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