrupted result, then the Agent
call's interrupted result (`non_execution_kind: "interrupted"`). The
parent's turn goes on and ends normally. |
| Background subagent | The same events and result, then Claude Code's
follow-up cycle about the stop. |
| Subagent with an open permission request | The request is withdrawn
(its `AbortSignal` fires), and both calls get results. |
| Subagent waiting on its own foreground subagent | The grandchild is
stopped too, with its own `killed` and `stopped`, which is the cascade
the RFD requires. |
## The change
- **The grant:** `announceNativeSubagent` grants `cancel` to each live
RFD child. A replayed child gets no capabilities, since it no longer
runs (the RFD's freshness rule for reconnection). A routing-only
ancestor that a restore announces gets none either.
- **The cancel:** `cancel` checks first whether the session id is a
child of one of its sessions (`NativeSubagentRuntime.cancelChild`), and
if so calls `Query.stopTask` with the child's agent id. It sends no
interrupt, so the parent's work goes on. A stop that Claude Code refuses
is logged.
- **The report comes from the SDK's events,** `killed` or `stopped`,
which `finishTask` already turns into `idle` with `cancelled`. Accepting
the stop is no evidence, as the RFD says, and a cancel that races with a
child's end does not rewrite its outcome.
- **Requests:** a request that the stop withdraws reports no `running`
before `idle cancelled`. `awaitingUser` now counts cancels of the child
and of its ancestors, as it counted the root's (#1337). An ancestor's
cancel stops the foreground subagents under it.
- **Unchanged:** AIR keeps its draft and grants nothing, and a cancel of
a root session works as before.
## Evidence
**Live, through the adapter** (Claude Code 2.1.296, Sonnet), every
update checked against the schema, with no violations:
| Run | Child | Parent |
|---|---|---|
| v1 RFD, foreground child running a command | `running`,
`requires_action`, `running`, `idle cancelled` 9 ms after the cancel;
its command `failed` | Agent call `failed`; turn ends `end_turn` |
| v1 RFD, background child while v1 holds the turn | `idle cancelled`;
its command `failed` | the held turn settles with `end_turn` |
| v1 RFD, child with its permission request open | `requires_action`,
then `idle cancelled`, with no `running` between | turn ends `end_turn`
|
| v2, foreground child | `idle cancelled` on the root's stream and its
own | Agent call `cancelled`; root `idle end_turn` |
No tool call was left open in any run.
**Tests:**
- `native-subagents.test.ts`:
- `cancelChild` stops the child's task and reports nothing itself, and
the SDK's `killed` reports `idle cancelled`;
- no stop for a session that is no child, or for AIR;
- no `running` for a request that the child's own cancel withdraws, or
that its parent's cancel withdraws.
- `acp-agent.test.ts`: `session/cancel` on a child calls `stopTask`, not
`interrupt`, and the turn ends `end_turn`; a refused stop is logged;
nothing happens for an unknown session id or an AIR child.
- `acp-v2.test.ts`: the same over v2, with the child's `idle cancelled`
on both streams.
- **Golden files:** the six v2 subagent golden files gain `capabilities:
{ cancel: {} }` on each announcement, and nothing else. No AIR golden
file changes.
- **Mutation checks:** removing each of these fails at least one test:
- the grant;
- the routing to the child;
- the child's cancel count;
- the ancestors in the count;
- the AIR exclusion;
- the error handling.
So does reporting `idle cancelled` as soon as the stop is accepted.
`npm run check`, build and the full suite (3014 tests) pass.
## Not changed, and a decision
- **A parent cancel still stops background subagents.** Claude Code does
that because the adapter does not declare the SDK's
`perTaskStopAffordance` option. Declared, an interrupt spares background
agents and background shell commands (checked live: both ran to
completion after the interrupt), as
agentclientprotocol/agent-client-protocol#2308 prefers. Background shell
commands would then outlive a cancel too, and ACP has no control to stop
them, so declaring it is left for a separate decision.
- **Order:** Claude Code sends the stopped child's command result right
after its `killed` event, so the client sees `idle cancelled` a few
milliseconds before that call ends. The RFD has the client mark the
child's unfinished calls cancelled at that point and still accept later
updates.
- **On v2,** the stopped child's command ends `failed`, not `cancelled`:
Claude Code's result for a subagent's command carries no non-execution
kind.
ACP adapter for the Claude Agent SDK
Use Claude Agent SDK from ACP-compatible clients!
