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

CodeMode has a full reference below; it is the flagship capability and the one this skill goes deep on. The rest ship today and each has its own README with API and examples.

Each capability lives in its own submodule and is imported from there (from pydanticaiharness.<module> import ...). Capabilities are not importable from the top-level pydanticaiharness package by design, so each one keeps its own optional dependencies isolated. CodeMode, FileSystem, Shell, and ManagedPrompt also have top-level re-exports (importable directly from pydanticaiharness).

APIs are subject to change between releases; breaking changes ship deprecation warnings where practical.

Still experimental: an ACP server adapter, imported from pydanticaiharness.experimental.acp. Importing it emits a HarnessExperimentalWarning.

The full, current list with links and status is in the capability matrix.

Install

uv add pydantic-ai-harness

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

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

Requires Python 3.10+ and pydantic-ai-slim>=2.18.0.

Quick Start

A harness capability is added to the agent like any other. Here CodeMode wraps locally registered tools into a single run_code tool that the model drives with Python.

```python {test="skip"} from pydantic_ai import Agent

from pydanticaiharness import CodeMode

agent = Agent('anthropic:claude-sonnet-4-6', capabilities=[CodeMode()])

@agent.toolplain def gettemperature_f(city: str) -> float: return {'Paris': 68.0, 'Tokyo': 77.0}[city]

@agent.toolplain def converttemp(fahrenheit: float) -> float: return round((fahrenheit - 32) * 5 / 9, 1)

result = agent.run_sync( 'Compare the weather in Paris and Tokyo, and report both temperatures in Celsius.' ) print(result.output) #> Paris is 20.0 C and Tokyo is 25.0 C.

The model writes a single Python script that fetches both temperatures with `asyncio.gather` and then
converts them -- performing four tool calls across two dependent stages in one `run_code` invocation.

## 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](./references/CODE-MODE.md)) 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. Use `native=False` for client-side dispatch that `CodeMode` can wrap, but do not treat a remote server as trusted or sandboxed; see the [Code Mode trust boundary](./references/CODE-MODE.md#sandbox-restrictions).
- **The Monty sandbox is a Python subset.** It has no third-party imports and only a small stdlib allowlist -- read [Code Mode](./references/CODE-MODE.md#sandbox-restrictions) before debugging generated code that fails to run.
- **`CodeMode` needs its extra.** Install `pydantic-ai-harness[codemode]`, not the bare package.