Introduction – Why API Testing Coding Questions Matter in Interviews
Modern QA and SDET interviews no longer stop at “What is an API?”. Interviewers increasingly ask api testing coding interview questions to evaluate whether candidates can:
- Write actual test code for APIs
- Validate responses using assertions
- Automate API tests instead of only using UI tools
- Debug real backend issues using code
For freshers, coding questions usually focus on basic API calls and validations.
For experienced candidates, interviewers expect framework-based API automation, assertions, and real-time scenarios.
This article is a complete coding-focused guide to help you prepare for api testing coding interview questions, with real examples, sample responses, Postman scripts, Rest Assured, and Python code.
What Is API Testing? (Clear & Simple)
API testing verifies whether an API:
- Accepts valid requests
- Enforces business rules
- Returns correct responses and status codes
- Handles errors gracefully
API testing coding means doing this validation using code, not only manual tools.
Example
For a login API:
- Valid credentials → 200 OK + token
- Invalid credentials → 401 Unauthorized
In coding interviews, you are often asked to write code to validate this behavior.
REST vs SOAP vs GraphQL (Interview Comparison)
| Feature | REST | SOAP | GraphQL |
| Transport | HTTP | HTTP/SMTP | HTTP |
| Payload | JSON / XML | XML | JSON |
| Complexity | Simple | Complex | Flexible |
| Usage | Most modern apps | Banking/Legacy | Modern APIs |
Most api testing coding interview questions focus on REST APIs.
API Testing Coding Interview Questions and Answers (80+)
Section 1: Core API & Coding Basics (Q1–Q20)
1. What are API testing coding interview questions?
Questions that require writing code to test APIs using tools or programming languages.
2. Why do interviewers ask API testing coding questions?
To assess real automation skills, logic, and debugging ability.
3. Which languages are commonly used for API testing coding?
- Java (Rest Assured)
- Python (requests)
- JavaScript (Postman / Newman)
4. What is REST API?
An architectural style using HTTP methods to access resources.
5. What HTTP methods are commonly tested?
GET, POST, PUT, PATCH, DELETE
6. What is an endpoint?
A URL representing an API resource.
7. What is request payload?
Data sent to API in request body.
8. What is response body?
Data returned by API.
9. What is JSON?
Lightweight data-interchange format.
{
“id”: 101,
“name”: “Ravi”,
“role”: “QA”
}
10. What is XML?
Markup format mostly used in SOAP APIs.
11. What is statelessness?
Each request contains all required information.
12. What is idempotency?
Multiple identical requests give same result.
13. What is authentication?
Verifying identity using token or credentials.
14. What is authorization?
Verifying access rights.
15. What is JWT token?
JSON Web Token used for secure access.
16. What is API schema?
Defines response structure and data types.
17. What is API chaining?
Using response of one API as input for another.
18. What is API automation?
Automating API tests using code.
19. What is assertion?
Validation condition in code.
20. What is negative API testing?
Testing invalid input scenarios.
HTTP Status Codes – Must Know for Coding Interviews
| Code | Meaning | Example |
| 200 | OK | Successful GET |
| 201 | Created | POST success |
| 204 | No Content | DELETE success |
| 400 | Bad Request | Invalid input |
| 401 | Unauthorized | Invalid token |
| 403 | Forbidden | No access |
| 404 | Not Found | Wrong URL |
| 409 | Conflict | Duplicate |
| 500 | Server Error | Backend issue |
API Validation Example (Coding-Oriented)
Sample Request
POST /api/login
Content-Type: application/json
{
“username”: “testuser”,
“password”: “pass123”
}
Sample Response
{
“token”: “abc.def.ghi”,
“expires_in”: 3600
}
Validations Required in Coding Interviews
- Status code == 200
- Token exists
- expires_in > 0
Coding Snippets Commonly Asked in API Testing Coding Interviews
1️⃣ Postman Test Script (JavaScript)
pm.test(“Status code is 200”, function () {
pm.response.to.have.status(200);
});
pm.test(“Token exists”, function () {
const jsonData = pm.response.json();
pm.expect(jsonData.token).to.not.be.undefined;
});
2️⃣ Rest Assured (Java)
given()
.contentType(“application/json”)
.body(payload)
.when()
.post(“/login”)
.then()
.statusCode(200)
.body(“token”, notNullValue());
3️⃣ Python Requests
import requests
response = requests.post(url, json=payload)
assert response.status_code == 200
assert “token” in response.json()
4️⃣ Coding Question: Validate Response Time
.then()
.time(lessThan(2000L));
5️⃣ Coding Question: Extract Token and Reuse
String token =
given()
.body(payload)
.when()
.post(“/login”)
.then()
.extract()
.path(“token”);
Advanced API Testing Coding Interview Questions (Q21–Q45)
