Skillget

pi-mcp-adapter

mcp
SUMMARY

Token-efficient MCP adapter for Pi coding agent

README.md

pi-mcp-adapter

Pi MCP Adapter

Use MCP servers with Pi without burning your context window.

https://github.com/user-attachments/assets/4b7c66ff-e27e-4639-b195-22c3db406a5a

Why This Exists

Mario wrote about why you might not need MCP. The problem: tool definitions are verbose. A single MCP server can burn 10k+ tokens, and you're paying that cost whether you use those tools or not. Connect a few servers and you've burned half your context window before the conversation starts.

His take: skip MCP entirely, write simple CLI tools instead.

But the MCP ecosystem has useful stuff - databases, browsers, APIs. This adapter gives you access without the bloat. One proxy tool (~200 tokens) instead of hundreds. The agent discovers what it needs on-demand. Servers only start when you actually use them.

Install

pi install npm:pi-mcp-adapter

Restart Pi after installation.

Quick Start

Create ~/.pi/agent/mcp.json:

{
  "mcpServers": {
    "chrome-devtools": {
      "command": "npx",
      "args": ["-y", "chrome-devtools-mcp@latest"]
    }
  }
}

Servers are lazy by default — they won't connect until you actually call one of their tools. The adapter caches tool metadata so search and describe work without live connections.

mcp({ search: "screenshot" })

chrome_devtools_take_screenshot
  Take a screenshot of the page or element.

  Parameters:
    format (enum: "png", "jpeg", "webp") [default: "png"]
    fullPage (boolean) - Full page instead of viewport

mcp({ tool: "chrome_devtools_take_screenshot", args: '{"format": "png"}' })

Note: args is a JSON string, not an object.

Two calls instead of 26 tools cluttering the context.

Config

Server Options

{
  "mcpServers": {
    "my-server": {
      "command": "npx",
      "args": ["-y", "some-mcp-server"],
      "lifecycle": "lazy",
      "idleTimeout": 10
    }
  }
}
Field Description
command Executable for stdio transport
args Command arguments
env Environment variables (${VAR} interpolation)
cwd Working directory
url HTTP endpoint (StreamableHTTP with SSE fallback)
auth "bearer" or "oauth"
bearerToken / bearerTokenEnv Token or env var name
lifecycle "lazy" (default), "eager", or "keep-alive"
idleTimeout Minutes before idle disconnect (overrides global)
exposeResources Expose MCP resources as tools (default: true)
directTools true, string[], or false — register tools individually instead of through proxy
debug Show server stderr (default: false)

Lifecycle Modes

  • lazy (default) — Don't connect at startup. Connect on first tool call. Disconnect after idle timeout. Cached metadata keeps search/list working without connections.
  • eager — Connect at startup but don't auto-reconnect if the connection drops. No idle timeout by default (set idleTimeout explicitly to enable).
  • keep-alive — Connect at startup. Auto-reconnect via health checks. No idle timeout. Use for servers you always need available.

Settings

{
  "settings": {
    "toolPrefix": "server",
    "idleTimeout": 10
  },
  "mcpServers": { }
}
Setting Description
toolPrefix "server" (default), "short" (strips -mcp suffix), or "none"
idleTimeout Global idle timeout in minutes (default: 10, 0 to disable)
directTools Global default for all servers (default: false). Per-server overrides this.

Per-server idleTimeout overrides the global setting.

Direct Tools

By default, all MCP tools are accessed through the single mcp proxy tool. This keeps context small but means the LLM has to discover tools via search. If you want specific tools to show up directly in the agent's tool list — alongside read, bash, edit, etc. — add directTools to your config.

Per-server:

{
  "mcpServers": {
    "chrome-devtools": {
      "command": "npx",
      "args": ["-y", "chrome-devtools-mcp@latest"],
      "directTools": true
    },
    "github": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-github"],
      "directTools": ["search_repositories", "get_file_contents"]
    },
    "huge-server": {
      "command": "npx",
      "args": ["-y", "mega-mcp@latest"]
    }
  }
}
Value Behavior
true Register all tools from this server as individual Pi tools
["tool_a", "tool_b"] Register only these tools (use original MCP names)
Omitted or false Proxy only (default)

To set a global default for all servers:

{
  "settings": {
    "directTools": true
  },
  "mcpServers": {
    "huge-server": {
      "directTools": false
    }
  }
}

Per-server directTools overrides the global setting. The example above registers direct tools for every server except huge-server.

Each direct tool costs ~150-300 tokens in the system prompt (name + description + schema). Good for targeted sets of 5-20 tools. For servers with 75+ tools, stick with the proxy or pick specific tools with a string[].

Direct tools register from the metadata cache (~/.pi/agent/mcp-cache.json), so no server connections are needed at startup. On the first session after adding directTools to a new server, the cache won't exist yet — tools fall back to proxy-only and the cache populates in the background. Restart Pi and they'll be available. To force it: /mcp reconnect <server> then restart.

