raw_mcp

A minimal Model Context Protocol server written in pure Python — no MCP SDK, no third-party libraries. It speaks JSON-RPC 2.0 over stdio, which is how MCP clients talk to servers they launch as a subprocess.

Prerequisites

• uv (brew install uv)

Related MCP server: Counting MCP Server

Layout

• server.py — the MCP server (stdlib only)

• test_server.py — a smoke test that drives the server through a full handshake

• pyproject.toml — uv project metadata (no runtime dependencies)

Run the tests

uv run test_server.py

Run the server manually

The server reads JSON-RPC from stdin and writes responses to stdout, so you can poke at it by hand:

uv run server.py

Then paste a line and press enter:

{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{}}}

Diagnostic logs go to stderr so they never corrupt the JSON on stdout.

Use it from an MCP client

Point any MCP client (Claude Desktop, etc.) at the server via uv. Example claude_desktop_config.json:

{
  "mcpServers": {
    "raw-mcp": {
      "command": "uv",
      "args": ["run", "server.py"],
      "cwd": "/Users/jameswhite/conductor/workspaces/raw_mcp/san-salvador"
    }
  }
}

Use it from Claude Code

Register the server with Claude Code's MCP support, then call the tool from a session.

Add the server

claude mcp add raw-mcp -- uv run --directory /Users/jameswhite/conductor/workspaces/raw_mcp/san-salvador server.py

• raw-mcp is the name it shows up as.

• Everything after -- is the launch command. Using uv run --directory <path> means it works regardless of which directory you start Claude Code from. If you always launch from this folder, claude mcp add raw-mcp -- uv run server.py is enough.

• Default scope is local (just you, this project). Add -s user to make it available everywhere, or -s project to share it via a committed .mcp.json.

Verify it connected

claude mcp list          # shows configured servers + connection status
claude mcp get raw-mcp   # shows the launch command for this one

raw-mcp should report ✔ Connected.

Use it

MCP servers are loaded at session startup, so open a fresh interactive Claude Code session (or restart your current one), then:

• Run /mcp — you should see raw-mcp connected with one tool, calculate_length.

• Ask Claude to use it, e.g. "Use the raw-mcp calculate_length tool on the string 'hello world'." The tool is exposed as mcp__raw-mcp__calculate_length.

Remove it when done

claude mcp remove raw-mcp

The one tool

calculate_length — returns the character count of a string.

How it works

1. initialize — the handshake. The server advertises protocolVersion and its tools capability.

2. notifications/initialized — the client acknowledges. It's a notification (no id), so the server sends nothing back.

3. tools/list — the server returns its tool definitions and JSON schemas.

4. tools/call — the client invokes a tool; the server returns content blocks.

Adding a tool is two steps: append its definition to TOOLS, then handle its name in call_tool.