What is pydantic-ai-harness AGENTS.md?

Repository guidance for Pydantic AI Harness, a library of reusable features for AI coding agents built on Pydantic AI. It distinguishes the harness's optional combinations—such as tools, guardrails, memory, planning, and verification—from Pydantic AI's core runtime.

Use it when adding or modifying agent tools, input checks, memory, context management, repository tools, plans, skills, sub-agents, or verification loops.

It helps developers place changes in the correct layer instead of duplicating core behavior in the harness. It also defines the project's terms and how reusable capabilities are organized.

How pydantic/pydantic-ai-harness configures Codex and OpenCode

Reuse

Borrowing it

Nothing to install: this file belongs to pydantic/pydantic-ai-harness. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file

curl -O https://raw.githubusercontent.com/pydantic/pydantic-ai-harness/main/AGENTS.md

Clone the repo

git clone --depth 1 https://github.com/pydantic/pydantic-ai-harness

Made for: Codex, OpenCode.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/instructions/pydantic/pydantic-ai-harness/agents-md.svg)](https://agentmods.dev/instructions/pydantic/pydantic-ai-harness/agents-md)

Your own site

<a href="https://agentmods.dev/instructions/pydantic/pydantic-ai-harness/agents-md"><img src="https://agentmods.dev/badge/instructions/pydantic/pydantic-ai-harness/agents-md.svg" alt="Measured on agentmods" height="20"></a>

Per session 2,651 This file is loaded in full into every session.

When invoked 2,651 The same file — it is already loaded in full.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.02651	$0.02651
Opus 5	$0.01326	$0.01326
Sonnet 5	$0.00530	$0.00530
Haiku 4.5	$0.00265	$0.00265

Measured 9d ago against content hash b9c7a30e00a0, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-08, from the pricing page.

Security

Grade A, and why

pydantic-ai-harness AGENTS.md scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 9d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

AGENTS.md · 203 lines

How it starts

The opening of the file, as written. The whole thing — 203 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Pydantic AI Harness

Repository purpose

pydantic-ai-harness is the first-party capability library for Pydantic AI.

Pydantic AI core owns the primitive runtime: agent loop semantics, normalized messages, model/provider/profile behavior, tool execution semantics, durable execution primitives, and generic capability hooks.

Harness owns optional, batteries-included compositions built from those primitives: coding-agent tools, guardrails, memory, context management, repo tools, verification loops, skills, planning, sub-agents, and other reusable agent behaviors.

When a change needs new core semantics, stop and propose the Pydantic AI core change instead of reimplementing core behavior in harness.

Vocabulary

Capability: an AbstractCapability subclass that bundles tools, hooks, instructions, and model settings into a reusable unit. This is the core abstraction of pydantic-ai-harness.
Hook: a lifecycle method on AbstractCapability that intercepts agent graph execution (e.g. before_model_request, wrap_run, after_tool_execute)
Toolset: a collection of tools that a capability can provide to the agent
Guard: a type of capability that validates inputs/outputs or controls tool access (e.g. InputGuard, OutputGuard)
Harness: this package -- a collection of pre-made capabilities for Pydantic AI.
AICA: AI Code Assistant -- the automated agent that implements issues, reviews plans, and handles PR feedback
Ralph loop: the state-machine-based workflow that drives AICA through phases (TRIAGE -> GOALS -> PLAN -> CODE -> VERIFY -> REVIEW -> PUBLISH)
DDD+ protocol: classification system for PR review comments (do, dismiss, discuss, waiting, done)

AICA preflight

Before implementing or reviewing a capability change:

Read agent_docs/index.md.
Read the linked agent_docs/ guide for the task.
Read the public Pydantic AI docs for every integration point you touch:
- capabilities: https://pydantic.dev/docs/ai/capabilities/overview/
- hooks: https://pydantic.dev/docs/ai/core-concepts/hooks/
- toolsets: https://pydantic.dev/docs/ai/tools-toolsets/toolsets/
- advanced tools: https://pydantic.dev/docs/ai/tools-toolsets/tools-advanced/
- agents: https://pydantic.dev/docs/ai/core-concepts/agent/
- testing: https://pydantic.dev/docs/ai/guides/testing/
Inspect the installed pydantic_ai package source for exact hook/toolset signatures when needed. Do not assume a contributor's local checkout layout.
Use pydantic_ai_harness.code_mode as the exemplar for capability shape, docs, tests, and public exports until another capability becomes a better example. Capabilities live in their own top-level submodule pydantic_ai_harness/<name>/ (module name = capability name; one module per capability or strategy) and are not re-exported from the root __init__.py, so each keeps its own optional dependencies. The experimental tier is retired; ACP is the sole remaining experimental capability (see agent_docs/capability-authoring.md, "Capability Submodules And Exports").

Read the full file on GitHub · 203 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

9d ago First seen · 203 lines · 2,651 tokens per session scan A b9c7a30e00a0

Subscribe to this mod's changes

pydantic-ai-harness AGENTS.md is an instructions file published in the GitHub repository pydantic/pydantic-ai-harness (864 stars, last pushed today), licensed MIT. It adds 2,651 tokens to every session, about $0.0133 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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Working notes and architecture documentation for the new esbuild-based build system in build/next. Use when making changes to the new build pipeline (transpile/bundle commands, NLS plugin, source-map handling, resource copying, or self-hosting watch tasks).

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pydantic-ai-harness: Instructions file for Codex