21. How do you validate JSON schema using code?
Using schema validators.
22. How do you perform data-driven API testing?
Read input data from CSV/JSON.
23. How do you handle authentication in API automation?
By generating and passing tokens dynamically.
24. How do you validate headers in code?
By asserting header values.
25. How do you validate array size in response?
Using length assertions.
26. How do you validate numeric ranges?
Boundary assertions.
27. How do you test pagination using code?
Loop through pages and validate counts.
28. How do you test API rate limiting?
Send multiple requests and expect 429.
29. How do you handle API retries?
Retry logic in framework.
30. How do you log request and response?
Using logging libraries.
31. How do you test API rollback?
Verify no partial data saved.
32. How do you test API concurrency?
Send parallel requests.
33. How do you test API security?
Validate unauthorized access.
34. How do you mock APIs in coding interviews?
Using stub servers.
35. How do you integrate API tests in CI/CD?
Run tests via Jenkins pipeline.
36. What is API contract testing?
Validating provider-consumer agreement.
37. How do you validate error messages?
Assert error code and message.
38. How do you handle environment-based testing?
Use config files and variables.
39. How do you test file upload APIs?
Send multipart requests.
40. How do you test file download APIs?
Validate response headers and content.
41. How do you handle flaky API tests?
Improve waits and environment stability.
42. How do you validate sorting?
Check response order.
43. How do you test filtering?
Use query parameters.
44. How do you validate default values?
Check response fields when input missing.
45. How do you test date fields?
Validate format and range.
Scenario-Based API Testing Coding Interview Questions (15)
These are very common coding + logic questions.
- API returns 200 but wrong data – what assertions will you add?
- Login API works but profile API fails – how do you debug?
- Token expired but API still accessible – what test to write?
- API works manually but fails in automation – why?
- Duplicate records created – how do you validate?
- API slow under load – what coding checks?
- Payment deducted but order not created – how to test rollback?
- API returns null fields – how to catch in code?
- API crashes for special characters – what test case?
- Same request returns different responses – what assertions?
- API schema changed – how will tests fail?
- Unauthorized user accesses secured API – how to validate?
- API fails only in CI pipeline – what logs to check?
- API returns XML instead of JSON – how to handle?
- Partial data saved – what validation missed?
How Interviewers Evaluate API Testing Coding Answers
Interviewers look for:
- Correct use of assertions
- Understanding of API behavior
- Clean, readable code
- Ability to explain failures
- Practical debugging mindset
👉 Writing correct code + explaining logic = success
API Testing Coding Interview Cheatsheet
- Validate more than status code
- Always assert response body
- Handle auth dynamically
- Test negative scenarios
- Use meaningful assertions
- Log request/response
FAQs – API Testing Coding Interview Questions
Q1. Are coding questions mandatory for API testing roles?
Yes, especially for automation and SDET roles.
Q2. Which language is best to prepare?
Java (Rest Assured) or Python.
Q3. Is Postman scripting enough?
Good for basics; coding frameworks preferred.
Q4. What is the biggest mistake candidates make?
Only checking status code.
Q5. Do freshers get coding questions?
Yes, but at a basic level.