REST Assured API Not Responding While Postman Works – Root Cause & Fix

Recently, I was debugging an issue for a teammate where an API was not responding when executed through Java code. Interestingly, the same API worked perfectly:

• From my machine
• From his Postman
• From other systems

But it failed only when triggered via Java using REST Assured.

The Problem

Here’s the request our code was making:

RestAssured.baseURI = "URL";

RestAssured.useRelaxedHTTPSValidation();

Response response = RestAssured.given()

        .auth()

        .preemptive()

        .basic(username, password)

        .header("Content-Type", "application/json")

        .body(requestBody)

        .post(endPoint)

        .then()

        .extract()

        .response();

                                          .

When executing this code:

• No response was returned
• The request remained stuck in execution
• Debugging didn’t reveal much

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The Root Cause

After about an hour of debugging and going through documentation, we discovered something subtle:

👉 By default, REST Assured automatically appends a charset to the Content-Type header.

This behavior can sometimes cause issues if the server is not expecting it.

---

The Fix

We resolved the issue by disabling the automatic charset addition using EncoderConfig:

given()
.config(RestAssured.config()
.encoderConfig(encoderConfig()
.appendDefaultContentCharsetToContentTypeIfUndefined(false)));

---

Additional Configuration (Optional)

You can also explicitly define charset behavior:

RestAssured.config = RestAssured.config()
        .encoderConfig(encoderConfig()
        .defaultContentCharset("US-ASCII"));

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Why This Works

• If no charset is specified:
  • Default content charset → ISO-8859-1
  • Query parameter charset → UTF-8
• Some servers reject or mishandle unexpected charset values
• Disabling or controlling this behavior resolves compatibility issues

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Interesting Observation

After applying the fix, the API started responding correctly.

However, when we later removed the configuration, it still worked.

We didn’t verify server logs, but it’s likely that:

• The responses were being served from cache
• Or the server state had changed after initial successful requests

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Key Takeaways

• REST Assured silently adds charset → can break APIs
• Always compare working tools like Postman vs code behavior
• When debugging API issues:
  • Check headers carefully
  • Look for hidden defaults

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Conclusion

Sometimes, the issue isn’t with your API or logic—but with default configurations you didn’t even know existed.

A small tweak in REST Assured configuration can save hours of debugging.