fastapi

Rules for building FastAPI web endpoints, a Python framework for creating web APIs.

In plain words
What is it for?
They help define response classes, handle HTTP errors, require existing database records, represent path IDs, and generate URLs with typed helpers.
Why use it?
They keep API responses, errors, database lookups, URL parameters, and generated links consistent and typed.

Cursor rule for Cursor

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add rules/iloveitaly/llm-ide-rules/fastapi
Clone the repo
git clone --depth 1 https://github.com/iloveitaly/llm-ide-rules

Made for: Cursor.

Per session 0 Nothing until a file matches its globs; then the whole rule loads.
When invoked 186 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
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.

ModelPer sessionOnce invoked
Fable 5 $0.00000 $0.00186
Opus 5 $0.00000 $0.00093
Sonnet 5 $0.00000 $0.00037
Haiku 4.5 $0.00000 $0.00019

Measured 3d ago against content hash 35ef0f64aa1a, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

fastapi 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 3d 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.

.cursor/rules/fastapi.mdc · 16 lines

What it actually says

Fastapi

  • When throwing a HTTPException, do not add a detail= and use a named status code (status.HTTP_400_BAD_REQUEST)
  • Do not return a dict, instead create a class RouteNameResponse
    • Locate these classes right above the def route_name(): function which uses them.
  • Use Model.one when a record must exist in order for the business logic to succeed.
  • Do not try/except Model.one when using a parameter from the request to pull a record. Let this exception bubble up.
  • Use model_id: Annotated[TypeID, Path()] to represent a model ID as a URL path parameter
  • Use the typed route helpers in app/generated/fastapi_typed_routes.py for all URL generation.
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.

  1. 3d ago First seen · 16 lines · 0 tokens per session scan A 35ef0f64aa1a

Subscribe to this mod's changes

fastapi is a cursor rule published in the GitHub repository iloveitaly/llm-ide-rules (13 stars, last pushed 3d ago), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 186 tokens. 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.