Simon Willison’s Weblog

MCP Run Python (via) Pydantic AI's MCP server for running LLM-generated Python code in a sandbox. They ended up using a trick I explored two years ago: using a Deno process to run Pyodide in a WebAssembly sandbox.

Here's a bit of a wild trick: since Deno loads code on-demand from JSR, and uv run can install Python dependencies on demand via the --with option... here's a one-liner you can paste into a macOS shell (provided you have Deno and uv installed already) which will run the example from their README - calculating the number of days between two dates in the most complex way imaginable:

ANTHROPIC_API_KEY="sk-ant-..." \
uv run --with pydantic-ai python -c '
import asyncio
from pydantic_ai import Agent
from pydantic_ai.mcp import MCPServerStdio

server = MCPServerStdio(
    "deno",
    args=[
        "run",
        "-N",
        "-R=node_modules",
        "-W=node_modules",
        "--node-modules-dir=auto",
        "jsr:@pydantic/mcp-run-python",
        "stdio",
    ],
)
agent = Agent("claude-3-5-haiku-latest", mcp_servers=[server])

async def main():
    async with agent.run_mcp_servers():
        result = await agent.run("How many days between 2000-01-01 and 2025-03-18?")
    print(result.output)

asyncio.run(main())'

I ran that just now and got:

The number of days between January 1st, 2000 and March 18th, 2025 is 9,208 days.

I thoroughly enjoy how tools like uv and Deno enable throwing together shell one-liner demos like this one.

Here's an extended version of this example which adds pretty-printed logging of the messages exchanged with the LLM to illustrate exactly what happened. The most important piece is this tool call where Claude 3.5 Haiku asks for Python code to be executed my the MCP server:

ToolCallPart(
    tool_name='run_python_code',
    args={
        'python_code': (
            'from datetime import date\n'
            '\n'
            'date1 = date(2000, 1, 1)\n'
            'date2 = date(2025, 3, 18)\n'
            '\n'
            'days_between = (date2 - date1).days\n'
            'print(f"Number of days between {date1} and {date2}: {days_between}")'
        ),
    },
    tool_call_id='toolu_01TXXnQ5mC4ry42DrM1jPaza',
    part_kind='tool-call',
)

I also managed to run it against Mistral Small 3.1 (15GB) running locally using Ollama (I had to add "Use your python tool" to the prompt to get it to work):

ollama pull mistral-small3.1:24b

uv run --with devtools --with pydantic-ai python -c '
import asyncio
from devtools import pprint
from pydantic_ai import Agent, capture_run_messages
from pydantic_ai.models.openai import OpenAIModel
from pydantic_ai.providers.openai import OpenAIProvider
from pydantic_ai.mcp import MCPServerStdio

server = MCPServerStdio(
    "deno",
    args=[
        "run",
        "-N",
        "-R=node_modules",
        "-W=node_modules",
        "--node-modules-dir=auto",
        "jsr:@pydantic/mcp-run-python",
        "stdio",
    ],
)

agent = Agent( 
    OpenAIModel(                          
        model_name="mistral-small3.1:latest",
        provider=OpenAIProvider(base_url="http://localhost:11434/v1"),                
    ),            
    mcp_servers=[server],
)

async def main():
    with capture_run_messages() as messages:
        async with agent.run_mcp_servers():
            result = await agent.run("How many days between 2000-01-01 and 2025-03-18? Use your python tool.")
    pprint(messages)
    print(result.output)

asyncio.run(main())'

Here's the full output including the debug logs.

# 4:51 am / deno, pydantic, uv, sandboxing, llm-tool-use, ai, llms, model-context-protocol, python, generative-ai, mistral, ollama, claude