Launching Replay: The Time Travel Debugger for the Web!

Jason Laster
Published in
4 min readSep 15, 2021


After hundreds of user interviews and 2.5 million recordings — we’re excited to share that is shipping today! Download Replay and let us know what you think!

Tl;dr What is Replay?

Replay lets you record your website and replay it down to a line of code. You can see when a function was called, what values were received, and what the application looked like at that point in time.

If you see a bug, you can record it with Replay, share it with your team, and anyone can debug it with familiar browser devtools.

What is hard about debugging today?

Most software is debugged with print statements. Print statements make it easy to see the order your code was called in, but can take a while to update because every time you add a new print statement, you need to refresh and reproduce the problem. And you can never be certain that the next time you re-run the program, it will run the same way.

Time-travel debugging is like Instant Replay for your software. You can record your software once and replay it as often as you like. This means when you add print statements in Replay, you no longer have to edit, refresh, or attempt to reproduce the bug. Replay will simply show the associated logs in the console.

We’ve all been adding print statements since our first “Hello World” so it is hard to imagine what it would feel like for them to be any different. If you give yourself five minutes in Replay, you’ll never go back.

Adding a print statement in Replay

The Maintenance Tax

The most difficult aspect of fixing a bug is often reproducing it in the first place. Bugs can also be incredibly distracting, requiring developers to context switch off of existing work in order to investigate the new problem.

Replay helps teams maintain their evolving software stack because anyone can record the problem and developers have all of the context they need to fix the problem. No reproductions needed.

We’ve been able to use Replay for customer support since day one because all of our users have Replay. For example, Mark, CTO of Glide, recorded a bug he was seeing and Holger, an engineer on our team, found the problem. It sounds simple, but the ability for Holger to debug Mark’s problem as if he were experiencing it in real time is magical!

Runtimes should be replayable

It would have been simpler to focus on one browser, but we designed Replay’s recorder to work across a range of runtimes and platforms from the start. We are also releasing Replay for Chrome and Node in Beta today. Join our Discord channel to share feedback.

Our goal is for runtimes to be replayable: we should be able to thoroughly understand our software no matter where it is running or what language it is written in. We explain how Replay works here and encourage you to check out our Firefox, Chromium, and Node forks on GitHub.

Our Roadmap

Launching today is our opportunity to open Replay up to a larger audience and learn from the community. We strive to make debugging more visual, exploratory, and fun. We can only do that with your help.

Our goal for 2021 is to help teams file debuggable bug reports. We have focused on JS bugs up to this point, but we will be turning our attention to visual and networking bugs post-launch and will be releasing the Elements Panel and Network Monitor this Fall.

We are working with XState, Excalidraw, and React DevTools so that post-launch they can update their issue templates to accept replays. Feel free to reach out if you would like to use Replay on your team.

Our fundraise and dream team

I’m thrilled to share that this past spring we raised a $5.7m seed-round led by Peter Levine at a16z. We feel incredibly fortunate to be able to share the journey with Peter and our original investors, Jesse Beyroutey at IA Ventures and Angela Tran at Version One.

We are also excited to welcome some pretty incredible angels: Tom Preston-Werner, Max Schoening, Amjad Masad, Shawn Wang, Ives van Hoorne, Rasmus Andersson, Beyang Liu, Desigan Chinniah, Kenneth Auchenberg, Johannes Schickling, and Christopher Chedeau. Their experience building and operating developer tools businesses has been invaluable and we’re so lucky to be able to work with them.

We’re hiring!

The Replay team is growing across many areas. With this new financing, we are hiring in engineering, design, developer relations, and more. Our team is fully-remote and building software for the long term.

Please see our jobs page for all open roles. If you do not see a role that matches your background, but you believe you would be an excellent fit, please write to and tell us a bit more about yourself.

— Jason (co-founder and CEO)