This tool implements an ACP agent by using the official Claude Agent SDK, supporting:
- Context @-mentions
- Images
- Tool calls (with permission requests)
- Compact file changes through the negotiated AIR diff patch extension
- Following
- Edit review
- TODO lists
- Nested subagent transcripts
- Interactive (and background) terminals
- Custom Slash commands
- Client MCP servers
/mcpin the chat: the MCP server status as a list. The adapter runs/mcp reconnect,/mcp enable, and/mcp disablethrough the SDK control API, because Claude Code refuses them in SDK mode. A reconnect of an ACP server that needs authentication starts MCP OAuth through URL elicitation- Session-scoped long-running goals for AIR through the goal extension under
_meta.jetbrains.air.goal - Structured errors, recovery, and warnings through the opt-in session failure extension
- Concrete model and effort defaults through the opt-in recommended config value extension
- Tool permission presentation, editable choices, and durable effects through the permission extension
- One fact per field in every tool call report, as the ACP tool call contract defines
- All AIR extensions, capabilities, and
_metakeys: AIR extensions
Learn more about the Agent Client Protocol.
To try changes that have landed on main but are not released yet, install from the
preview channel — every push to main publishes one. See
docs/RELEASES.md.
npm install @agentclientprotocol/claude-agent-acp@preview
Subagent sessions
A client that declares clientCapabilities.subagents, other than JetBrains AIR, gets each Agent or
Task subagent as its own child session, following the subagents RFD:
subagent_updateon the parent announces the child before any of its traffic, with the parent'stitleanddescriptionfor it, and reports its work state:running,requires_actionwhile one of its requests is open (andrunningagain when the last one ends, unless a cancel withdrew it: the child is stopping then), andidlewhen its work ends, withend_turnorcancelledwhen the SDK gives the reason. A failure isidlewith theerrorstop reason and the SDK's error text as itserror. The child keeps its session for every delegation to it.- The prompt of the Agent or Task call that launches the child, and the message of each
SendMessage call that resumes it, is a
session_messagein the child's transcript. The tool call stays a tool call of the session that made it. session/cancelon the parent interrupts Claude Code, which stops the foreground children that the turn waits on and, currently, background children too. Each stopped child reportsidlewithcancelledfrom Claude Code's own events, and the open requests of the foreground ones are withdrawn. The adapter ends no child at a cancel itself, so a child that keeps running keeps its open requests and its open tool calls. A tool call of a stopped child that Claude Code reports no result for, such as the command it was running, ends in the child's session before the cancelled prompt resolves. When Claude Code stops a background child after that, or while no prompt runs, the tool call ends right after the child reportsidle. A child that still runs when Claude Code exits isidlewith no stop reason.- A child whose parent is not yet known when it ends is not exposed, since the RFD forbids guessing its parent.
- A replay restores each child under its live session id, from its own transcript: its prompt and
each SendMessage message are
session_messages, and its end is anidlebefore the response. A launch without Claude Code's metadata file for the child stays an ordinary tool call of the parent, and a notice, for a client that takes notices, says that subagent history is missing. - After a load or resume, with or without replay, a SendMessage that resumes an agent from before it runs that agent in its session from before. Before the agent's traffic, each of its ancestors that the client was not told about in this connection is announced, parent before child and with no work state, as the RFD requires. An agent whose launch the session's history does not record stays unexposed.
- Each running child is announced with the
cancelcapability.session/cancelwith the child's session id stops that child's current work, through the SDK'sQuery.stopTask, and not its parent's: Claude Code also stops the foreground children it waits on, and the parent's turn goes on. Each stopped child reportsidlewithcancelledfrom Claude Code's own events, and its requests that the stop withdraws report norunningfirst. A replayed child, which no longer runs, gets no capabilities. A client cannot prompt or otherwise change a child.
An ACP v2 client gets the same without declaring anything, since subagents
are part of v2's baseline. Each child also reports its work state as its own state_update, right
after the parent's copy.
JetBrains AIR keeps the earlier draft of the RFD (subagent_spawned, subagent_state_update),
which it enables with nativeSubagentSessions in _meta.jetbrains.air.capabilities. See
AIR extensions.
Without either signal, Agent and Task stay ordinary tool calls and child interactions stay on the
root session. Clients that use the historical _meta["subagent-transcript"] capability or the
forwardSubagentText session option retain the flattened child transcript.
Experimental ACP v2
Started with CLAUDE_AGENT_ACP_EXPERIMENTAL_V2=1, the adapter also serves the
draft ACP v2: a client that asks for
protocol version 2 or later in initialize gets v2, and every other client gets v1, as before.
The draft can still change, and this surface changes with it.
On v2 the adapter serves the session baseline (session/new, session/list, session/resume
with replay, session/close, session/prompt, session/cancel), session/delete,
session/fork, additional directories, config options (the mode is one), plans, notices,
compaction, display terminals for commands, permission requests, elicitation, providers, auth, and
subagents. Steering and the AIR extensions are v1 only for now.
Contribution Policy
This project does not require a Contributor License Agreement (CLA). Instead, contributions are accepted under the following terms:
By contributing to this project, you agree that your contributions will be licensed under the Apache License, Version 2.0. You affirm that you have the legal right to submit your work, that you are not including code you do not have rights to, and that you understand contributions are made without requiring a Contributor License Agreement (CLA).