The Complete Production SOP: Deploying FastAPI & Pydantic v2 Architect in Autonomous AI Coding Agents
Step-by-step SOP to install and run cursor-fastapi in Codex, Claude Code, and Cursor. Enforce Pydantic v2, async SQLAlchemy 2.0, and strict type safety.
Technical Overview & Architectural Context
Without strict architecture constraints, autonomous coding agents frequently hallucinate obsolete Pydantic v1 patterns (such as `@validator` instead of `@field_validator`, or `.dict()` instead of `.model_dump()`), mix blocking synchronous database sessions into async FastAPI endpoints, and generate untyped dependency injection functions. This leads to silent event loop starvation, runtime deprecation warnings, and critical OpenAPI contract mismatches. The FastAPI & Pydantic v2 Architect skill (`cursor-fastapi`) establishes rigid procedural boundaries for Codex, Claude Code, Cursor, and Windsurf. It forces agents to utilize pure SQLAlchemy 2.0 async session generators, Pydantic v2 configuration objects (`ConfigDict`), typed `Annotated` dependencies, and comprehensive test verification before committing code changes.
Prerequisites & Environment Requirements
Verified Search Intents & Long-Tail Queries Solved
This operational tutorial is engineered to provide deterministic solutions for these developer search queries:
Step-by-Step Execution Playbook
Local Environment Setup & SKILL.md Mounting
Mount the cursor-fastapi skill into agent-readable rule paths for deterministic context loading.
Agent runtimes scan specific workspace directories for system instructions. For Cursor, rules reside in `.cursor/rules/` as `.mdc` files or root `.cursorrules`. For Claude Code and Codex, instructions are discovered via `.agents/skills/cursor-fastapi/SKILL.md` or `CLAUDE.md`. We fetch and link the canonical skill definition directly.
mkdir -p .cursor/rules ~/.agents/skills/cursor-fastapi && curl -fsSL https://askills.dev/api/skills/cursor-fastapi/raw -o .cursor/rules/cursor-fastapi.mdc && cp .cursor/rules/cursor-fastapi.mdc ~/.agents/skills/cursor-fastapi/SKILL.md---
description: FastAPI & Pydantic v2 Architect Rules
globs: ["**/*.py"]
---
# FastAPI & Pydantic v2 Architect
- Always use Pydantic v2 syntax: `model_dump()`, `@field_validator`, `ConfigDict`.
- Disallow blocking I/O inside `async def` endpoints; use `AsyncSession` with `await session.execute()`.
- Annotate dependencies with `typing.Annotated[T, Depends(dep)]`.
- Verify all endpoint contracts using `pytest-asyncio` and `httpx.AsyncClient`.Using glob pattern matching `["**/*.py"]` inside `.cursor/rules` ensures the rule is only injected into LLM context when editing Python files, saving thousands of tokens during front-end and config edits.
Avoid committing hardcoded database connection strings inside your `.cursorrules` or `.mdc` manifests. Ensure agents read credentials strictly via `pydantic-settings`.
Environment Secrets & Dependency Validation
Provision production-ready asynchronous dependencies and typed runtime settings.
The skill mandates SQLAlchemy 2.0 async engines, `asyncpg`, and modern Pydantic v2 settings. Configure your workspace dependencies with `uv` to ensure fast, deterministic package resolution without agent hallucination of non-existent packages.
uv pip install "fastapi>=0.110.0" "pydantic>=2.7.0" "pydantic-settings>=2.2.0" "sqlalchemy[asyncio]>=2.0.29" "asyncpg>=0.29.0" "uvicorn[standard]>=0.29.0" "pytest-asyncio>=0.23.0" "httpx>=0.27.0"Instruct the agent to inspect `uv.lock` or `pyproject.toml` before running imports to prevent unpinned library collisions.
Make sure `.env` and `.env.local` are declared in `.gitignore` before allowing agents to generate config files containing development secrets.
Intent Matching & Trigger Invocation
Trigger deterministic architecture design using precise, intent-aligned prompt verbs.
Autonomous agents trigger skills most reliably when prompts invoke explicit vocabulary matched to domain keywords: 'Pydantic v2 schemas', 'AsyncSession dependency', 'OpenAPI router', and 'Annotated Depends'.
Structure prompts using Role-Task-Constraint syntax (e.g., 'Using cursor-fastapi guidelines, scaffold the User authentication router with Pydantic v2 validation').
Verification, Linting & Output Quality Gate
Run automated linters and asynchronous test suites to validate code contracts.
The agent must be forced to run static analysis and unit testing before marking any task as complete. This validates that no Pydantic v1 methods crept into the codebase.
ruff check . && ruff format --check . && mypy app/ && pytest -vSet your agent execution workflow to run `ruff check --fix` autonomously upon error before asking for human intervention.
Real-World Scenario: Implement High-Throughput Async Order Processing Router
“Apply cursor-fastapi rules: Implement an async POST /orders endpoint that accepts an OrderCreate schema, validates positive quantity and item SKU format using Pydantic v2, writes to an async SQLAlchemy 2.0 session with transactional rollback on failure, and returns an OrderRead schema with status 201.”
Runtime Troubleshooting & Failure Modes
Root Cause: The agent generated legacy Pydantic v1 code (`.dict()`) inside a Pydantic v2 runtime environment.
# Replace legacy calls:
# data = order_in.dict()
# With Pydantic v2 idiom:
data = order_in.model_dump(exclude_unset=True)Root Cause: Lazy loading triggered on an SQLAlchemy relationship attribute inside an async route without pre-fetching (`selectinload` or `joinedload`).
from sqlalchemy.orm import selectinload
stmt = select(Order).options(selectinload(Order.items)).where(Order.id == order_id)
result = await session.execute(stmt)
order = result.scalar_one_or_none()Frequently Asked Questions (FastAPI & Pydantic v2 Architect SOP)
Q:Can I use FastAPI & Pydantic v2 Architect across Codex, Claude Code, and Cursor simultaneously?
Yes. The underlying rule is distributed as a universal SKILL.md and `.cursor/rules` `.mdc` file compatible with Cursor, Windsurf, Claude Code, and Codex CLI runners.
Q:How does FastAPI & Pydantic v2 Architect prevent context window overflow during long agent sessions?
The skill applies fine-grained file globbing (`**/*.py`) and concise procedural constraints rather than injecting entire boilerplate repositories into prompt memory, reducing input token usage by up to 60%.
Q:Does this skill require escalated root or container permissions?
No. The skill operates completely within userland workspace bounds, audited with a 98/100 security score and requiring zero sudo privileges or container breakouts.
Ready to equip your autonomous agent with FastAPI & Pydantic v2 Architect?
Copy the complete audited SKILL.md specification or download the 1-click zip bundle directly from our verified registry.