pi-mono/packages/coding-agent/docs/mcp.md

MCP Servers

Pi connects to Model Context Protocol servers over stdio or streamable HTTP and makes their tools available to the model.

Configure servers

Add servers to ~/.pi/agent/mcp.json, or to .pi/mcp.json in a project. The format matches other MCP clients, so existing mcpServers entries can be copied over:

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "."]
    },
    "docs": {
      "url": "https://example.com/mcp",
      "headers": { "Authorization": "Bearer ${DOCS_TOKEN}" },
      "exposure": "direct"
    }
  }
}

• stdio servers take command, args, env, and cwd. Relative cwd resolves against the session directory.
• HTTP servers take url, headers, and oauth (see Sign in with OAuth). The legacy SSE transport is not supported.
• env and headers values can reference environment variables (${NAME}) or commands (!command), like provider API keys.
• timeout sets the per-request timeout in seconds (default 60). Progress notifications from the server reset it.
• enabled: false keeps an entry without connecting to it.

Project entries replace global entries with the same name. A project mcp.json is only read after the project is trusted, because stdio servers run commands.

Rules that are easy to get wrong:

• Server names may only contain letters, digits, _, and -. Tools are named mcp__<server>__<tool>.
• type is optional: a command makes a stdio server and a url a streamable HTTP server. When present, it must be stdio, http, or streamable-http. sse is rejected; most servers that document an SSE endpoint also serve streamable HTTP, often at /mcp instead of /sse.
• command is a single executable and args its arguments, not one shell string.
• Keep secrets out of the file: use ${NAME} for environment variables or !command to run a command, for example "Authorization": "Bearer !op read op://vault/github/token".
• Invalid entries are skipped and reported; the other servers still connect.

Set up servers

When asked to add an MCP server, the agent should:

1. Put personal servers and servers with credentials in ~/.pi/agent/mcp.json. Use the project .pi/mcp.json only for servers the project itself needs, and only in trusted projects.
2. Convert entries written for other clients:
   • Claude Desktop, Claude Code, and Cursor use the same mcpServers shape; copy the entry.
   • VS Code uses a top-level servers object and inputs prompts; move the entry under mcpServers and replace ${input:...} with ${NAME} environment variables.
   • Codex uses TOML ([mcp_servers.<name>] with command, args, env, or url); write the same fields as JSON.
   • opencode uses "type": "local" with command as an array (split it into command and args), "type": "remote" for URLs, environment for env, and {env:NAME} for ${NAME}.
3. Run pi mcp list to check the entry. It connects to every enabled server and prints the state, the tools, and errors such as the stderr of a stdio server that failed to start. It exits with 1 while anything is wrong.
4. For a server that needs a sign-in, run pi mcp login <server>. It opens the authorization page in the user's browser and waits until the user approves access; tell the user to approve it. A running session uses the new credentials on its next turn.
5. Tell the user to run /reload (or start a new session) so the running session connects to added or changed servers.

Pi connects when a session starts. The first prompt waits up to 10 seconds for startup connections; the tools of servers that take longer become available once they connect. HTTP connections that fail with a network error or a transient status (408, 429, 5xx) are retried twice. A server that drops its connection shows as disconnected and is reconnected on the next call. When a server announces that its tool list changed, new tools are added and withdrawn tools become unreachable until the server offers them again.

Config errors, servers that failed to connect, and servers that need a sign-in are reported once after startup.

Manage servers

/mcp opens the server manager. It lists every configured server with its state, tool count, exposure, and whether it comes from the global or the project mcp.json; servers that need attention come first. Select a server to:

• sign in, for OAuth servers that need it (see Sign in with OAuth)
• see its tools, its command or URL, and the full connection error, including the tail of a stdio server's stderr
• reconnect
• sign out, which deletes the stored OAuth credentials
• change its exposure (see Exposure)
• disable or enable it

Exposure changes and enabling or disabling are saved to the mcp.json that defines the server; other content of the file is kept. Disabled servers stay listed so they can be enabled again.

Outside the interactive TUI, /mcp prints the server status. /mcp login <server>, /mcp logout <server>, and /mcp reconnect <server> run those actions directly.

From a shell, pi mcp list, pi mcp login <server>, and pi mcp logout <server> do the same without a session (see MCP commands).

