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## Summary The draft ACP v2 is no longer opt-in. The adapter always connects through the SDK's protocol router (as codex-acp does), so a client that asks for protocol version 2 or later in `initialize` gets v2, and every other client gets v1 as before. v2 is still experimental and follows the draft as it changes. - Removes `CLAUDE_AGENT_ACP_EXPERIMENTAL_V2`, which only turned on what is now the default. - Adds `CLAUDE_AGENT_ACP_PROTOCOL` to limit the adapter to one version: - unset, empty, or `auto`: both, picked by the client's `initialize`. - `v1`: a client that asks for v2 is negotiated down to v1. This is the escape hatch if a client's v2 handling breaks. - `v2`: a v1 client's `initialize` is rejected with `-32022 Unsupported protocol version`. - Any other value fails at startup, so a typo like `CLAUDE_AGENT_ACP_PROTOCOL=1` doesn't silently serve both. - Fixes the comment in `src/acp-subagents.ts` that said v2 clients don't get subagents. Note: a client that already sends `protocolVersion >= 2` switches to the v2 surface as soon as this lands on `main`, which publishes a preview. ## Testing - New routing tests: down-negotiation to v1, rejecting v1 when limited to v2, env parsing. - `npm run check`, `npm run build`, `npm run test:run` pass. - Ran the built binary over stdio with each setting: default → v2 for a v2 client, `v1` → protocol version 1, `v2` → `-32022` for a v1 client, invalid value → exits with an error at startup. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
117 lines
8.2 KiB
Markdown
117 lines
8.2 KiB
Markdown
# ACP adapter for the Claude Agent SDK
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[](https://www.npmjs.com/package/@agentclientprotocol/claude-agent-acp)
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Use [Claude Agent SDK](https://platform.claude.com/docs/en/agent-sdk/overview#branding-guidelines) from [ACP-compatible](https://agentclientprotocol.com) clients!
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This tool implements an ACP agent by using the official [Claude Agent SDK](https://platform.claude.com/docs/en/agent-sdk/overview), supporting:
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- Context @-mentions
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- Images
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- Tool calls (with permission requests)
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- Compact file changes through the negotiated [AIR diff patch extension](docs/air-extensions.md#diff-patch)
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- Following
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- Edit review
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- TODO lists
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- Nested subagent transcripts
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- Interactive (and background) terminals
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- Custom [Slash commands](https://docs.anthropic.com/en/docs/claude-code/slash-commands)
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- Client MCP servers
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- `/mcp` in the chat: the MCP server status as a list. The adapter runs `/mcp reconnect`, `/mcp enable`, and `/mcp disable` through 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
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- Session-scoped long-running goals for AIR through the [goal extension](docs/air-extensions.md#goal) under `_meta.jetbrains.air.goal`
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- Structured errors, recovery, and warnings through the opt-in [session failure extension](docs/air-extensions.md#session-failure)
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- Concrete model and effort defaults through the opt-in [recommended config value extension](docs/air-extensions.md#recommended-config-values)
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- Tool permission presentation, editable choices, and durable effects through the [permission extension](docs/air-extensions.md#permission-presentation)
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- One fact per field in every tool call report, as the [ACP tool call contract](docs/air-extensions.md#tool-call-contract) defines
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- All AIR extensions, capabilities, and `_meta` keys: [AIR extensions](docs/air-extensions.md)
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Learn more about the [Agent Client Protocol](https://agentclientprotocol.com/).
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To try changes that have landed on `main` but are not released yet, install from the
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`preview` channel — every push to `main` publishes one. See
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[`docs/RELEASES.md`](docs/RELEASES.md#preview-releases).
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```sh
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npm install @agentclientprotocol/claude-agent-acp@preview
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```
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### Subagent sessions
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A client that declares `clientCapabilities.subagents`, other than JetBrains AIR, gets each Agent or
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Task subagent as its own child session, following the [subagents RFD](https://github.com/agentclientprotocol/agent-client-protocol/blob/main/docs/rfds/subagents.mdx):
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- `subagent_update` on the parent announces the child before any of its traffic, with the
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parent's `title` and `description` for it, and reports its work state: `running`,
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`requires_action` while one of its requests is open (and `running` again when the last one ends,
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unless a cancel withdrew it: the child is stopping then), and `idle` when its work ends, with
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`end_turn` or `cancelled` when the SDK gives the reason. A failure is `idle` with the `error`
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stop reason and the SDK's error text as its `error`. The child keeps its session for every
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delegation to it.
