---
title: "JMeter HTTP 400 Bad Request & 415 Unsupported Media Type"
description: "Fix JMeter HTTP 400 Bad Request and 415 Unsupported Media Type errors. Configure HTTP Header Manager, Content-Type headers, and JSON body payloads correctly."
url: https://docs.jmeter.ai/topics/errors/http-415-400-bad-request/
lastUpdated: 2026-10-18
source: docs.jmeter.ai
---

# JMeter HTTP 400 Bad Request & 415 Unsupported Media Type

## Symptom

When executing REST API requests in View Results Tree or JTL results, samplers return client-side HTTP 4xx error codes:

```text
Response code: 415 (Unsupported Media Type)
Response message: Unsupported Media Type
```

Or for payload validation issues:

```text
Response code: 400 (Bad Request)
Response message: Bad Request
```

The response body typically explains that the server cannot parse the incoming payload or that the `Content-Type` header is missing or unexpected.

## Quick diagnosis (TL;DR)

- **HTTP 415**: The API endpoint requires `Content-Type: application/json` (or `application/xml`), but JMeter sent the request without a `Content-Type` header (or sent default `text/plain`). Add an **HTTP Header Manager**.
- **HTTP 400**: The payload syntax is malformed (e.g. unescaped quotes in JSON strings, missing required parameters, or invalid variable interpolation like `{"name": \${var}}` without quotes).

## Common causes

| Cause | Status | Why it happens |
| --- | --- | --- |
| Missing `Content-Type` header | 415 | JMeter HTTP Request does not include `Content-Type` by default without an HTTP Header Manager |
| Wrong Header Manager scope | 415 | HTTP Header Manager placed under a different Thread Group or sampler, leaving child samplers without headers |
| Unquoted variable in JSON | 400 | Writing `{"id": \${userId}}` where `userId` is a string value, resulting in invalid JSON `{"id": abc}` |
| Multipart form data checkbox | 415 / 400 | Checking “Use multipart/form-data” for standard raw JSON body payloads |
| Malformed JSON quotes / newlines | 400 | CSV variables containing unescaped double quotes (`"`) or literal newlines breaking JSON schema validation |

## Fix (ordered)

### 1. Add HTTP Header Manager at the Test Plan or Thread Group Level

To ensure every REST API request transmits the proper headers:

1. Right-click your **Thread Group** -> **Add** -> **Config Element** -> **HTTP Header Manager**.
2. Add the standard REST API headers:

| Name | Value |
| --- | --- |
| `Content-Type` | `application/json; charset=UTF-8` |
| `Accept` | `application/json` |

```xml
<!-- HTTP Header Manager configuration in JMX -->
<HeaderManager guiclass="HeaderPanel" testclass="HeaderManager" testname="HTTP Header Manager">
  <collectionProp name="HeaderManager.headers">
    <elementProp name="" elementType="Header">
      <stringProp name="Header.name">Content-Type</stringProp>
      <stringProp name="Header.value">application/json</stringProp>
    </elementProp>
    <elementProp name="" elementType="Header">
      <stringProp name="Header.name">Accept</stringProp>
      <stringProp name="Header.value">application/json</stringProp>
    </elementProp>
  </collectionProp>
</HeaderManager>
```

### 2. Configure the “Body Data” Tab (Do Not Mix with Parameters)

In your **HTTP Request** sampler:

1. Select the **Body Data** tab (not the **Parameters** tab).
2. Uncheck **Use multipart/form-data** (unless performing actual file uploads).
3. Ensure strings inside JSON payloads are wrapped in quotes:

```json
{
  "username": "${username}",
  "email": "${email}",
  "age": ${age}
}
```

### 3. Handle Escaped JSON Strings with `__groovy()`

If CSV data contains special characters, quotes, or newlines, use Groovy’s `JsonOutput.toJson()` to safely escape strings:

```text
${__groovy(groovy.json.JsonOutput.toJson(vars.get('csv_text_column')))}
```

### 4. Inspect Request Headers in View Results Tree

To verify what headers JMeter actually sent over the wire:

1. Run with 1 thread in GUI.
2. Select the failed sampler in **View Results Tree**.
3. Open the **Request** tab -> **Request Headers** sub-tab.
4. Verify that `Content-Type: application/json` is present and matches API documentation.

## Related tools and topics

| Resource | Use when |
| --- | --- |
| [401/403 after recording](/topics/errors/401-403-after-recording/) | Authentication and authorization header issues |
| [API load testing](/topics/api-load-testing/) | Full end-to-end API load testing tutorial |
| [Correlation & dynamic values](/topics/correlation-dynamic-values/) | Extracting and passing dynamic tokens in headers |
| [Properties Cheat Sheet](/tools/properties-cheatsheet/) | Quick lookup of JMeter configuration elements |

## Frequently asked questions

### Can I set different Content-Type headers for different samplers?

Yes. Place an HTTP Header Manager directly as a child of a specific HTTP Request sampler to override the parent Thread Group’s headers for that individual request.

### Why does JMeter change my JSON payload into form parameters?

If you add rows to the “Parameters” tab of an HTTP Request, JMeter switches to `application/x-www-form-urlencoded`. Always clear the Parameters tab when using the “Body Data” tab for raw JSON payloads.

## Continue Learning

→

### Next Practical Step

Add an HTTP Header Manager with `Content-Type: application/json` to your Thread Group and verify the Body Data tab in your samplers.

📖

### Related Reference

- [API load testing](/topics/api-load-testing/)
- [401/403 after recording](/topics/errors/401-403-after-recording/)
- [Functions & variables](/topics/functions-and-variables/)