fix(server): serialize non-finite floats as null in text content blocks to ensure valid RFC 8259 JSON

Author: pylrn

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

@@ -203,7 +203,9 @@ async def render(
                 elif isinstance(msg, str | ContentBlock | Image | Audio):  # bare content is one user message
                     messages.append(UserMessage(msg))
                 else:  # pragma: no cover
-                    content = pydantic_core.to_json(msg, fallback=str, indent=2).decode()
+                    content = pydantic_core.to_json(
+                        msg, fallback=str, indent=2, inf_nan_mode="null"
+                    ).decode()
                     messages.append(Message(role="user", content=content))
 
             return messages

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

@@ -100,7 +100,9 @@ async def read(self) -> str | bytes:
         elif isinstance(result, str):
             return result
         else:
-            return pydantic_core.to_json(result, fallback=str, indent=2).decode()
+            return pydantic_core.to_json(
+                result, fallback=str, indent=2, inf_nan_mode="null"
+            ).decode()
 
     @classmethod
     def from_function(

src/mcp/server/mcpserver/utilities/func_metadata.py

@@ -666,6 +666,8 @@ def _convert_to_content(result: Any) -> list[ContentBlock]:
         )
 
     if not isinstance(result, str):
-        result = pydantic_core.to_json(result, fallback=str, indent=2).decode()
+        result = pydantic_core.to_json(
+            result, fallback=str, indent=2, inf_nan_mode="null"
+        ).decode()
 
     return [TextContent(type="text", text=result)]

tests/server/mcpserver/test_func_metadata.py

@@ -1520,3 +1520,19 @@ def fn() -> StepA | StepB: ...  # pragma: no branch
 
     meta = func_metadata(fn)
     assert meta.output_schema is None
+
+
+def test_convert_to_content_non_finite_floats():
+    import json
+    import math
+    from mcp.server.mcpserver.utilities.func_metadata import _convert_to_content
+
+    result = {"mean": 1.5, "stddev": math.nan, "max": math.inf, "min": -math.inf}
+    content = _convert_to_content(result)
+    assert len(content) == 1
+    assert content[0].type == "text"
+    text = content[0].text
+    # Must be valid RFC 8259 JSON parseable by standard JSON parsers
+    parsed = json.loads(text)
+    assert parsed == {"mean": 1.5, "stddev": None, "max": None, "min": None}
+