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

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

When your API changes, the contract your consumers depend on changes too. API Contract Diff is a free online tool that helps you compare API contracts — for example, the version you shipped last month against the one you are about to release — so you can see exactly what is different before it surprises anyone.

Try it free: API Contract Diff

API Contract Diff is a focused developer utility for comparing API contracts. You paste in your contract content, run the tool's parser and standards-based checks, review the findings, and then copy or download a report of the differences. It is designed as a fast first-pass inspection: useful when you are preparing a release, reviewing a pull request, or documenting what changed between two versions of an API.

It is not a replacement for your CI pipeline or production scanners — the tool itself is explicit about that. Think of it as the quick check you run before the formal checks: paste, review, share the result.

Step 1 — Prepare your two contract versions. Grab the "before" and "after" versions of your API contract. This could be the spec from your previous release and the spec from the branch you are about to merge. Having both ready makes the comparison meaningful.

Step 2 — Paste the content into the tool. Open the API Contract Diff page in your browser. There is no account or sign-up step — just paste your contract input into the tool directly. The page is built for real input, not demos.

Step 3 — Run the checks. Run the tool's parser and standards-based checks on your input. The tool processes your content and produces clear findings about what differs between the two contract versions.

Step 4 — Review the findings. Read through the results carefully. Look especially for changes that could break existing clients: removed fields, changed types, renamed endpoints. The findings are meant to be scannable so you can triage quickly.

Step 5 — Copy or download the report. Once you are done reviewing, copy the output to your clipboard or download it as a text report. The report works well as an attachment to a pull request, a QA handoff note, or release documentation.

1. Diff before you merge. Make the contract diff part of your pre-merge routine for API changes. Running the comparison takes a minute and catches the accidental breaking change that is easy to miss in a long spec diff on GitHub.

2. Keep the report with the release. Download the text report and attach it to your release notes or the PR that changed the API. Months later, when someone asks "when did this field disappear?", the report answers the question instantly.

3. Use it in code review prep. Before asking a teammate to review an API change, run the contract diff yourself and mention the key differences in your PR description. It shows you have already thought about backward compatibility, and it makes the review faster.

Q: Does API Contract Diff upload my input? A: No. The normal workflow runs locally in your browser. This matters when your contract describes internal or sensitive endpoints — the content never leaves your device.

Q: Is this a full replacement for CI or production scanners? A: No. It is designed for fast local inspection before you run your project's own compiler, CI checks, or infrastructure scanners. Use it as the first pass, not the last word.

Q: Can I copy or download the result? A: Yes. Run the tool, then copy the output or download a text report to share with your team.

Q: Do I need an account? A: No. The tool works without sign-up — open the page, paste, and compare.

API Contract Diff runs 100% in your browser — files and pasted content never leave your device. There is no server processing your contract and no account tracking what you compared. For API work, where contracts often describe internal systems, that local-only behavior is the point.