I launched PasteCheck last week a free, no-signup code error checker that runs entirely in the browser. No AI, no server, just dedicated language parsers that tell you exactly what's wrong and where.
Week 1 was about getting it live. Week 2 was about making it good.
Here's what I actually built, why I built it, and what I learned.
CSS support — because people kept hitting "unknown language"
The original version supported JavaScript, TypeScript, Python, and HTML. That covered most pastes but standalone CSS kept falling through to an "unknown language" error. Users weren't doing anything wrong. The tool just didn't speak CSS yet.
So I added it. CSS now gets its own purple badge, its own parser, and its own set of checks unknown or misspelled properties with "did you mean?" suggestions,
!important overuse warnings, and unresolved TODO/FIXME comments.
The misspelling dictionary covers 70+ common typos. dipslay, colour, backgrond-color all flagged with the correct property suggested alongside.
JS multi-error reporting because one error at a time isn't enough
This one was embarrassing once I noticed it. If your JavaScript had two syntax errors on different lines, PasteCheck would find the first one, throw, and stop. You'd fix it, paste again, find the second one. Fix it. Paste again.
That's not a linter. That's a guessing game.
I rebuilt the error recovery so acorn now does a line-by-line pass after the initial failure, surfacing every error it can find independently. Paste once, see everything.
Session history — because stateless tools waste time
Every time you hit "Check New Code", the previous result was gone. For a student pasting one snippet that's fine. For a developer debugging across multiple files in a session, it's friction.
I added session history. Your last 10 checks are stored in localStorage no backend, no login, no account. A "Recent Checks" button appears once you have history. Each entry shows the first line of your code plus the error and warning count. Tap any entry to reload it instantly.
Zero infrastructure cost. Meaningful quality of life improvement.
What I'm building toward
The free tier is now solid. Five languages, multi-error reporting, session history, mobile-first throughout.
The next milestone is a Pro tier. The anchor feature is GitHub integration connect a repo and PasteCheck scans every push or PR for syntax errors automatically. No copy-pasting. No app switching. It just runs as part of your workflow.
That's the feature that turns a useful free tool into something a working developer would pay for.
The constraints that shape everything
I build entirely from an Android phone. No laptop, no desktop, no Replit agent. Every commit goes through SPCK Editor and GitHub directly.
That constraint forces a certain discipline — no unnecessary dependencies, no over-engineered solutions, no "I'll set up the dev environment later." If it can't be built lean, it doesn't get built.
PasteCheck is live at pastecheck.co.uk free, no signup, works on any device.
If you're building something similar or have thoughts on the GitHub integration direction, I'd genuinely like to hear it.
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