Hello Elixir community!

I’m excited to announce the first release of ReqCassette, a VCR-style record-and-replay library specifically designed for Req.

What is ReqCassette?

ReqCassette captures HTTP responses to JSON files (“cassettes”) and replays them in subsequent test runs, making your tests:

• Faster - No network calls after initial recording
• Deterministic - Same response every time
• Offline-capable - Work without internet once cassettes are recorded
• Cost-effective - Perfect for testing paid APIs (like LLM services!)

Why build this

ReqCassette leverages Req’s native testing infrastructure instead of global mocking, making it process-isolated and fully compatible with ExUnit’s async testing.

Quick Example

defmodule MyApp.APITest do
  use ExUnit.Case, async: true

  test "fetches user data" do
    # First run: records to cassette
    # Subsequent runs: instant replay!
    response = Req.get!(
      "https://api.example.com/users/1",
      plug: {ReqCassette.Plug, %{cassette_dir: "test/cassettes"}}
    )

    assert response.status == 200
    assert response.body["name"] == "Alice"
  end
end

Created with ReqLLM in mind

One of my favorite use cases is testing LLM applications with
ReqLLM:

{:ok, response} = ReqLLM.generate_text(
  "anthropic:claude-sonnet-4-20250514",
  "Explain recursion",
  max_tokens: 100,
  req_http_options: [
    plug: {ReqCassette.Plug, %{cassette_dir: "test/cassettes"}}
  ]
)

First call costs money and it’s slow, but every subsequent test is fast and
FREE - the response is replayed from the cassette! This makes testing with LLMs
much more practical.

How It Works

ReqCassette uses Req’s :plug option to intercept requests:

1. First request → Forwards to real server → Saves response to JSON →
   Returns response
2. Subsequent requests → Loads JSON cassette → Returns saved response

Cassettes are matched by HTTP method, path, query string, and request body,
creating a unique MD5 hash for the filename.

What’s Next?

Some ideas for future versions:

• Additional recording modes (:once, :none, :all)
• Custom cassette naming
• Header-based matching
• Cassette expiration/TTL
• Request filtering/sanitization helpers

Feedback Welcome!

This is the first release, and I’d love to hear your thoughts:

• Are there features you’d like to see?
• Any issues or bugs to report?

Thanks for reading, and happy testing!

Resources

• Hex: hex.pm/packages/req_cassette
• Documentation: hexdocs.pm/req_cassette
• ReqLLM Integration Guide:
  ReqLLM Integration
• GitHub:

https://github.com/lostbean/req_cassette

Note: Special thanks to the Req maintainers for building such a fantastic HTTP
client with great testing support, and to the ReqLLM maintainers for creating a
promising and foundational library that makes Elixir shine in the new LLM
space!