pydantic-ai-harness

Original：🇺🇸 English

Translated

Extend Pydantic AI agents with batteries-included capabilities from pydantic-ai-harness — currently Code Mode, which collapses many tool calls into one sandboxed Python execution. Use when the user mentions pydantic-ai-harness, CodeMode, Monty, code mode, or tool sandboxing, when they want an agent to run agent-written Python, or when a Pydantic AI agent would benefit from orchestrating multiple tool calls in a single sandboxed script.

6installs

Sourcepydantic/skills

Added on2026-06-02

NPX Install

npx skill4agent add pydantic/skills pydantic-ai-harness

SKILL.md Content

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Building with Pydantic AI Harness

Pydantic AI Harness is the official capability library for Pydantic AI. Capabilities that need model or framework support — and those fundamental to every agent — live in core

pydantic-ai

; optional, batteries-included capabilities live here. Both are composed onto an agent through the same

capabilities=[...]

API.

This skill covers the capabilities shipped by

pydantic-ai-harness

. For the core framework — agents, tools, structured output, hooks, and testing — use the

building-pydantic-ai-agents

skill instead.

When to Use This Skill

Invoke this skill when:

The user mentions
```
pydantic-ai-harness
```
,
```
CodeMode
```
, code mode, or the Monty sandbox
An agent makes many sequential tool calls that could collapse into one sandboxed Python execution
The user wants the model to write Python that loops, branches, aggregates, or parallelizes tool calls with
```
asyncio.gather
```
The user asks to sandbox or constrain the code an agent runs

Do not use this skill for:

Core Pydantic AI usage — building agents, adding tools, structured output, streaming, or testing (use
```
building-pydantic-ai-agents
```
)
Capabilities that ship in core
```
pydantic-ai
```
, such as web search, tool search, and thinking
The Pydantic validation library on its own (
```
pydantic
```
/
```
BaseModel
```
without agents)

Supported Capabilities

Capability	Description	Reference
`CodeMode`	Wraps eligible tools into a single sandboxed `run_code` tool so the model orchestrates them in Python	Code Mode

More capability areas are tracked in the capability matrix; as they stabilize, this skill grows to cover them.

Install

bash

uv add pydantic-ai-harness

Each capability declares its own extra. Code Mode needs the Monty sandbox:

bash

uv add "pydantic-ai-harness[codemode]"   # `code-mode` is also accepted as an alias

Requires Python 3.10+ and

pydantic-ai-slim>=1.95.1

.

Quick Start

A harness capability is added to the agent like any other. Here

CodeMode

wraps an MCP server's tools into a single

run_code

tool that the model drives with Python.

python

from pydantic_ai import Agent
from pydantic_ai.capabilities import MCP  # MCP ships in core pydantic-ai

from pydantic_ai_harness import CodeMode

agent = Agent(
    'anthropic:claude-sonnet-4-6',
    capabilities=[
        # native=False routes the MCP tools through a local toolset so CodeMode can wrap them.
        # Without it, providers with native MCP run the tools server-side and bypass the sandbox.
        MCP('https://hn.caseyjhand.com/mcp', native=False),
        CodeMode(),
    ],
)

result = agent.run_sync(
    'Across the top and best Hacker News feeds, find the most-discussed story with at '
    'least 100 points and summarize its comment thread in one paragraph.'
)
print(result.output)
#> The most-discussed story clearing 100 points is ...

Instead of one model round-trip per tool call, the model writes a single Python script that fetches both feeds with

asyncio.gather

, dedupes and ranks them in plain Python, and pulls the winning thread — collapsing many calls into one

run_code

.

Key Practices

Confirm a harness capability is actually needed. If core Pydantic AI tools and capabilities are enough, use the
```
building-pydantic-ai-agents
```
skill instead — don't reach for the harness by default.
Read the reference before writing code. Each capability has its own configuration, constraints, and gotchas — load the linked reference (e.g. Code Mode) first.
Install the capability's extra. Importing
```
CodeMode
```
without
```
pydantic-ai-harness[codemode]
```
raises an
```
ImportError
```
; the Monty sandbox is an optional dependency.

Common Gotchas

native=True
tools bypass
CodeMode
. Provider-native MCP servers and web search execute server-side, so
```
run_code
```
never sees them. Construct them with
```
native=False
```
to keep them local and wrappable.
The Monty sandbox is a Python subset. No class definitions, no third-party imports, and only a small stdlib allowlist — read Code Mode before debugging generated code that fails to run.
CodeMode
needs its extra. Install
```
pydantic-ai-harness[codemode]
```
, not the bare package.