mcp

Bundled MCP extension

This package is the D18 addition to upstream pi: an MCP client built on the official Go SDK and registered through the same native ExtensionAPI used by every other extension. MCP does not enter the agent loop, provider layer, or built-in tool registry directly. With no enabled server in settings, the CLI does not construct the extension and performs no MCP work.

Settings

Add a top-level mcpServers object to global or trusted-project settings.json. Each key is the server's local name and each value selects exactly one transport:

{
  "mcpServers": {
    "local": {
      "command": "my-mcp-server",
      "args": ["--stdio"],
      "env": { "API_TOKEN": "..." },
      "cwd": ".",
      "timeoutMs": 10000
    },
    "remote": {
      "url": "https://example.test/mcp",
      "headers": { "Authorization": "Bearer ..." },
      "maxRetries": 2,
      "timeoutMs": 10000
    },
    "temporarily-off": {
      "command": "another-server",
      "enabled": false
    }
  }
}

command selects stdio and accepts args, env, and cwd; url selects streamable HTTP and accepts headers and maxRetries. Omit maxRetries for the SDK default of 5 reconnect attempts; an explicit 0 (or any negative value) disables retries so a dropped connection fails fast. cwd is resolved from pigo's working directory. The process inherits the current environment before applying env overrides. timeoutMs defaults to 10 seconds and bounds connect, initialization, and initial tool discovery. "disabled": true — the convention used by Cline, Roo, and Claude Desktop configs — is honored exactly like "enabled": false. Other unknown fields are ignored so settings remain forward-compatible. Invalid entries are skipped with a per-entry warning while the remaining valid servers still load (ParseSettingsWithWarnings reports them); only a malformed mcpServers value itself is an error. Project entries are invisible until the existing project-trust flow accepts that project. Setting goExtensions.mcp to false or passing --no-extensions disables the bundled extension as a whole.

Lifecycle and tool mapping

All enabled servers connect concurrently during extension loading, each bounded by its own timeoutMs, so one unavailable server is reported by /mcp without preventing the session from starting and without delaying the other servers. Startup failures are also printed as warnings. (A stdio child that ignores shutdown still adds the SDK's ~5s kill grace on top of timeoutMs before its process is killed.) /mcp reconnect [server] recreates one or every connection; re-running the extension factory against a fresh registry re-registers the already-discovered tools without reconnecting. A call that fails because the connection died (closed connection, EOF or a broken pipe from a dead child) deactivates that server's tools immediately. Session shutdown closes the SDK sessions and stdio children; a child's exit status (for example signal: terminated after the kill grace) is expected there and not reported as an error. Server tool-list notifications add new tools and deactivate removed ones without retrying calls whose side effects may already have run.

Remote names are exposed as stable, provider-safe names of the form mcp__<server>__<tool>_<hash>. The hash prevents collisions after sanitizing or truncating long names. JSON input schemas pass through unchanged. Text and image results map to pigo's native tool result blocks, structured content and MCP metadata remain in Details, and MCP progress notifications become normal tool execution updates. MCP tool errors are returned through the agent's ordinary error path so providers receive an error tool result.

Test strategy

There is no upstream pi implementation to extract a conformance golden from. The package tests instead run the official SDK example-server pattern end to end, including progress and mixed text/image output, and separately exercise real stdio and streamable-HTTP transports. Dynamic tool changes, reconnect/error isolation, disabled configuration, deterministic naming, and the unconfigured zero-work path are ordinary Go tests.