API Contract Diff: Free Online API Contract Comparison — How-To Guide

Try it free: API Contract Diff

When an API changes, the question is never just "what changed?" — it is "what did we break?" A renamed field, a removed endpoint, or a type that quietly switched from a number to a string can break every client that depends on the old contract. API Contract Diff is a free online tool that compares two JSON API contracts or schemas and summarizes the added, removed, and changed paths, so you can see the full impact of a change before it ships.

What the tool does

API Contract Diff takes two versions of an API contract — typically two JSON schemas or OpenAPI-style contract files — and produces a clear summary of the differences between them. Instead of scrolling through two long documents side by side, you get a focused list of what was added, what was removed, and what changed, path by path.

It is built as a serious first-pass local tool: paste real input, run the parser or standards-based checks, review clear findings, then copy or download the output. Use it for quick reviews, debugging notes, code review prep, QA handoff, and documentation examples. It is designed for fast local inspection — for regulated production changes, verify final results with your project's own compiler, CI checks, or infrastructure scanner.

How to use it

1. Open the tool. Go to the API Contract Diff page. No account is needed and nothing needs to be installed.
2. Paste the old contract. Put the earlier version of your JSON API contract or schema into the first input area.
3. Paste the new contract. Put the updated version into the second input area.
4. Run the comparison. Start the diff and let the tool parse both contracts and work out the differences.
5. Review the summary. Read the list of added, removed, and changed paths. Removed paths and type changes are the ones that usually break clients, so check those first.
6. Copy or download the result. Copy the output into a pull request description, a review comment, or a QA handoff note, or download a text report to keep with the change.

Practical tips

• Diff before you merge, not after you deploy. Run the comparison as part of your code review prep, while the change is still easy to adjust. A breaking change caught in review costs minutes; one caught in production costs much more.
• Treat removals and type changes as breaking by default. New fields are usually safe to add, but a removed endpoint, a renamed property, or a field that changed type will break existing clients. Scan the removed and changed lists first and confirm each one is intentional.
• Use the diff as your changelog. Paste the summary into your release notes or API documentation. It tells consumers exactly what changed in plain terms, which is far more useful than "various improvements."

FAQ

Does API Contract Diff upload my input?

No. The normal workflow runs locally in your browser. Your contracts stay on your own device.

Is this a full replacement for CI or production scanners?

No. It is designed for fast local inspection before you run project-specific checks. For production changes, still verify with your project's compiler, CI checks, or infrastructure scanner.

Can I copy or download the result?

Yes. Run the tool, then copy the output or download a text report to share with your team or attach to a pull request.

What kind of input does it accept?

Two JSON API contracts or schemas — for example, the before and after versions of an API definition you are about to change.

Privacy note

API Contract Diff runs 100% in your browser — your files never leave your device. There is no upload, no account, and no server-side processing of your contracts, so you can safely compare internal or unreleased API definitions.

Try it free: API Contract Diff