diff --git a/src/mcp/server/mcpserver/utilities/func_metadata.py b/src/mcp/server/mcpserver/utilities/func_metadata.py
index be4afb4e9..26aae518e 100644
--- a/src/mcp/server/mcpserver/utilities/func_metadata.py
+++ b/src/mcp/server/mcpserver/utilities/func_metadata.py
@@ -33,6 +33,23 @@ def _is_input_required_type(obj: Any) -> bool:
     return isinstance(obj, type) and issubclass(obj, InputRequiredResult)
 
 
+def _is_optional_str(annotation: Any) -> bool:
+    """Whether `annotation` is `str | None` (`Optional[str]`), modulo `None`.
+
+    Used by `pre_parse_json` to skip JSON pre-parsing for such annotations:
+    unlike e.g. `str | list[str]`, no member of this union other than `str`
+    could ever be produced by decoding a string as JSON, so attempting to
+    parse it only risks corrupting a valid string value that happens to look
+    like a JSON object/array (e.g. a JSON-serialized payload passed as a
+    plain string parameter).
+    """
+    origin = get_origin(annotation)
+    if not is_union_origin(origin):
+        return False
+    non_none_args = [arg for arg in get_args(annotation) if arg is not type(None)]
+    return non_none_args == [str]
+
+
 class StrictJsonSchema(GenerateJsonSchema):
     """A JSON schema generator that raises exceptions instead of emitting warnings.
 
@@ -169,7 +186,11 @@ def pre_parse_json(self, data: dict[str, Any]) -> dict[str, Any]:
                 continue
 
             field_info = key_to_field_info[data_key]
-            if isinstance(data_value, str) and field_info.annotation is not str:
+            if (
+                isinstance(data_value, str)
+                and field_info.annotation is not str
+                and not _is_optional_str(field_info.annotation)
+            ):
                 try:
                     pre_parsed = json.loads(data_value)
                 except json.JSONDecodeError:
diff --git a/tests/server/mcpserver/test_func_metadata.py b/tests/server/mcpserver/test_func_metadata.py
index 62a9612b9..2c8cd8e49 100644
--- a/tests/server/mcpserver/test_func_metadata.py
+++ b/tests/server/mcpserver/test_func_metadata.py
@@ -204,6 +204,35 @@ def func_with_str_types(str_or_list: str | list[str]):  # pragma: no cover
     assert result["str_or_list"] == ["hello", "world"]
 
 
+def test_optional_str_is_never_json_pre_parsed():
+    """`str | None` must never be JSON-decoded, even if the value looks like a
+    JSON object/array (e.g. a JSON-serialized payload passed as a plain string).
+
+    Regression test: `field_info.annotation is not str` used to be True for
+    `str | None`, so a value like a JSON-serialized template body would get
+    silently turned into a dict and fail pydantic validation downstream.
+    """
+
+    def func_with_optional_str(body: str | None = None):  # pragma: no cover
+        return body
+
+    meta = func_metadata(func_with_optional_str)
+
+    json_payload = '{"blocks": [{"type": "text", "value": "hi"}]}'
+    result = meta.pre_parse_json({"body": json_payload})
+    assert result["body"] == json_payload
+
+    json_list_payload = '["hello", "world"]'
+    result = meta.pre_parse_json({"body": json_list_payload})
+    assert result["body"] == json_list_payload
+
+    # Plain strings and None are unaffected.
+    result = meta.pre_parse_json({"body": "hello"})
+    assert result["body"] == "hello"
+    result = meta.pre_parse_json({"body": None})
+    assert result["body"] is None
+
+
 def test_skip_names():
     """Test that skipped parameters are not included in the model"""