Improve json handling

src/fastmcp/resources/types.py

@@ -1,5 +1,6 @@
 """Concrete resource implementations."""
 
+import pydantic_core
 import asyncio
 import json
 from pathlib import Path
@@ -58,8 +59,8 @@ class FunctionResource(Resource):
             if isinstance(result, str):
                 return result
             try:
-                return json.dumps(result, default=pydantic.json.pydantic_encoder)
-            except TypeError:
+                return json.dumps(pydantic_core.to_jsonable_python(result))
+            except (TypeError, pydantic_core.PydanticSerializationError):
                 # If JSON serialization fails, try str()
                 return str(result)
         except Exception as e:

src/fastmcp/server.py

@@ -1,5 +1,6 @@
 """FastMCP - A more ergonomic interface for MCP servers."""
 
+import pydantic_core
 from typing import Any, Literal, Optional, Union
 
 from mcp.server import RequestContext
@@ -14,7 +15,6 @@ from typing import Callable, Sequence
 import inspect
 import re
 
-import pydantic.json
 from mcp.server import Server as MCPServer
 from mcp.server.stdio import stdio_server
 from mcp.server.sse import SseServerTransport
@@ -397,7 +397,7 @@ def _convert_to_content(value: Any) -> Sequence[Union[TextContent, ImageContent]
                 result.append(
                     TextContent(
                         type="text",
-                        text=json.dumps(item, default=pydantic.json.pydantic_encoder),
+                        text=json.dumps(pydantic_core.to_jsonable_python(item)),
                     )
                 )
         return result
@@ -414,7 +414,7 @@ def _convert_to_content(value: Any) -> Sequence[Union[TextContent, ImageContent]
     return [
         TextContent(
             type="text",
-            text=json.dumps(value, indent=2, default=pydantic.json.pydantic_encoder),
+            text=json.dumps(pydantic_core.to_jsonable_python(value)),
         )
     ]
 

tests/resources/test_function_resources.py

@@ -1,3 +1,4 @@
+from pydantic import BaseModel
 import pytest
 from fastmcp.resources import FunctionResource
 
@@ -80,6 +81,20 @@ class TestFunctionResource:
         with pytest.raises(ValueError, match="Error reading resource function://test"):
             await resource.read()
 
+    async def test_basemodel_conversion(self):
+        """Test handling of BaseModel types."""
+
+        class MyModel(BaseModel):
+            name: str
+
+        resource = FunctionResource(
+            uri="function://test",
+            name="test",
+            func=lambda: MyModel(name="test"),
+        )
+        content = await resource.read()
+        assert content == '{"name": "test"}'
+
     async def test_custom_type_conversion(self):
         """Test handling of custom types."""