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New agent runtime expectations: ACP over stdio

This note documents the preferred integration shape for a new Open Design agent runtime.

Recommendation

New agent runtimes should expose an ACP over stdio CLI mode.

In practice, Open Design expects to spawn a local executable and speak JSON-RPC over the child process streams:

Open Design daemon
  └─ spawn your-agent acp
       ├─ stdin  <- ACP JSON-RPC requests/responses
       ├─ stdout -> ACP JSON-RPC responses/notifications
       └─ stderr -> logs and diagnostics only

If the runtime's real implementation is a local or remote server, keep that detail behind a thin CLI wrapper:

your-agent acp
  └─ connects to your runtime server / SDK / model backend

That wrapper keeps Open Design on the standard ACP subprocess transport and avoids requiring a daemon-side network transport adapter.

Why stdio, not an ACP server?

The ACP protocol uses JSON-RPC, but transport matters.

The ACP transport documentation defines stdio as communication over standard input and standard output. In that transport, the client launches the agent as a subprocess, the agent reads from stdin, writes protocol messages to stdout, and writes logs to stderr.

ACP's remote HTTP/WebSocket transport is still described as a draft/proposal rather than the established compatibility path. Open Design's implemented ACP adapters therefore use stdio subprocesses today.

Messages Open Design sends

For streamFormat: 'acp-json-rpc', Open Design currently drives a session with these JSON-RPC methods:

  1. initialize
    • Sent first.
    • Includes Open Design client metadata and clientCapabilities.
  2. session/new
    • Creates a working session.
    • Includes the project working directory.
    • May include MCP server descriptors when the runtime is allowed to use Open Design-provided tools.
  3. session/set_config_option or session/set_model (optional)
    • Sent when the user selected a non-default model.
    • Open Design prefers session/set_config_option when session/new reports a model config option; otherwise it falls back to session/set_model.
  4. session/prompt
    • Sends the composed user/system prompt as text content.
    • A successful response marks the prompt as complete.
  5. session/cancel
    • Sent on user cancellation when a session exists and stdin is still writable.

Messages Open Design expects from the agent

The runtime should support the corresponding JSON-RPC responses and notifications:

  1. Response to initialize.
  2. Response to session/new.
    • Must include a usable sessionId.
    • Should report the current model if available.
    • Should report model config options if model selection is supported through config options.
  3. Notifications using session/update.
    • Open Design currently maps:
      • agent_thought_chunk to thinking output.
      • agent_message_chunk to assistant text output.
  4. Optional session/request_permission requests.
    • Open Design auto-selects an approve/allow-style option when available.
    • If no acceptable option is present, the turn fails fast.
  5. Response to session/prompt.
    • Should include usage metadata when available.
    • This response tells Open Design the turn is finished.

Process lifecycle expectations

  • Keep protocol messages on stdout parseable as JSON-RPC lines.
  • Write human-readable logs and diagnostics to stderr.
  • Return clear JSON-RPC errors for protocol failures.
  • After session/prompt completes, either exit cleanly when stdin closes or tolerate Open Design sending SIGTERM after a short grace period.
  • Implement session/cancel if possible. Open Design falls back to process termination when the transport is no longer usable.
  • Avoid interactive terminal prompts. If permission is required, use ACP permission requests instead.

Open Design adapter shape

An ACP runtime definition in Open Design is intentionally small:

export const myAgentDef = {
  id: 'my-agent',
  name: 'My Agent',
  bin: 'my-agent',
  versionArgs: ['--version'],
  fallbackModels: [{ id: 'default', label: 'default' }],
  buildArgs: () => ['acp'],
  streamFormat: 'acp-json-rpc',
} satisfies RuntimeAgentDef;

Existing examples include Devin, Hermes, Kimi, Kiro, Kilo, and Vibe runtime definitions under apps/daemon/src/runtimes/defs/.

Fact sources

  • ACP transport documentation: https://agentclientprotocol.com/protocol/transports
    • ACP uses JSON-RPC.
    • The stdio transport uses standard input and standard output.
    • In stdio mode, the client launches the agent as a subprocess.
    • The agent reads from stdin, writes protocol messages to stdout, and uses stderr for logs.
  • ACP remote transport RFD: https://agentclientprotocol.com/rfds/streamable-http-websocket-transport
    • Describes Streamable HTTP / WebSocket as the proposed remote transport direction.
    • Notes that ACP's standard transport has historically been stdio and that a standard remote transport is still being defined.
  • Open Design implementation:
    • apps/daemon/src/acp.ts implements the ACP JSON-RPC session lifecycle.
    • apps/daemon/src/server.ts spawns ACP runtimes as child processes with piped stdio.
    • apps/daemon/src/runtimes/defs/*.ts contains existing ACP runtime definitions using streamFormat: 'acp-json-rpc'.