When performing API testing, understanding API status codes is one of the most fundamental skills every software tester should master. Status codes provide immediate feedback about whether an API request was successful, redirected, contained a client-side error, or failed due to a server issue.
Instead of relying solely on the response body, testers should always verify the HTTP status code first. A correct status code helps confirm that the application behaves as expected and makes it easier to identify issues quickly.
In this article, we’ll explore the most important API status codes, explain what they mean, and discuss how software testers can use them to improve API testing.
What Are API Status Codes?
API status codes are three-digit HTTP response codes returned by a web server after it processes a client’s request. These codes indicate the outcome of the request and help developers and testers understand whether the operation succeeded or failed.
Each status code belongs to one of five categories:
- 1xx – Informational: The request has been received and processing continues.
- 2xx – Success: The request was successfully processed.
- 3xx – Redirection: Additional action is required to complete the request.
- 4xx – Client Errors: The request contains invalid data or cannot be processed.
- 5xx – Server Errors: The server encountered an unexpected problem.
For software testers, the 2xx, 4xx, and 5xx categories are the most commonly encountered during API testing.
2xx Success Status Codes
A 2xx status code indicates that the server successfully processed the request.
200 OK
The 200 OK status code is the most common response during API testing. It indicates that the request was successfully completed and the requested resource has been returned.
Example:
A user requests their profile information.
GET /api/users/15
Response:
200 OK
As a tester, verify:
- The status code is 200.
- The response body contains the expected data.
- Response time meets performance expectations.
201 Created
The 201 Created response indicates that a new resource has been successfully created.
Example:
POST /api/users
Response:
201 Created
A tester should also verify that:
- The new resource exists.
- The response includes the generated ID if applicable.
- The data is correctly stored in the database.
204 No Content
The server successfully processed the request but doesn’t return any response body.
This is commonly used after successful DELETE operations.
Example:
DELETE /api/users/15
Response:
204 No Content
Testers should confirm that:
- No response body is returned.
- The resource was actually deleted.
- Future requests for that resource return an appropriate error.
3xx Redirection Status Codes
Although less common in API testing, redirection codes are still important.
301 Moved Permanently
This status code indicates that the requested resource has permanently moved to another URL.
Modern APIs rarely use this internally, but testers should verify that clients correctly follow redirects when necessary.
304 Not Modified
Used with caching mechanisms.
If the requested resource hasn’t changed since the last request, the server returns 304 Not Modified, reducing unnecessary data transfer.
Testers working on performance optimization should understand this behavior.
4xx Client Error Status Codes
A 4xx status code means the problem originated from the client request rather than the server.
400 Bad Request
The request contains invalid syntax or missing required information.
Common causes include:
- Missing required fields
- Invalid JSON
- Incorrect parameter values
- Invalid request format
Testers should intentionally send invalid requests to verify that the API returns 400 Bad Request along with a meaningful error message.
401 Unauthorized
The request lacks valid authentication credentials.
Example scenarios include:
- Missing authentication token
- Expired access token
- Invalid API key
A tester should verify that protected endpoints reject unauthorized users.
403 Forbidden
Unlike 401, the user is authenticated but doesn’t have permission to access the resource.
Example:
A regular user attempts to access an administrator endpoint.
Expected response:
403 Forbidden
Permission testing is an essential part of API security testing.
404 Not Found
The requested resource doesn’t exist.
Example:
GET /api/users/999999
If the user doesn’t exist, the API should return:
404 Not Found
A tester should ensure that nonexistent resources return the correct error instead of misleading success responses.
409 Conflict
This status code occurs when a request conflicts with existing data.
Example:
Creating an account using an email address that already exists.
Proper conflict handling improves application reliability and user experience.
5xx Server Error Status Codes
A 5xx status code indicates that the server encountered an unexpected problem while processing the request.
These errors are generally not caused by the client.
500 Internal Server Error
One of the most common server-side failures.
Possible causes include:
- Application bugs
- Database failures
- Unexpected exceptions
- Misconfigured services
Testers should report:
- Steps to reproduce
- Request payload
- Response body
- Response headers
- Server logs (if available)
502 Bad Gateway
Occurs when one server receives an invalid response from another server.
This often appears in microservices architectures where multiple services communicate with each other.
503 Service Unavailable
The server is temporarily unavailable due to:
- Maintenance
- High traffic
- Resource exhaustion
Testers performing load or performance testing frequently encounter this status code.
Best Practices for Testing API Status Codes
Simply checking whether an endpoint returns 200 OK is not enough. Effective API testing requires validating both the status code and the behavior behind it.
Here are some best practices:
- Always verify the expected status code for every test case.
- Validate the response body along with the status code.
- Test positive and negative scenarios.
- Confirm that error messages are meaningful.
- Verify authentication and authorization responses.
- Check response headers where applicable.
- Ensure APIs don’t expose sensitive information in error responses.
- Include status code validation in your automated API tests.
Why API Status Codes Matter
Understanding API status codes helps software testers quickly identify whether an issue originates from the client, server, authentication, permissions, or application logic.
Rather than treating status codes as simple numbers, testers should view them as valuable diagnostic tools that reveal how well an API handles different scenarios. Correct validation of status codes improves test coverage, speeds up debugging, and contributes to building reliable, secure, and high-quality applications.
Whether you’re performing manual API testing or building automated API test suites, mastering API status codes is an essential step toward becoming a more effective software tester.