charmlibs-interfaces-mcp

Charm library for the MCP (Model Context Protocol) Juju interface.

Provides McpProvider and McpRequirer classes that charm authors can use to integrate with the mcp relation interface.

Installation

pip install charmlibs-interfaces-mcp

Usage

Provider (principal charm)

import ops
from charmlibs.interfaces.mcp import McpProvider, McpDefinitions, Tool, ExecHandler

class MyCharm(ops.CharmBase):
    def __init__(self, framework):
        super().__init__(framework)
        self.mcp = McpProvider(self, "mcp")
        framework.observe(self.on.mcp_relation_joined, self._on_mcp_joined)

    def _on_mcp_joined(self, event):
        self.mcp.set_definitions(McpDefinitions(
            tools=[
                Tool(
                    name="list-files",
                    description="List files in a directory",
                    input_schema={
                        "type": "object",
                        "properties": {
                            "dir": {"type": "string", "description": "Directory path"},
                        },
                        "required": ["dir"],
                    },
                    handler=ExecHandler(command=["ls", "-la", "{{dir}}"]),
                ),
            ],
        ))

Requirer (mcp-server charm)

import ops
from charmlibs.interfaces.mcp import McpRequirer

class McpServerCharm(ops.CharmBase):
    def __init__(self, framework):
        super().__init__(framework)
        self.mcp = McpRequirer(self, "mcp")

    def _on_relation_changed(self, event):
        definitions = self.mcp.collect_definitions()
        # definitions = {"tools": [...], "prompts": [...], "resources": [...]}

Data models

All models are plain dataclasses (no pydantic):

• Tool — an MCP tool with name, description, input_schema, and handler
• Prompt — a prompt template with arguments
• Resource — a readable data source
• ExecHandler — runs a command on the machine
• HttpHandler — calls a local HTTP endpoint
• McpDefinitions — container for tools, prompts, and resources