Request body is not JSON-stringified when Content-Type: application/json is specified

[ryota-iso]

Summary

When specifying header: { 'content-type': 'application/json' }, the request body is not stringified.
Even when application/json is explicitly set, the body should be parsed and serialized accordingly.

Steps to Reproduce

Run the following code and inspect the request body being sent.

betterFetch("https://example.com", {
  headers: {
    "content-type": "application/json"
  },
  body: {
    "foo": "bar",
  },
  method: "POST"
});

Expected Behavior

The request body should be serialized using JSON.stringify before being sent.

Example:

{ "foo": "bar" }

Actual Behavior

The body is sent as [object Object].

Environment

• Library version: 1.1.21
• OS: macOS 15.7.3
• Runtime: Node.js 24.12.0
• Package manager: pnpm 10.25.0