API Works Locally But Not Live

An API that works locally but not live is usually failing because production changes the origin, URL, headers, secrets, runtime or network path. Local success does not prove the deployed app is allowed to make the same request from the live domain.

Generated apps may call localhost, use example tokens, skip CORS handling or place private requests in browser components. That is where AI code repair and deployment diagnosis can overlap.

Inspect the live request

Open the live app, reproduce the failing action and inspect Network. Look at the full request URL, method, status, response body and headers. A 401 means something different from CORS, and a 404 means something different from a timeout.

Do not rely on the app’s visible message. Generated frontends often display the same “something went wrong” text for every kind of failure.

Local and live differences

Local development may use relaxed origins, proxy settings, local .env values and a development server that hides cross-origin details. Production uses HTTPS, the live domain, host variables and often serverless execution limits.

External service dashboards may need the live domain, redirect URL, webhook endpoint or allowed origin added before requests are accepted.

• Frontend still calls localhost.
• CORS allows local origin but blocks production.
• Token exists locally but not live.
• Serverless function misses a secret.
• Provider dashboard lacks the live callback URL.

Browser calls versus server calls

A common AI-generated mistake is calling a private API directly from the browser. It may seem to work during testing, but production exposes CORS and secret handling.

If the API requires a private key, the app should usually call it from a server route or function, validate the input, call the provider privately and return a safe response.

What not to do

Do not disable CORS blindly, paste private keys into frontend code, switch to an unprotected endpoint or suppress the error in the UI. Those changes may make the demo appear to work while weakening the app.

Change one thing at a time so you know whether the request was malformed, unauthorised, blocked, missing a route or timing out.

When it is wider than one API

If forms, auth, database actions and APIs all fail live, the issue may be a wider production configuration problem: missing variables, wrong base URL, unsupported runtime or deployment settings copied from the wrong environment.

A focused AI app repair pass can check API failures alongside routing, variables and live user flow.

API checklist

Record the live request exactly as it happens:

• Request URL and method.
• Status code and response body.
• Origin and domain.
• Auth headers or token source.
• Serverless logs if used.
• Provider dashboard domain settings.

Short conclusion

APIs that work locally but not live are usually fixable. Diagnose the production request, not the preview assumption.

Why localhost success can mislead

Local development often runs from a trusted origin with local variables, local proxies and a developer server. The live app runs from a public HTTPS domain with different headers, domains, secrets and runtime limits.

An API provider may allow your local test, then reject the production origin because it has not been added to an allowed list. That is not a frontend design issue; it is a production access issue.

Read the status code properly

Status codes narrow the problem. A 401 usually points to missing or invalid credentials. A 403 points to permissions or domain rules. A 404 points to the wrong endpoint or route. A 429 points to rate limits. A 500 points to server-side failure or provider error.

Do not ask AI to rewrite the whole API layer until you know which class of failure you have. A 401 and a CORS block need very different repairs.

• 401: auth or token issue.
• 403: permission or allowed-origin issue.
• 404: wrong URL or route.
• 429: rate limit.
• 500: server or provider failure.

Server functions need their own logs

If the browser calls a serverless function which then calls the API, the browser may only show that the function failed. You need the function log to see whether the private provider call was missing a secret, timed out or returned an error.

This is common in AI-built apps because the visible UI and the server function are generated separately. Both can look correct while the handoff between them fails.

A clean API repair

A proper repair leaves private calls on the server, validates incoming data, handles provider errors clearly and returns useful safe messages to the frontend. It should not leak tokens or show raw provider errors to users.

After the repair, test success, failure and empty-response cases. Production APIs are not guaranteed to return the happy path every time.

How to test API repairs without false positives

After an API repair, test more than one successful request. Check an unauthorised request, an empty response, a slow provider response and a validation failure. AI-generated apps often handle the happy path and then collapse when the provider returns anything else.

Use the live domain for these tests. A local request can still pass because it uses local variables or a development proxy. The important proof is whether the production browser, production server function and external provider agree on the same URL, headers, origin and credentials.

If the API sends business-critical data, also check what is stored or forwarded. A successful HTTP status does not prove the payload is correct. Forms, booking tools, AI assistants, CRMs and payment flows can all return success while saving incomplete or mislabelled information.

When several API-backed features fail together, compare them with production environment variable checks. Shared base URLs, keys or provider domains often explain multiple live failures at once.

• Test success, validation failure and empty result.
• Check the live request and the server/function log.
• Confirm payload fields are correct.
• Confirm provider dashboard allows the production domain.
• Retest after redeploy, not only locally.

Owner handoff notes for live APIs

After the API works live, document the final production endpoint, provider dashboard setting, required variables and whether the request runs from browser code or a server function. This makes the next failure easier to diagnose.

Also record the expected response shape. AI-generated interfaces often assume fields that can change. If the provider response changes later, a clear expected shape helps you repair the app without redesigning the whole feature.

• Production endpoint.
• Allowed domain or origin.
• Server or browser request path.
• Expected response fields.