MCP Integration (Python)

Python exposes two different MCP stories:

Secure a local FastMCP transport with jacs.mcp
Expose JACS operations as MCP tools with jacs.adapters.mcp

Use the first when you already have an MCP server or client. Use the second when you want the model to call JACS signing, agreement, A2A, or trust helpers as normal MCP tools.

What Is Supported

Local FastMCP server wrapping with JACSMCPServer
Local FastMCP client wrapping with JACSMCPClient
One-line server creation with create_jacs_mcp_server()
FastMCP tool registration with register_jacs_tools(), register_a2a_tools(), and register_trust_tools()

Important Constraints

JACSMCPClient, JACSMCPServer, and jacs_call() enforce loopback-only URLs
Unsigned fallback is disabled by default
strict=True is about config loading and failure behavior, not an opt-in to security

1. Secure A FastMCP Server

The shortest path is the factory:

from jacs.mcp import create_jacs_mcp_server

mcp = create_jacs_mcp_server("My Server", "./jacs.config.json")

@mcp.tool()
def hello(name: str) -> str:
    return f"Hello, {name}!"

If you already have a FastMCP instance:

from fastmcp import FastMCP
from jacs.mcp import JACSMCPServer

mcp = JACSMCPServer(FastMCP("Secure Server"), "./jacs.config.json")

2. Secure A FastMCP Client

from jacs.mcp import JACSMCPClient

client = JACSMCPClient("http://localhost:8000/sse", "./jacs.config.json")

async with client:
    result = await client.call_tool("hello", {"name": "World"})

To allow unsigned fallback explicitly:

client = JACSMCPClient(
    "http://localhost:8000/sse",
    "./jacs.config.json",
    allow_unsigned_fallback=True,
)

3. Register JACS As MCP Tools

This is the better fit when the model should be able to ask for signatures, agreements, A2A cards, or trust-store operations directly.

from fastmcp import FastMCP
from jacs.client import JacsClient
from jacs.adapters.mcp import (
    register_jacs_tools,
    register_a2a_tools,
    register_trust_tools,
)

client = JacsClient.quickstart(name="mcp-agent", domain="mcp.local")
mcp = FastMCP("JACS Tools")

register_jacs_tools(mcp, client=client)
register_a2a_tools(mcp, client=client)
register_trust_tools(mcp, client=client)

The core tool set includes document signing, verification, agreements, audit, and agent-info helpers. The A2A and trust helpers are opt-in registrations.

Useful Helper APIs

From jacs.mcp:

jacs_tool to sign a specific tool's response
jacs_middleware() for explicit Starlette middleware
jacs_call() for one-off authenticated local MCP calls

Example Paths In This Repo

jacspy/examples/mcp/server.py
jacspy/examples/mcp/client.py
jacspy/examples/mcp_server.py
jacspy/tests/test_adapters_mcp.py

When To Use Adapters Instead

Choose Python Framework Adapters instead of MCP when:

the model and tools already live in the same Python process
you only need signed LangChain, LangGraph, CrewAI, or FastAPI boundaries
you do not need MCP clients to connect from outside the app

JACS Usage Documentation