ProxyCeptor for Backend and API Developers: Test Contract Changes Against the Real UI
Backend and API developers use ProxyCeptor to try a response change against the real frontend before deploying: deep-merge a new field, remove a deprecated one, or return a new error code, and watch the UI react. The same workspace includes Commander, a browser-based REST client, and a server-side forwarder for CORS-restricted calls.
Problems backend & api developers run into
Breaking the UI with a contract change
Renaming or removing a field looks safe in the API but breaks a screen nobody remembered.
Slow feedback loops
Deploying to staging just to see how the UI handles a change takes too long.
Different tools for everything
Separate apps for sending requests, mocking and proxying.
Workflows that fix them
Preview a contract change
Use a JavaScript transform to remove a field or rename a key in the live response and click through the UI. See transforming API responses with JavaScript.
Roll out new error codes safely
Before shipping a new 409 or 422, return it from a rule and check that the UI shows something sensible.
Send requests with Commander
Commander is a Postman-style API client at proxyceptor.com/test with a JSON viewer and an optional CORS forwarder.
Server-to-server overrides
Route calls through the ProxyCeptor forwarder with a workspace key so rules apply without any browser. See overriding API responses server to server.
Example
{ "name": "Remove legacyPrice", "match": { "urlPattern": "*/api/products/*", "matchType": "wildcard" }, "response": { "body": { "enabled": true, "mode": "execute-js", "jsTransform": "function transform(response) { delete response.legacyPrice; return response; }" } }}Frequently asked questions
Where does the JavaScript transform run?
In the Chrome extension or the SDK, inside your own browser or app, on the response your front end receives. The transform gets the parsed JSON (or text) and returns the new body.
Can I use ProxyCeptor instead of WireMock?
For front-end-facing checks, often yes. For backend-to-backend contract tests in CI, WireMock is still the better fit. See ProxyCeptor vs WireMock.
Does it support GraphQL?
Yes. Match the GraphQL endpoint and transform or merge the JSON response. You can check the operation name inside a JavaScript transform.
Related guides
Try these workflows free
Create a free ProxyCeptor account to mock, delay, block and rewrite API traffic, then share the same rules with your team.