Automated API Testing in Postman with Charles Proxy Tricks

Hi, I’m Nadeem — an ISTQB‑certified QA engineer who loves speed, data, and smart tooling. In this guide I’ll share how automated API testing in Postman becomes even more powerful when you pair it with advanced tips from Charles Proxy.

Why Postman for API Automation?

Postman offers collections, environments, pre‑request scripts, and test assertions — everything you need for scalable API checks. It’s scriptable in JavaScript, CI‑friendly, and great for sharing with devs.

Collections: Group related endpoints and run them as a suite.
Environment Variables: Swap base URLs for staging, QA, prod.
Collection Runner / Newman: CI‑ready execution and JUnit reports.

But some API bugs hide only in real network conditions. That’s where Charles Proxy comes in.

Why Charles Proxy?

Charles Proxy is a local HTTP/HTTPS proxy that lets you inspect, rewrite, throttle, or repeat requests. Used together, Postman and Charles create a full-stack API testing workflow:

Inspect: See raw traffic for validation.
Rewrite: Mock headers or responses without changing server code.
Throttle: Simulate 3G, high‑latency, or packet loss scenarios.

Setting Up Charles as a Proxy for Postman

Open Charles → Proxy ▸ Proxy Settings → note the listening port (default 8888).
In Postman ▸ Settings ▸ Proxy → enable “Use System Proxy” or set:
- Proxy host: 127.0.0.1
- Proxy port: 8888
Send a request in Postman → watch it appear in Charles.

Now every automated run pipes through Charles for inspection or manipulation.

Example: Automating a Login Flow

1. Create the Collection

POST {{baseUrl}}/login
Body: {
  "email": "{{userEmail}}",
  "password": "{{userPwd}}"
}

2. Add Tests in the “Tests” Tab

pm.test("Status is 200", () => {
  pm.response.to.have.status(200);
});
pm.test("Token present", () => {
  const json = pm.response.json();
  pm.expect(json.token).to.exist;
  pm.environment.set("authToken", json.token);
});

3. Run with Newman (CI)

newman run login-collection.json -e staging.json --reporters cli,junit

Charles Proxy Tricks for Advanced Testing

Mock a 500 Error

Right‑click the request in Charles → Enable Rewrite.
Add a rule that changes the response status to 500.

Re‑run the Postman test. Now you can assert graceful error handling.

Throttle to 3G

Charles ▸ Throttle Settings → choose preset “Regular 3G.”
Execute your Postman collection again.

Measure latency, timeout handling, and retry logic.

Repeat a Request Burst

Select a request in Charles → Repeat Advanced.
Set count = 50, concurrency = 10.

Great for quick load‑test smoke checks before real JMeter/Gatling runs.

Data‑Driven Assertions in Postman

Pairing automated API testing in Postman with data files (CSV/JSON) helps cover edge cases:

newman run login-collection.json \
  -e staging.json \
  -d credentials.csv \
  --iteration-count 20

Each row in credentials.csv triggers a login test with unique inputs‑—handy for password rules, account states, or A/B backends.

Best Practices

Keep proxy configs versioned (Charles rewrites can export XML).
Tag tests by priority: smoke, regression, performance.
Log requests in Charles during CI only on failure to save disk.
Rotate auth tokens via pre‑request scripts.

Final Thoughts

By combining automated API testing in Postman with Charles Proxy tricks, you unlock deeper visibility, better mocking, and realistic network scenarios — without setting up complex stubs or environments. Start with small experiments: throttle a service call, rewrite a header, or burst repeat a request. You’ll quickly see bugs that plain automation suites often miss.

Need help integrating Postman, Newman, and Charles into your pipeline? Feel free to reach out — I’m always up for a good testing chat.

Automated API Testing in Postman with Charles Proxy Tricks