Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsTo debug a broken API request, first capture exactly what was sent and what came back. Then check the request’s URL, method, authentication, headers and body; use the response or browser error to narrow down the cause; and change one thing at a time. This checklist works especially well when a web page’s API call fails, because it separates problems in the request from browser restrictions, transport failures and server responses.
1. Reproduce the failure and capture the request
Repeat the same action that triggers the failure. Open your browser’s developer tools, select the Network panel, and find the request associated with that action. Record whether it appears at all, its method and final URL, request headers and body, status, response headers and body, and timing. Check the Console for JavaScript errors or browser security messages.
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If no request appears in Network, the page may not have initiated it. Check whether the action ran, whether the application logged a runtime error, and whether an extension or page state interfered. A URL entered in the browser’s address bar is not necessarily equivalent to an application request: the application may add authentication, headers, cookies or a body.
Postman’s Console can show the request sent, underlying headers, variable values, redirects, proxy and certificate configuration, network details, and the raw response before Postman processes it. Its documentation 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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Keep credentials and private information safe: redact bearer tokens, cookies, personal data and private endpoints before sharing screenshots, copied requests, or logs.
2. Check the request that actually went out
Inspect the final request, not just a URL template or source-code snippet. Compare it with the API’s own documentation or a known working example.
- URL: Verify the host, path, version prefix, query parameters and any substituted variables or path parameters. Look for unresolved variables, misspellings and an unintended environment.
- Scheme: Confirm whether the endpoint requires
httpsrather thanhttp. - Method: Check that the request uses the documented method, such as
GETorPOST. - Authentication: Confirm the credential is present, current and sent in the expected format. Check that the account or token has the required permissions.
- Headers: Compare headers such as
Content-TypeandAcceptwith the API requirements. - Body: Check that the payload uses the expected format and field names, types and structure. Confirm that the request actually includes a body when one is required.
Postman lists incorrect URLs, unresolved variables and an incorrect HTTP/HTTPS scheme among common troubleshooting paths. See its API request troubleshooting guide.
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3. Read the response, not just the status code
Record the HTTP status, response headers, response body, cookies where relevant, and duration. A status code narrows the investigation, but the API’s response body and documentation often explain what the service rejected or what failed. Note whether the response is an API error object, empty, malformed, or HTML that may have come from a proxy or gateway.
Postman’s response viewer exposes the body, headers, cookies and HTTP status, along with network information and response time. Its troubleshooting documentation also recommends examining the request and response details when diagnosing a failure.
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These status-code patterns are diagnostic heuristics, not universal definitions. Follow the particular API’s documentation for the meaning of its responses and its retry rules.
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| What you see | Check first | What it may indicate |
|---|---|---|
| No request in Network | Page action, JavaScript/runtime errors, and extensions or other blockers | The request may never have been initiated. |
| CORS error in the browser | Console message, failed request, any preflight OPTIONS request, and the server’s CORS response |
Browser cross-origin rules or server CORS configuration may be involved. A successful non-browser request is a clue, not proof that the API is otherwise correct. |
| 400 or validation error | Method, route, query, required fields, payload shape, content type and error body | The request may not match the API’s expected structure or validation rules. |
| 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 missing or invalid identity from insufficient access. |
| 404 | Host, base path, route, version prefix, trailing slash and environment | The route may not exist at that address, or routing may differ by environment. |
| 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 retry interval. |
| 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 | Connectivity, proxy, VPN or firewall, timeout, TLS and certificates; note whether any response arrived | The failure may have occurred before an HTTP response. A timeout alone does not show whether the server processed the request. |
5. Find out whether the browser is part of the problem
If page JavaScript fails, try sending an equivalent request through an appropriate request client. Keep the method, URL, headers and body the same where possible, while protecting credentials. A difference in outcome can help isolate browser behavior, but it is not a definitive comparison: the client may use different cookies, authentication, origin, proxy or network route.
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Postman documents different behavior across its agents: its Browser Agent may encounter CORS restrictions, while its Desktop Agent routes requests through the local machine and avoids browser CORS limitations. The Cloud Agent also avoids browser CORS limits but cannot reach private or local network resources and has plan-based usage limits. Check Postman’s agent documentation for current behavior and availability.
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6. Check transport and environment when there is no HTTP response
If the request never returns an HTTP response, investigate the route between the client and server before treating it as an API-level error.
- Check whether other sites or services are reachable.
- Consider whether a VPN, corporate proxy, firewall, DNS setting or network policy could block the destination.
- Inspect HTTPS certificate validation and whether the endpoint requires a client certificate.
- Check whether the client’s timeout is too short for the operation.
- If the issue varies by environment, record the browser, operating system, network, request agent and HTTP version.
Do not routinely disable certificate verification to make a request succeed. A verification failure is evidence to inspect certificate configuration and the trust chain. Postman lists connectivity, firewalls, proxies, SSL or client certificates, timeouts, TLS compatibility, and malformed response encoding or headers among possible causes in its troubleshooting guide. Its Console documentation also describes selecting HTTP/1.x or HTTP/2 for diagnostic comparison, with constraints on when each is used: Postman Console.
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7. Choose a tool that matches the debugging question
Free browser tools and request clients answer different questions. Start with the browser when you need to know what the page actually attempted; use a standalone client to test a request outside the page’s browser context.
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- Browser developer tools: Best starting point for a request triggered by a website. Network shows the page’s attempted traffic; Console surfaces browser and runtime errors.
- Postman Browser Tool: Captures requests generated while you interact with a web application, then lets you open a captured request as an HTTP request for further exploration. Its documentation describes it as a feature in the Postman desktop app at workspace level. See Inspect traffic with the Browser Tool.
- Postman agents: The Browser Agent can be subject to browser CORS restrictions. The Desktop Agent uses the local machine and avoids those restrictions, but does not support Safari. The Cloud Agent cannot access private or local network resources and has plan-based monthly usage limits. Postman describes a Free-plan Interceptor Agent as available while it gathers feedback; it captures and inspects traffic and can send HTTP requests, but does not support WebSocket, Socket.IO, gRPC, MQTT or GraphQL. Because feature availability can change, verify current agent details in Postman’s agent documentation.
curl: Useful for creating a minimal command-line reproduction. It does not reproduce browser CORS enforcement, so a successfulcurlrequest does not by itself explain whether page JavaScript can make the call.
8. Change one thing at a time and preserve a safe reproduction
Once you have a baseline, change one value, resend, and capture the new request and response. Compare the result with the original instead of changing several settings at once. Postman’s Console history can help compare earlier configurations when response history was enabled and the request is in a supported workspace.
Be cautious with retries for non-idempotent operations, such as creating payments or records. A timeout does not establish that the server did no work; retry only when the API documents safe behavior or provides a suitable way to verify the original operation.
If a server-side error continues with a correctly formed request, give the API owner a redacted reproduction, timestamp, endpoint, request ID if present, and the relevant response evidence. Do not include live tokens, session cookies or personal data.
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