How does the Accept header differ from Content-Type?

Think of two separate questions. Accept says, “What response formats can I handle or prefer?” Content-Type says, “What format is the content in this message?” In a request, Content-Type describes the body you are sending; in a response, it describes the body returned. Accept expresses a response-format preference, not a label for the request body. [S1] [S2]

So if you send a JSON request and want a JSON response, both headers may appear, but they describe different things.

Here is a simplified example. Assume the endpoint accepts the JSON body shown and can return JSON; those are example conditions, not guarantees about any particular service.

POST /items HTTP/1.1
Accept: application/json
Content-Type: application/json

{"name":"notebook"}

The request’s Content-Type labels the JSON body being sent. Accept expresses that the client can handle, and is asking for, a JSON response. If the server returns a response body, check that response’s Content-Type to see how the returned content is identified. The server selects a response format from the formats proposed through Accept and identifies its chosen format with Content-Type. [S1] [S2]

This example doesn’t establish that a real endpoint supports JSON, that it will return JSON, or what it will do if it cannot meet that preference. Check that service’s documentation and the actual response.

Check the direction and the body separately:

1. Sending a body? Read the request’s Content-Type. Ask whether it describes the body actually sent.
2. Requesting a response format? Read Accept. Ask whether it lists a format the client can handle or prefers.
3. Received a body? Read the response’s Content-Type. That describes the returned content—not the client’s earlier preference. [S1] [S2]

For example, if the request body is JSON but its Content-Type says text/plain, investigate the request’s body label. If Accept says application/json but the response is identified as another type, compare the response with the preference you sent. Accept is negotiation input, not a guarantee that the server will return the preferred type. [S1]

Use this before changing a header:

• Locate the message you’re checking: request or response.
• If it has a body, compare that body with its Content-Type.
• For a request, read Accept as the response format or formats the client can handle or prefers.
• If there is a response body, inspect the response’s Content-Type to identify its media type.
• If there is no request body, don’t treat Accept as a substitute body label; check the service’s instructions for which headers it expects.

Quick rule: Content-Type describes content in this message; Accept expresses a response-format preference. Accept values can include specific media types, wildcards, and quality weights. Which formats a service supports, and how it handles every negotiation case, depends on that service. [S1] [S2]