0c8de1380f
Found by running the ACP TCK's v2 suite against the adapter (ACP-ADDDIRS-202). The code is shared, so it also fixes v1's `session/load` and `session/resume`. ## The bug A `session/resume` or `session/load` can change a live session's settings: its `cwd`, additional directories, MCP servers, or the other settings in its fingerprint. When that happens, `getOrCreateSession` tears the live query down and starts a new one with `resume: sessionId`. Claude Code writes the conversation only at the first prompt, so for a session that was never prompted, the resume failed with `-32002` "Resource not found". The live session was already torn down by then, so the client lost a session it had a moment earlier. ## The fix The sign-out path already handled the same case: it tries `resume`, and on `resourceNotFound` it starts a fresh query under the same id with `reuseSessionId`. That logic is now one helper, `recreateLiveSession`, used by both: - **`getOrCreateSession`** uses it when the session was live. A session that was never prompted then starts a fresh query under its own id. It loses no history, because it had none, and it keeps its mode. - **A session that isn't loaded here** still resumes directly. If it has no stored conversation, that still fails with `-32002`, since the session really is unknown. - **`recreateSignedOutQuery`** keeps its own handling: if both attempts fail, it puts the old session object back. ## Checks - **ACP TCK v2** (`agentclientprotocol/acp-tck` at `fa00ccd`) against this build: ACP-ADDDIRS-202 now passes, where it failed before. ACP-ADDDIRS-201 and ACP-RESUME-201 to 205 still pass. - **Live v1 `session/load`**, Claude Code 2.1.296: create a session, then load it with a new additional directory before any prompt. - On `main`, the load failed with `-32002`. - With this fix, the load succeeds. A prompt then read a file that exists only in the new directory, and the transcript was saved under the same session id. - **Tests:** three new tests: - a live session that was never prompted starts a fresh query with its mode; - an unknown session still fails; - another failure of a live session's resume doesn't start a fresh query. Removing the fallback, applying it to every error, or applying it to sessions that aren't loaded each fails a test. The sign-out tests pass through the shared helper. - **Suite:** `npm run check`, build and all 3068 tests pass, with no golden changes. **Not covered:** a model chosen with `session/set_config_option` before the first prompt resets to the default when such a session is recreated, as it already did after a sign-out. The mode is kept.
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. Background children keep running, as the RFD prefers; a client stops one with its owncancel. Each stopped child reportsidlewithcancelledfrom Claude Code's own events, and its open requests 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. To spare background children, the adapter declares the SDK optionperTaskStopAffordance, which a client can turn off in_meta.claudeCode.options. That option spares background workflows too, which a client cannot stop, so the adapter stops them after the interrupt, as Claude Code did without it. Background commands keep running after a cancel either way. 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).