Replay Browser

Replay Browser is a fork of Chromium that records everything a browser session does, so the session can be replayed deterministically later. It is what Replay QA drives when it runs a journey, what the replayio CLI and the Playwright plugin launch, and what produces every recording you open in Replay DevTools or analyze with Replay MCP.

Chromium version

Replay Browser is built on Chromium 108. The recorder hooks in at the library-call level (memory allocation, sockets, files, timers, and the other sources of nondeterminism) rather than instrumenting JavaScript, so the Chromium changes are small and the browser behaves like a normal Chromium build while recording. See How does time travel work? for the recording model and the replayio/chromium repo for the source.

Run replayio info to see the exact build installed on your machine.

What can be recorded

Most web applications record without changes. Web Workers, Canvas, and WebAssembly work. WebGL and Web Audio are not yet supported; pages that depend on them may render incorrectly when replayed.

Recording overhead

Recording adds a small amount of overhead to a session, because intercepting library calls is cheap and the browser otherwise runs as it normally does. The one deliberate cost is that V8's baseline JIT is disabled in Replay Browser, so CPU-heavy pages run somewhat slower than in stock Chrome.

Platform support

Linux x86-64, macOS x86-64, and macOS ARM. There is no native Windows build; the Linux build runs under WSL, and Replay QA records in the cloud regardless of your OS.

Installing and updating

The replayio CLI installs Replay Browser on first use and updates it with replayio update. Replay QA and the Playwright plugin manage their own copies. See How to record and the CLI reference.

Launching the executable directly

replayio record is the supported way to launch the browser. If you need to pass Chromium flags, for example to load an unpacked extension, you can launch the executable that the CLI installed:

Terminal
replayio update
RECORD_ALL_CONTENT=1 ~/.replay/runtimes/Replay-Chromium.app/Contents/MacOS/Chromium --load-extension=./extension
replayio upload

On Linux the executable is ~/.replay/runtimes/chrome-linux/chrome. RECORD_ALL_CONTENT=1 turns recording on for every tab; without it the browser runs without recording. See replayio-public/chrome-extension-example for a worked example.