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To debug a failing API request, capture the exact request and response first, then check request construction, browser restrictions, and network transport in that order. Browser developer tools are the best starting point for a call made by a website; a separate API client can help isolate browser-specific behavior, but it does not prove the API is healthy or reproduce the browser’s security rules.
1. Capture the failure before changing anything
Repeat the same action that triggers the problem. Note when it occurs, which endpoint is involved, and whether the browser shows an HTTP response or a network error. Open the browser’s developer tools and inspect the Network panel. Preserve the failed request and its response details; check the Console for JavaScript errors or browser security messages.
Record the final URL, method, request headers and body, status code, response headers and body, cookies where relevant, and duration. A status code on its own rarely explains an API-specific failure. The body may contain a useful error object, HTML from a gateway or proxy, no content, or malformed data.
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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Postman’s response viewer exposes the body, headers, cookies, HTTP status, network information, and response time. Its Console shows the request sent, underlying headers, variable values, redirects, proxy and certificate configuration, and raw response before Postman processes it. Postman says, “Every request sent by Postman is logged in the Postman Console, so you can view the details of what happened when you sent the request.” Postman: Troubleshooting API requests.
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Do not share unredacted screenshots, copied requests, or logs: authorization headers, cookies, personal information, and private endpoints may be sensitive.
2. Verify the request that actually went out
Check the transmitted request, not just the code or URL template you intended to send. Compare it with the API’s documentation or a known working example.
- URL: Confirm the host, path, version prefix, query parameters, and any substituted variables or path parameters. Make sure variables resolved to the expected values.
- Scheme and method: Check whether the request uses
httporhttps, and whether it uses the method the endpoint expects. - Authentication and headers: Check that the expected credentials are present and current, and that headers such as
Content-TypeandAcceptmatch the API’s requirements. - Body: Confirm the payload format, field names, required values, and JSON structure. A syntactically valid body can still violate the API’s schema.
Postman’s troubleshooting guide specifically calls out incorrect URLs, unresolved variables, and an incorrect HTTP/HTTPS scheme as issues to check. Postman: Troubleshooting API requests.
3. Use the response to narrow the cause
Read the status, headers, and body together, then compare them with the API’s own documented error behavior. The following are general diagnostic heuristics, not universal meanings: individual APIs can use status codes and error fields differently.
| What you see | First checks | What it may suggest |
|---|---|---|
| No request in the Network panel | JavaScript or runtime errors, whether the action triggered a call, and whether an extension blocked it | The request may not have been initiated. |
| 400 or validation response | Method, route, query, required fields, payload shape, content type, and error body | The request structure or a field may not meet the API’s contract. |
| 401 or 403 | Credential presence and expiry, authentication scheme, scopes or permissions, cookies, and origin | Authentication or authorization may have been rejected. Use the API documentation to distinguish an invalid identity from insufficient access. |
| 404 | Host, base path, route, API version, trailing slash, and environment | The route may not exist at that address, or routing may differ between environments. |
| 429 | Response headers and the API’s rate-limit policy | The service may be limiting requests. Reduce request frequency and follow documented retry guidance; do not assume a fixed wait time. |
| 5xx | Request ID, timestamp, response body, repeatability, and the API’s status page if available | A server or upstream failure is plausible. Preserve the evidence for the API owner. |
| Timeout, connection reset, or TLS error | Whether any response arrived; connectivity, proxy, VPN, firewall, timeout, TLS, and certificate settings | The request may have failed before an HTTP response. A timeout alone does not establish whether the server processed the work. |
4. Find out whether the browser is part of the problem
If page JavaScript fails, try the same method, URL, headers, and body in an appropriate request client. Keep credentials private. If the client succeeds while the page fails, inspect the browser Console for a CORS error and the Network panel for a preflight OPTIONS request, if one was sent.
This comparison is a clue, not proof that the API is otherwise correct. The page and client may differ in cookies, authentication, origin, proxy, or network route. A command-line request made with curl, for example, is not a browser request and does not enforce browser CORS rules.
Postman documents that its Browser Agent may encounter CORS restrictions, while its Desktop Agent sends requests through the local machine and avoids browser CORS limitations. Its Cloud Agent also avoids browser CORS limitations, but cannot access private or local network resources and has plan-based usage limits. Agent features and availability can change; check Postman’s agent documentation for current details.
5. Choose a tool that matches the question
| Tool | Best use | Important limitation |
|---|---|---|
| Browser developer tools | See what the page attempted, inspect browser errors, and examine requests and responses in the Network panel. | They observe page behavior; they do not automatically provide a clean standalone replay. A URL entered in the browser bar is not necessarily equivalent to an authenticated application request. |
| Postman Browser Tool | Capture requests generated while interacting with a web application, then open a captured request for further exploration. | Postman describes it as a workspace-level feature in its desktop app. See Postman: Inspect traffic with the Browser Tool. |
| Postman Desktop Agent | Send requests through the local machine, including when browser CORS restrictions are getting in the way. | It does not support Safari. A successful request still may differ from the page’s request in origin, cookies, or other context. |
| Postman Cloud Agent | Send requests without relying on browser CORS behavior. | It cannot reach private or local network resources; usage limits depend on the plan. |
| Postman Interceptor Agent | Capture and inspect traffic and send HTTP requests. | Postman describes Free-plan availability as being offered while it gathers feedback. It does not support WebSocket, Socket.IO, gRPC, MQTT, or GraphQL; verify current availability before relying on it. |
curl |
Make a minimal command-line reproduction and inspect the result. | It does not reproduce browser CORS enforcement or automatically carry the page’s cookies and authentication context. |
Agent distinctions and feature status are described in Postman’s agent documentation. Use a tool that can answer the question at hand: observe what the browser did, replay a request, or test connectivity from a particular network.
6. Check transport and environment when no response arrives
If the request fails before an HTTP response, investigate the path between the client and server rather than interpreting it as an API error. Check whether other sites work, and whether a VPN, corporate proxy, firewall, DNS or network policy is involved. Confirm the timeout is reasonable and whether the endpoint requires a client certificate.
For HTTPS failures, inspect certificate validation and the trust chain. Do not routinely disable certificate verification: a verification failure is evidence to investigate, not a safe fix. Postman’s troubleshooting guide also lists connectivity, firewalls, proxies, SSL or client certificates, timeouts, TLS compatibility, and malformed responses among possible causes. Postman: Troubleshooting API requests.
If the result differs by environment, record the browser and operating system, network, request agent, and HTTP version. Postman allows HTTP/1.x or HTTP/2 to be selected for diagnostic comparison, with constraints on when each is used; see its troubleshooting documentation.
7. Change one thing at a time and preserve a reproducible case
Once you have a baseline, change one variable, resend, and capture the new request and response. Compare the new evidence with the original rather than making several changes at once. Postman Console history can help compare configurations when response history was enabled and the request is in a supported workspace.
Be cautious with retries for operations that create records, charge payments, or otherwise change state. A timeout does not prove that the server failed to process the request. Retry only when the API documents safe retry behavior, such as an appropriate idempotency mechanism.
If a correctly formed request continues to produce a server-side error, send the API owner a redacted reproduction, timestamp, endpoint, request ID if present, and relevant response evidence. Remove bearer tokens, cookies, personal data, and other secrets.
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