replace inefficient use of to_jsonable_python (#545)

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

@@ -1,7 +1,6 @@
 """Base classes for FastMCP prompts."""
 
 import inspect
-import json
 from collections.abc import Awaitable, Callable, Sequence
 from typing import Any, Literal
 
@@ -155,7 +154,9 @@ class Prompt(BaseModel):
                         content = TextContent(type="text", text=msg)
                         messages.append(UserMessage(content=content))
                     else:
-                        content = json.dumps(pydantic_core.to_jsonable_python(msg))
+                        content = pydantic_core.to_json(
+                            msg, fallback=str, indent=2
+                        ).decode()
                         messages.append(Message(role="user", content=content))
                 except Exception:
                     raise ValueError(

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

@@ -9,7 +9,7 @@ from typing import Any
 import anyio
 import anyio.to_thread
 import httpx
-import pydantic.json
+import pydantic
 import pydantic_core
 from pydantic import Field, ValidationInfo
 
@@ -59,15 +59,12 @@ class FunctionResource(Resource):
             )
             if isinstance(result, Resource):
                 return await result.read()
-            if isinstance(result, bytes):
+            elif isinstance(result, bytes):
                 return result
-            if isinstance(result, str):
+            elif isinstance(result, str):
                 return result
-            try:
-                return json.dumps(pydantic_core.to_jsonable_python(result))
-            except (TypeError, pydantic_core.PydanticSerializationError):
-                # If JSON serialization fails, try str()
-                return str(result)
+            else:
+                return pydantic_core.to_json(result, fallback=str, indent=2).decode()
         except Exception as e:
             raise ValueError(f"Error reading resource {self.uri}: {e}")
 

src/mcp/server/fastmcp/server.py

@@ -3,7 +3,6 @@
 from __future__ import annotations as _annotations
 
 import inspect
-import json
 import re
 from collections.abc import AsyncIterator, Callable, Iterable, Sequence
 from contextlib import (
@@ -551,10 +550,7 @@ def _convert_to_content(
         return list(chain.from_iterable(_convert_to_content(item) for item in result))  # type: ignore[reportUnknownVariableType]
 
     if not isinstance(result, str):
-        try:
-            result = json.dumps(pydantic_core.to_jsonable_python(result))
-        except Exception:
-            result = str(result)
+        result = pydantic_core.to_json(result, fallback=str, indent=2).decode()
 
     return [TextContent(type="text", text=result)]
 

tests/server/fastmcp/resources/test_function_resources.py

@@ -100,7 +100,7 @@ class TestFunctionResource:
             fn=lambda: MyModel(name="test"),
         )
         content = await resource.read()
-        assert content == '{"name": "test"}'
+        assert content == '{\n  "name": "test"\n}'
 
     @pytest.mark.anyio
     async def test_custom_type_conversion(self):

tests/server/fastmcp/resources/test_resource_template.py

@@ -185,4 +185,4 @@ class TestResourceTemplate:
 
         assert isinstance(resource, FunctionResource)
         content = await resource.read()
-        assert content == "hello"
+        assert content == '"hello"'