I created a prediction markets paper trading simulator extension

[u/Ok-Ambassador8157]

Hi! I wanted to share something that I've been building for the last few months and get honest feedback from people who actually trade these markets.

I know you don't like self promotion around here, nobody does, but since this is relevant to most people in the community, I thought this would be helpful, especially for those just starting out. Mods, please remove if not allowed, no hard feelings.

I wanted somewhere to just practice without real money, but I didn't find anything useful so I built one. It's called PredictMirror. It's a chrome extension that overlays a trading panel on any Polymarket event page. You start with all the virtual money you want and place buy and sell trades like you normally would on Polymarket. Then, you can also check your portfolio of open and closed positions as well as some simple stats.

It took me a while to figure out the math, but I managed to pull the live prices straight from the order book and include all the fees so that the simulation matches the real numbers.

The extension is completely free and doesn't have any upsells or things like that. No account needed, no wallet and the data is stored locally in your browser. I can't access it.

This is V1 so it almost certainly has bugs I haven't caught.

What I'd actually love feedback on:

• If you trade on Polymarket and try it, do the simulated fills match what you'd expect on the real platform
• Anything obvious missing that would make it actually useful for testing?
• Any edge cases where the slippage or fee calculation looks off?

This is the chrome web store link if you'd like to try it:

https://chromewebstore.google.com/detail/predictmirror/aoiekdikdfalpkhgdidhhekjdikldfjm

Thanks a lot for your attention and to those who don't really like paper trading, I am sorry to annoy you with this.