Stopping a stdio server closes its stdin, then sends SIGTERM and finally SIGKILL to its whole process group, so servers started through wrappers such as npx or uvx do not linger.

Sign in with OAuth

Remote servers that use OAuth, such as Sentry, need no credentials in mcp.json:

{
  "mcpServers": {
    "sentry": { "url": "https://mcp.sentry.dev/mcp" }
  }
}

When such a server rejects the connection, /mcp shows it as needing sign-in. Select it and choose "Sign in" (or run /mcp login sentry, or pi mcp login sentry in a shell) to open the authorization page in your browser. After you approve access, the browser redirects to a temporary server on 127.0.0.1 and pi connects. If the browser runs on another machine, for example over SSH, paste the URL it was redirected to into the sign-in screen instead.

Pi registers itself with the authorization server (dynamic client registration), stores tokens in ~/.pi/agent/mcp-auth.json, and refreshes access tokens automatically when they expire or the server rejects them. If the server later asks for more scope than was granted, it shows as needing sign-in again, and signing in requests the new scope. "Sign out" in /mcp (or /mcp logout sentry) deletes the stored credentials.

OAuth applies to HTTP servers without an Authorization header. For authorization servers that do not support dynamic client registration, configure a pre-registered client:

{
  "mcpServers": {
    "example": {
      "url": "https://mcp.example.com/mcp",
      "oauth": { "clientId": "my-client", "clientSecret": "${EXAMPLE_SECRET}", "callbackPort": 8765 }
    }
  }
}

callbackPort fixes the redirect URI to http://127.0.0.1:<port>/oauth/callback, which must match the redirect URI registered for the client. clientSecret is optional and can reference environment variables or commands.

Exposure

Each server's tools are registered as mcp__<server>__<tool>. The exposure setting controls how the model reaches them:

• codemode (default): the tools are callable from codemode scripts and listed in the codemode tool's description, but are not declared to the model. Large MCP tool lists stay out of the model's tool declarations, and scripts can call several MCP tools, in parallel if needed, while returning only the part of the result the model needs. Pi activates the codemode tool when such a server connects. Large servers do not fill the description: declarations share a token budget, and scripts find the remaining tools with searchTools() (see codemode).
• deferred: like codemode, but the tools are not listed in the codemode tool's description either. Scripts can still call them by name and find them with searchTools() or in ALL_TOOLS. With the tool_search tool enabled, the model can also load them as declared tools.
• direct: the tools are declared to the model like built-in tools, and are also callable from codemode.
• hidden: the tools are registered but cannot be called.

Codemode-only tools do not depend on the active tool set, so they stay callable after /tree, resume, and fork. To keep pi from activating the codemode tool, set "autoEnableCodemode": false at the top level of mcp.json, next to mcpServers. A project mcp.json value overrides the global one. Pi then warns once that codemode-only tools cannot be called until codemode is activated.

Codemode scripts receive an MCP tool's whole CallToolResult (content blocks as sent by the server, structuredContent, and isError), and the codemode description declares it as CallToolResult<T>. A result with isError resolves in scripts and is reported to the model as an error for direct calls. image(result.content[0]) forwards an image block to the model. The server's instructions describe its tools in the codemode description.

Every MCP call goes through pi's tool pipeline, so tool_call and tool_result extension handlers, including permission gates, apply to MCP tools. Calls made from codemode scripts carry the codemode call's id as parentToolCallId.

Servers from extensions

Extensions can add servers for the current session with pi.registerMcpServer(name, config), using the same config shape as mcp.json (see Extensions). They connect like configured servers and appear in /mcp with the extension as their source. Enabling, disabling, and exposure changes for them apply to the current session only. A server in mcp.json with the same name takes precedence; /mcp lists the overridden registration. pi mcp shell commands do not load extensions and only see mcp.json servers.

Other MCP extensions

An installed extension that registers the /mcp command, such as pi-mcp-adapter, replaces the built-in MCP support: pi then neither reads mcp.json in sessions nor connects servers, and /mcp belongs to that extension. Remove the extension to use the built-in support. Likewise, an extension that registers a tool named codemode or tool_search replaces the built-in tool of that name. pi mcp shell commands always use the built-in support.

SDK

SDK sessions do not load the built-in extensions. Add the MCP extension, and the codemode extension for codemode and deferred servers, to the resource loader. See SDK.