fastapi-evil 0.1.2

A drop-in FastAPI wrapper that adds intentional request slowdowns — for chaos engineering, testing, and mayhem.

fastapi-evil

😈 Your FastAPI. Now with more suffering. 😈

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A drop-in replacement for FastAPI that adds intentional request slowdowns. One import change. Zero other code changes. Maximum mayhem.

# before 😇
from fastapi import FastAPI
app = FastAPI()

# after 😈
from fastapi_evil import EvilAPI
app = EvilAPI()  # requests now take 10 seconds. you're welcome.

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Install

pip install fastapi-evil
# or with uv
uv add fastapi-evil

To use the fastapi dev CLI (Uvicorn + hot reload), install with the standard extra:

pip install "fastapi-evil[standard]"
# or with uv
uv add "fastapi-evil[standard]"

Then run your app:

# pip
fastapi dev main.py

# uv
uv run fastapi dev main.py

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Why Would Anyone Do This?

• Chaos engineering — stress-test your clients without a service mesh
• Reproduce timeout bugs — "it only happens in prod" → now it happens everywhere
• Test retry logic — make your HTTP clients earn it
• Time-limited degradation — slow things down for exactly one hour, then stop
• Convince your boss the servers are overloaded — we do not endorse this

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Quickstart

from fastapi_evil import EvilAPI

app = EvilAPI()

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    return {"item_id": item_id}

# Every request now waits 10 seconds before responding.
# uvicorn main:app --reload

---

Parameters

EvilAPI accepts all standard FastAPI constructor arguments plus:

Parameter	Type	Default	Description
slow_for_millis	int	10_000	Milliseconds to delay each request (10 seconds)
slow_for_methods	list[str] | None	None	HTTP methods to slow. None = ALL.
slow_until	datetime | None	None	Stop slowing after this datetime. None = slow forever.

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Examples

Slow only GET requests by 3 seconds

from fastapi_evil import EvilAPI

app = EvilAPI(
    slow_for_millis=3_000,
    slow_for_methods=["GET"],
)

Time-limited chaos — slow everything for the next hour, then stop

from datetime import datetime, timedelta, timezone
from fastapi_evil import EvilAPI

app = EvilAPI(
    slow_for_millis=5_000,
    slow_until=datetime.now(timezone.utc) + timedelta(hours=1),
)

Precise method targeting — slow writes only

from fastapi_evil import EvilAPI

app = EvilAPI(
    slow_for_millis=2_000,
    slow_for_methods=["POST", "PUT", "PATCH", "DELETE"],
)

Use the named constant for readability

from fastapi_evil import EvilAPI, ALL_METHODS

app = EvilAPI(slow_for_millis=8_000, slow_for_methods=ALL_METHODS)

Pass any standard FastAPI kwargs — they all work

from fastapi_evil import EvilAPI

app = EvilAPI(
    title="My Suffering API",
    version="6.6.6",
    description="This API is evil and slow.",
    slow_for_millis=1_000,
)

---

How It Works

EvilAPI subclasses FastAPI and wires a pure ASGI middleware (SlowMiddleware) that calls asyncio.sleep before dispatching each matched request. It uses a pure ASGI implementation (not Starlette's BaseHTTPMiddleware) to avoid known ContextVar propagation bugs. WebSocket and lifespan scopes are passed through untouched.

Request → SlowMiddleware → check method + slow_until → asyncio.sleep → your routes

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slow_until and Timezones

slow_until accepts any Python datetime. Timezone-aware datetimes are recommended:

from datetime import datetime, timezone, timedelta

# Good — explicit UTC
slow_until=datetime.now(timezone.utc) + timedelta(hours=2)

# Also works — but emits a UserWarning (assumed UTC)
slow_until=datetime(2026, 12, 31, 23, 59, 59)

---

Dev Setup

git clone https://github.com/mohsin/fastapi-evil
cd fastapi-evil
uv sync
uv run pytest

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Publishing

uv build
uv publish --token pypi-<your-token>

Get your token at pypi.org/manage/account/token.

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License

MIT — do whatever evil you want with it.