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- The prompt of the Agent or Task call that launches the child, and the message of each
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SendMessage call that resumes it, is a `session_message` in the child's transcript. The tool
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call stays a tool call of the session that made it.
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- `session/cancel` on the parent interrupts Claude Code, which stops the foreground children that
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the turn waits on. Background children keep running, as the RFD prefers; a client stops one with
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its own `cancel`. Each stopped child reports `idle` with `cancelled` from Claude Code's own
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events, and its open requests are withdrawn. The adapter ends no child at a cancel itself, so a
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child that keeps running keeps its open requests and its open tool calls. To spare background
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children, the adapter declares the SDK option `perTaskStopAffordance`, which a client can turn
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off in `_meta.claudeCode.options`. That option spares background workflows too, which a client
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cannot stop, so the adapter stops them after the interrupt, as Claude Code did without it.
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Background commands keep running after a cancel either way. A tool call of a stopped child that Claude Code reports no
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result for, such as the command it was running, ends in the child's session before the cancelled
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prompt resolves. When Claude Code stops a background child after that, or while no prompt runs,
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the tool call ends right after the child reports `idle`. A child that still runs when Claude Code
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exits is `idle` with no stop reason.
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- A child whose parent is not yet known when it ends is not exposed, since the RFD forbids
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guessing its parent.
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- A replay restores each child under its live session id, from its own transcript: its prompt and
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each SendMessage message are `session_message`s, and its end is an `idle` before the response.
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A launch without Claude Code's metadata file for the child stays an ordinary tool call of the
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parent, and a notice, for a client that takes notices, says that subagent history is missing.
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- After a load or resume, with or without replay, a SendMessage that resumes an agent from before
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it runs that agent in its session from before. Before the agent's traffic, each of its ancestors
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that the client was not told about in this connection is announced, parent before child and
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with no work state, as the RFD requires. An agent whose launch the session's history does not
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record stays unexposed.
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- Each running child is announced with the `cancel` capability. `session/cancel` with the child's
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session id stops that child's current work, through the SDK's `Query.stopTask`, and not its
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parent's: Claude Code also stops the foreground children it waits on, and the parent's turn goes
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on. Each stopped child reports `idle` with `cancelled` from Claude Code's own events, and its
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requests that the stop withdraws report no `running` first. A replayed child, which no longer
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runs, gets no capabilities. A client cannot prompt or otherwise change a child.
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An [ACP v2](#experimental-acp-v2) client gets the same without declaring anything, since subagents
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are part of v2's baseline. Each child also reports its work state as its own `state_update`, right
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after the parent's copy.
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JetBrains AIR keeps the earlier draft of the RFD (`subagent_spawned`, `subagent_state_update`),
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which it enables with `nativeSubagentSessions` in `_meta.jetbrains.air.capabilities`. See
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[AIR extensions](docs/air-extensions.md#native-subagent-sessions).
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Without either signal, Agent and Task stay ordinary tool calls and child interactions stay on the
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root session. Clients that use the historical `_meta["subagent-transcript"]` capability or the
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`forwardSubagentText` session option retain the flattened child transcript.
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### Experimental ACP v2
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The adapter also serves the [draft ACP v2](https://agentclientprotocol.com/protocol/v2/overview):
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a client that asks for protocol version 2 or later in `initialize` gets v2, and every other client
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gets v1. The draft can still change, and this surface changes with it.
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`CLAUDE_AGENT_ACP_PROTOCOL` limits the adapter to one version. With `v1`, a client that asks
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for v2 is negotiated down to v1. With `v2`, the adapter rejects the `initialize` of a v1 client.
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`auto`, the default, serves both.
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On v2 the adapter serves the session baseline (`session/new`, `session/list`, `session/resume`
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with replay, `session/close`, `session/prompt`, `session/cancel`), `session/delete`,
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`session/fork`, additional directories, config options (the mode is one), plans, notices,
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compaction, display terminals for commands, permission requests, elicitation, providers, auth, and
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[subagents](#subagent-sessions). Steering and the AIR extensions are v1 only for now.
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## Contribution Policy
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This project does not require a Contributor License Agreement (CLA). Instead, contributions are accepted under the following terms:
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> By contributing to this project, you agree that your contributions will be licensed under the [Apache License, Version 2.0](https://www.apache.org/licenses/LICENSE-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).
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