fix: allow MIME type parameters in resource validation (RFC 2045) (#1755)

Co-authored-by: Claude <noreply@anthropic.com>

src/mcp/server/fastmcp/resources/base.py

@@ -28,7 +28,7 @@ class Resource(BaseModel, abc.ABC):
     mime_type: str = Field(
         default="text/plain",
         description="MIME type of the resource content",
-        pattern=r"^[a-zA-Z0-9]+/[a-zA-Z0-9\-+.]+$",
+        pattern=r"^[a-zA-Z0-9]+/[a-zA-Z0-9\-+.]+(;\s*[a-zA-Z0-9\-_.]+=[a-zA-Z0-9\-_.]+)*$",
     )
     icons: list[Icon] | None = Field(default=None, description="Optional list of icons for this resource")
     annotations: Annotations | None = Field(default=None, description="Optional annotations for the resource")

tests/issues/test_1754_mime_type_parameters.py

@@ -0,0 +1,70 @@
+"""Test for GitHub issue #1754: MIME type validation rejects valid RFC 2045 parameters.
+
+The MIME type validation regex was too restrictive and rejected valid MIME types
+with parameters like 'text/html;profile=mcp-app' which are valid per RFC 2045.
+"""
+
+import pytest
+from pydantic import AnyUrl
+
+from mcp.server.fastmcp import FastMCP
+from mcp.shared.memory import (
+    create_connected_server_and_client_session as client_session,
+)
+
+pytestmark = pytest.mark.anyio
+
+
+async def test_mime_type_with_parameters():
+    """Test that MIME types with parameters are accepted (RFC 2045)."""
+    mcp = FastMCP("test")
+
+    # This should NOT raise a validation error
+    @mcp.resource("ui://widget", mime_type="text/html;profile=mcp-app")
+    def widget() -> str:
+        raise NotImplementedError()
+
+    resources = await mcp.list_resources()
+    assert len(resources) == 1
+    assert resources[0].mimeType == "text/html;profile=mcp-app"
+
+
+async def test_mime_type_with_parameters_and_space():
+    """Test MIME type with space after semicolon."""
+    mcp = FastMCP("test")
+
+    @mcp.resource("data://json", mime_type="application/json; charset=utf-8")
+    def data() -> str:
+        raise NotImplementedError()
+
+    resources = await mcp.list_resources()
+    assert len(resources) == 1
+    assert resources[0].mimeType == "application/json; charset=utf-8"
+
+
+async def test_mime_type_with_multiple_parameters():
+    """Test MIME type with multiple parameters."""
+    mcp = FastMCP("test")
+
+    @mcp.resource("data://multi", mime_type="text/plain; charset=utf-8; format=fixed")
+    def data() -> str:
+        raise NotImplementedError()
+
+    resources = await mcp.list_resources()
+    assert len(resources) == 1
+    assert resources[0].mimeType == "text/plain; charset=utf-8; format=fixed"
+
+
+async def test_mime_type_preserved_in_read_resource():
+    """Test that MIME type with parameters is preserved when reading resource."""
+    mcp = FastMCP("test")
+
+    @mcp.resource("ui://my-widget", mime_type="text/html;profile=mcp-app")
+    def my_widget() -> str:
+        return "<html><body>Hello MCP-UI</body></html>"
+
+    async with client_session(mcp._mcp_server) as client:
+        # Read the resource
+        result = await client.read_resource(AnyUrl("ui://my-widget"))
+        assert len(result.contents) == 1
+        assert result.contents[0].mimeType == "text/html;profile=mcp-app"