Bug Description

The MIME type validation regex in FastMCP's resource handler doesn't support MIME type parameters, which are valid per RFC 2045.

Code Location

File: src/mcp/server/fastmcp/resources/base.py, line 31

mime_type: str = Field(
    default="text/plain",
    description="MIME type of the resource content",
    pattern=r"^[a-zA-Z0-9]+/[a-zA-Z0-9\-+.]+$",  # ← Doesn't allow params
)

Problem

The regex ^[a-zA-Z0-9]+/[a-zA-Z0-9\-+.]+$ only matches type/subtype but rejects valid MIME types with parameters like:

• text/html;profile=mcp-app
• application/json; charset=utf-8

Why This Matters

The MCP Apps SDK requires resources to use the MIME type text/html;profile=mcp-app to identify MCP App widgets.

When trying to register a resource with this MIME type in Python:

@mcp.resource("ui://my-app/widget.html", mime_type="text/html;profile=mcp-app")
def widget() -> str:
    return Path("widget.html").read_text()

It fails with:

pydantic_core._pydantic_core.ValidationError: 1 validation error for FunctionResource
mime_type
  String should match pattern '^[a-zA-Z0-9]+/[a-zA-Z0-9\-+.]+$' 
  [type=string_pattern_mismatch, input_value='text/html;profile=mcp-app', input_type=str]

Suggested Fix

Update the regex to support optional parameters per RFC 2045:

pattern=r"^[a-zA-Z0-9]+/[a-zA-Z0-9\-+.]+(;\s*[a-zA-Z0-9\-]+=[a-zA-Z0-9\-]+)*$"

Or remove the pattern validation entirely and trust the developer.

Environment

• Python SDK version: 1.23.1
• Python: 3.11