# Pydantic AI Foundation

> Use when porting or building an agent runtime's core machinery — deferred/external tool calls, human-in-the-loop approval, pause/resume envelopes, barrier-segmented parallel execution, structured-output schema resolution, capability middleware with ordering constraints, toolset composition (wrapper/combined/prefixed/renamed/prepared/filtered/dynamic/approval/external), streaming partial→final validation, the durable-execution shared kernel (string-only model round-trip with credential-leak rejection, replay-unit enqueue/cancel guards, live-vs-replay event-stream split), the durability-engine adapter plane (anyio-shielded cancel forwarding, boundary-guarded context rehydration, sequence-keyed cache keys, legacy-replay dual paths), static tool-choice deadlock gates, transport-level tenacity retries honoring Retry-After, session cancellation as typed resumable exceptions, and replayed-history hardening for partially-rejected provider turns. Source code and direct tests are ground truth.

- Skill: `ryan-brosas/pydantic-ai-foundation` (Agent Skill, multi-file: 221 files)
- Install (CLI): `npx skillmds@latest add ryan-brosas/pydantic-ai-foundation`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ryan-brosas/pydantic-ai-foundation/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: ryan-brosas (https://skillmd.com/u/ryan-brosas)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ryan-brosas/pydantic-ai-foundation

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# pydantic-ai: Agent Runtime Foundation

## Foundation contract

This is a cold, source-specific foundation, not an operational procedure. It is
historical, revision-pinned evidence rather than timeless guidance. Current
project source, tests, requirements, and runtime behavior outrank this
projection and decide what ships.

## Provenance and revision

- Source record: `pydantic-ai`; portable upstream identity, license, coverage caveats, and retrieval history are preserved in the index.
- Recorded revision: `main@a5b5fb7a247f863599d61dfa9159bc2ebc786255`.
- Full provenance, coverage caveats, boundaries, and preserved evidence:
  `references/index.md`.

## Topic map

- Representative topics: Deferred envelope; Step-final deferral ladder;
  Approval dispatch; End strategies; Parallel execution; Retry budget;
  Availability refusals; Declarative deferral.
- Scope and retrieval questions: `references/index.md#use-this-for`.
- Complete capsule chooser: `references/index.md#load-the-matching-source-dump`.
- Detailed grouped map: `references/index.md#capsule-map`.
- Source limitations and portability boundaries: `references/index.md#boundaries`.

## Retrieval

Open the index, choose one capsule matching the active question, and load only
that capsule. Revalidate its cited source and revision before relying on it. Do
not bulk-load the inventory, treat historical claims as current truth, or apply
this foundation as a procedure.