How can you tell whether an HTTP response contains JSON?

No. A successful status and the body’s format answer different questions. The status does not establish that the returned representation is JSON. Inspect the response’s Content-Type header first. For a JSON response, a common value is application/json. The header is useful metadata and a clue about the representation, not proof that the body is valid JSON. [S1]

With browser JavaScript Fetch, check the status and header separately, then attempt JSON parsing only when the header indicates JSON. Be ready for parsing to fail if the response body is unexpected or invalid. [S2]

Read the response’s Content-Type value and look for a JSON media type, commonly application/json. Treat that as guidance for choosing how to read the body. If the value indicates another format, do not automatically call a JSON parser: handle the response according to the format your application expects. The header describes the representation, but it cannot by itself guarantee that the body matches or is valid. [S1]

For example, if an endpoint is expected to return JSON but the response says text/html, pause before parsing it as JSON. That is a hypothetical example, not a claim about what any particular server returns. Check that you are examining the intended response and record the status, header, and a safe description of the body when investigating.

Here is an illustrative pattern for browser JavaScript. It checks the status separately, uses the header to decide whether to try JSON parsing, and handles a parse failure:

const response = await fetch("/api/example");
const contentType = response.headers.get("Content-Type") ?? "";

// Check response.status separately for your application.
if (!contentType.includes("application/json")) {
  throw new Error(`Expected JSON; received ${contentType || "no Content-Type"}`);
}

try {
  const data = await response.json();
  console.log(data);
} catch (error) {
  console.error("The response could not be parsed as JSON", error);
}

The application/json check is a simple example; adjust it to the response format your application expects. A matching header is a parsing clue, not a guarantee that parsing will succeed. Fetch separates checking the response status from reading its body, and its guidance supports checking Content-Type before trying to read JSON. [S1][S2]

Capture the status, the exact Content-Type value (or note that it is absent), and what happened when parsing was attempted. If you can inspect the body, describe whether it appears to be JSON or something else, but avoid sharing secrets or personal data from it. Then compare those observations with the response format your application expects.

A successful status, a JSON-looking header, or a JSON-looking sample alone does not establish that the entire body is valid JSON. The header is a clue for choosing how to parse; unexpected or invalid bodies still need to be handled. These observations do not by themselves explain why a mismatch occurred or what a particular server should return. [S1][S2]