resource mimetype not set to JSON automatically

[ilia-kats]

As far as I understand the docs, if a resource function's return value is converted to JSON, also the mimetype will be automatically set to application/json. However, that is not the case: It's always text/plain, both in my own code and in the desktop.py example.

Am I misunderstanding the docs or is there something missing here?

[LarryHu0217]

JSON serialization and the resource's MIME type are separate here. Returning a dict can produce JSON text while the resource still advertises the default text/plain. A -> dict return annotation does not change that default.

Set the MIME type explicitly on the resource:

from fastmcp import FastMCP

mcp = FastMCP("example")

@mcp.resource("data://config", mime_type="application/json")
def config() -> dict:
    return {"ok": True}

There is no need to call json.dumps() for that dictionary. If your function already returns a JSON string, keep the explicit MIME type as well.

The current return-value documentation now explains that JSON-native return values retain the resource's declared MIME type. So the automatic JSON conversion does not imply automatic application/json metadata.

I verified both resources/list and resources/read through an in-memory FastMCP client/server at source commit b0bbca6: without the decorator argument, both reported text/plain; with it, both reported application/json. Both payloads parsed as the same dictionary. I have not reproduced your specific installed version or desktop client.