How to Troubleshoot Failed REST API Calls ? Interview Q&A for QA , SDET, SDEt 2025

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How to Troubleshoot Failed REST API Calls?

When a REST API call fails, it can be due to various reasons such as network issues, incorrect request formatting, authentication failures, or server-side errors. Below is a structured approach to debugging failed REST API calls effectively.

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1. Check the HTTP Status Code

The HTTP status code in the response provides an initial clue about the failure:

Status Code	Meaning	Possible Cause
4xx (Client Error)	Request issue	Bad request, authentication failure, missing parameters
5xx (Server Error)	API issue	Internal server error, downtime, or bug in the backend

👉 Example: If you get 401 Unauthorized, check authentication headers.

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2. Verify the API Endpoint & Request Method

• Ensure the URL is correct (typo errors are common).

• Check if you are using the right HTTP method (GET, POST, PUT, DELETE).

• Confirm that query parameters and path variables are correct.

👉 Example: If calling GET /users/123 but meant GET /user/123, you’ll get a 404 Not Found error.

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3. Check API Authentication & Authorization

If the API requires authentication:

• Verify the API key, OAuth token, or session credentials.

Ensure correct headers are set, e.g.:

Authorization: Bearer <your-token>

• 
  

• Check if the token is expired or missing permissions.

👉 Example: If you get 403 Forbidden, your user might not have access to the requested resource.

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4. Validate Headers & Content-Type

• Ensure the correct Content-Type header is used (application/json, application/xml, etc.).

• Check for required custom headers (e.g., X-API-KEY).

👉 Example: Sending JSON data but missing Content-Type: application/json could result in 415 Unsupported Media Type.

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5. Inspect Request Payload (For POST/PUT Requests)

• Ensure the request body is properly formatted (especially JSON or XML).

• Validate required fields are not missing or null.

• Use an online JSON validator or Postman to check payload correctness.

👉 Example: If sending { "name": "John", "age": "twenty" } instead of { "age": 20 }, the API might return 400 Bad Request.

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6. Test the API Manually (Postman/cURL)

• Use Postman or cURL to isolate the issue from your application code.

Example cURL command:

curl -X GET "https://api.example.com/users/123" -H "Authorization: Bearer <token>"

• 
  

• If Postman works but your code doesn't, check your code logic.

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7. Enable Debugging & Logging

• Check server logs for backend issues.

• Enable verbose logging in your client (Postman, browser dev tools, application logs).

Example: In JavaScript fetch API, log the error response:

fetch("https://api.example.com/data")

  .then(response => response.json())

  .catch(error => console.error("API Error:", error));

• 
  

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8. Check Network Issues & CORS Errors

• Open Chrome DevTools (F12 → Network tab) to inspect the request/response.

• Look for CORS-related errors (browser blocks cross-origin requests if not allowed).

👉 Example: If you see CORS policy: No 'Access-Control-Allow-Origin', the API needs to allow cross-origin requests.

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9. Verify API Rate Limits & Throttling

• Some APIs have rate limits (e.g., 1000 requests/hour).

• Check response headers like X-RateLimit-Limit or Retry-After.

👉 Example: If you get 429 Too Many Requests, wait before retrying.

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10. Check for API Deprecation or Changes

• Verify API documentation for recent updates or deprecations.

• If using a third-party API, check status pages (e.g., https://status.example.com).

👉 Example: If an old endpoint was replaced (/v1/users → /v2/users), update your request.

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Final Step: Contact API Support or Debug Server Logs

If all else fails:

• Contact API support with logs and request details.

• If you control the API, check server logs for deeper debugging.

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