Interactive configuration: Run /mcp to open an interactive panel showing all servers with connection status, tools, and direct/proxy toggles. You can reconnect servers, initiate OAuth, and toggle tools between direct and proxy — all from one overlay. Changes are written to your config file; restart Pi to apply.

Subagent integration: If you use the subagent extension, agents can request direct MCP tools in their frontmatter with mcp:server-name syntax. See the subagent README for details.

MCP UI Integration

MCP servers can ship interactive UIs via the MCP UI standard. When you call a tool that has a UI resource, the adapter opens it in a native macOS window via Glimpse if available, otherwise falls back to the browser.

How it works:

  1. Agent calls a tool like launch_dashboard
  2. The tool's metadata includes _meta.ui.resourceUri pointing to a UI resource
  3. pi-mcp-adapter fetches the UI HTML and opens it in an iframe
  4. The UI can call MCP tools and send messages back to the agent

Native rendering: On macOS, if Glimpse is installed (pi install npm:glimpseui), UIs open in a native WKWebView window instead of a browser tab. Set MCP_UI_VIEWER=browser to force the browser, or MCP_UI_VIEWER=glimpse to require native rendering.

Bidirectional communication: The UI talks back. When it sends a prompt or intent, the message is stored and triggerTurn() wakes the agent. The agent retrieves messages via mcp({ action: "ui-messages" }) and responds, enabling conversational UIs where the app and agent collaborate in real-time.

Session reuse: When the agent calls the same tool again while its UI is already open, the adapter pushes the new result to the existing window instead of replacing it. This enables live updates — the agent can refine a chart, add data, or respond to user input without losing the current view. Different tools still replace the session as before.

Message types from UI:

Type Purpose
prompt User message that triggers an agent response
intent Structured action with name + params
notify Fire-and-forget notification
message Generic message payload
(custom) Any other type forwarded as intent

Retrieving UI messages:

mcp({ action: "ui-messages" })

Returns accumulated messages from UI sessions. Each message includes type, sessionId, serverName, toolName, and timestamp. Prompt messages include prompt, intent messages include intent and params.

Browser controls:

  • Cmd/Ctrl+Enter — Complete and close
  • Escape — Cancel and close
  • Done/Cancel buttons — Same as keyboard shortcuts

Technical notes:

  • Tool consent gates whether UIs can call MCP tools (never/once-per-server/always)
  • Works with both stdio and HTTP MCP servers
  • Uses a local 408KB AppBridge bundle (MCP SDK + Zod) for browser↔server communication

Local Example: Interactive Visualizer

A minimal MCP UI example at examples/interactive-visualizer demonstrating charts, bidirectional messaging, and streaming. From that directory:

npm install
npm run build
npm run install-local

Restart pi, then ask the agent to show a chart — it calls show_chart and opens the UI in Glimpse (macOS) or the browser. Use npm run uninstall-local to remove the MCP entry.

Import Existing Configs

Already have MCP set up elsewhere? Import it:

{
  "imports": ["cursor", "claude-code", "claude-desktop"],
  "mcpServers": { }
}

Supported: cursor, claude-code, claude-desktop, vscode, windsurf, codex

Project Config

Add .pi/mcp.json in a project root for project-specific servers. Project config overrides global and imported servers.

Usage

Mode Example
Status mcp({ })
List server mcp({ server: "name" })
Search mcp({ search: "screenshot navigate" })
Describe mcp({ describe: "tool_name" })
Call mcp({ tool: "...", args: '{"key": "value"}' })
Connect mcp({ connect: "server-name" })
UI messages mcp({ action: "ui-messages" })

Search includes both MCP tools and Pi tools (from extensions). Pi tools appear first with [pi tool] prefix. Space-separated words are OR'd.

Tool names are fuzzy-matched on hyphens and underscores — context7_resolve_library_id finds context7_resolve-library-id.

Commands

Command What it does
/mcp Interactive panel (server status, tool toggles, reconnect)
/mcp tools List all tools
/mcp reconnect Reconnect all servers
/mcp reconnect <server> Connect or reconnect a single server
/mcp-auth <server> OAuth setup

How It Works

  • One mcp tool in context (~200 tokens) instead of hundreds
  • Servers are lazy by default — they connect on first tool call, not at startup
  • Tool metadata is cached to disk so search/list/describe work without live connections
  • Idle servers disconnect after 10 minutes (configurable), reconnect automatically on next use
  • npx-based servers resolve to direct binary paths, skipping the ~143 MB npm parent process
  • MCP server validates arguments, not the adapter
  • Keep-alive servers get health checks and auto-reconnect
  • Specific tools can be promoted from the proxy to first-class Pi tools via directTools config, so the LLM sees them directly instead of having to search

Limitations

  • OAuth tokens obtained externally (no browser flow)
  • No automatic token refresh
  • Cross-session server sharing not yet implemented (each Pi session runs its own server processes)

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