Pydantic AI Harness

What it does

When your agent uses it

• The user mentions pydantic-ai-harness
• Tool sandboxing
• They want first-party filesystem/shell/sub-agent/planning/compaction capabilities for a Pydantic AI agent
• They want an agent to run agent-written Python

Example prompts

• “/pydantic-ai-harness”

Requirements

• Python 3
• Compatibility (from SKILL.md): Requires Python 3.10+ and pydantic-ai-slim>=2.18.0

What it can do on your machine

Read from SKILL.md and the folder at commit 238d971. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  Shell commands in SKILL.md call:

  • uv

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  Links to these hosts (documentation or services it may open):

  • github.com

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

• Compatibility

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

  From compatibility in the SKILL.md frontmatter.

Context cost

Pydantic AI Harness loads about 1.9k tokens when it runs, and up to ~3.6k if it reads all its reference files. Until then it costs about 158 tokens; SKILL.md has 710 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~158

When it runs · the whole SKILL.md, loaded when a task matches

~1.9k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~3.6k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from pydantic/skills at commit 238d971, republished under its MIT licence (© pydantic). 710 words, ~1,899 tokens.

name

pydantic-ai-harness

description

Extend Pydantic AI agents with batteries-included capabilities from pydantic-ai-harness -- Code Mode (collapse many tool calls into one sandboxed Python execution), a filesystem and shell, sub-agents, planning, context compaction, and more. Use when the user mentions pydantic-ai-harness, CodeMode, Monty, code mode, or tool sandboxing, when they want first-party filesystem/shell/sub-agent/planning/compaction capabilities for a Pydantic AI agent, 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.

compatibility

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

license

MIT

metadata.version

0.1.0

metadata.author

pydantic

uv add pydantic-ai-harness

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

from pydantic_ai import Agent

from pydantic_ai_harness import CodeMode

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


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


@agent.tool_plain
def convert_temp(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.

© pydantic, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 1 other file (references) in skills/pydantic-ai-harness of pydantic/skills.

• SKILL.md
• references/CODE-MODE.md

Open the folder on GitHubat commit 238d971

Used in 1 other repository

We found 2 copies of this SKILL.md (exact, near-identical or edited) in other folders. This page covers the copy in pydantic/skills, which our catalogue first saw on October 7, 2026.

• pydantic/pydantic-ai-harnesspydantic_ai_harness/.agents/skills/pydantic-ai-harness