Cassetter¶

Cassetter is a Rust powered HTTP cassette recorder for Python tests. Safe by default.

The first time your test runs, Cassetter records the real HTTP interactions to a cassette file. Every run after that, it replays the recorded responses. No network needed. Fast, deterministic tests.

The key features are:

• Safe by default: sensitive headers, query parameters, and body fields are filtered at write time. Cassettes never contain secrets.
• Fast: parsing, matching, and serialization run in a Rust core. Loading cassettes is 3 to 6 times faster than VCR.py.
• Secure: cassette YAML is parsed in Rust. There is no way for a cassette file to execute Python code.
• Readable cassettes: JSON bodies are stored as structured YAML, not escaped strings. Diffs stay clean.
• Multi protocol: HTTP, gRPC, WebSockets, and streaming (SSE) responses.
• Drop in: designed as a replacement for VCR.py and pytest-recording, with the same @pytest.mark.vcr marker.

Requirements¶

Python 3.10+

Cassetter has no required runtime dependencies. It intercepts the HTTP libraries you already have installed.

Installation¶

$ uv add cassetter

Or with pip:

$ pip install cassetter

Example¶

Mark a test with @pytest.mark.vcr:

import httpx
import pytest


@pytest.mark.vcr(record_mode="once")
async def test_get_users():
    async with httpx.AsyncClient() as client:
        response = await client.get("https://api.example.com/users")
    assert response.status_code == 200

Now run it:

$ pytest

The first run performs the real request and saves the interaction to tests/cassettes/test_get_users/test_get_users.yaml.

Run it again. This time the response comes from the cassette. No network. Same result.

That's it.

What the cassette looks like¶

Open the file. It is plain YAML, and JSON bodies are stored as structure, not as escaped strings:

version: 1
interactions:
  - request:
      method: GET
      uri: https://api.example.com/users
      headers:
        accept:
          - '*/*'
    response:
      status: 200
      headers:
        content-type:
          - application/json
      body:
        type: json
        content:
          users:
            - id: 1
              name: Alice
    recorded_at: '2026-02-20T10:30:01Z'

Notice what is not there: no authorization header, no cookies, no API keys. Cassetter strips them before writing. You will read more about it in Safe by default.

Supported libraries¶

Cassetter detects which libraries are installed and intercepts them automatically. You can also select them explicitly, see Use the context manager.

License¶

This project is licensed under the terms of the MIT license.