fix: forbid extra arguments in FastMCP tool arg models (#3067)

Author: elbachir-salik

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

@@ -60,7 +60,7 @@ def model_dump_one_level(self) -> dict[str, Any]:
             kwargs[output_name] = value
         return kwargs
 
-    model_config = ConfigDict(arbitrary_types_allowed=True)
+    model_config = ConfigDict(arbitrary_types_allowed=True, extra="forbid")
 
 
 class FuncMetadata(BaseModel):

tests/client/test_client.py

@@ -156,6 +156,7 @@ async def test_client_list_tools(app: MCPServer):
                         description="Greet someone by name.",
                         input_schema={
                             "properties": {"name": {"title": "Name", "type": "string"}},
+                            "additionalProperties": False,
                             "required": ["name"],
                             "title": "greetArguments",
                             "type": "object",

tests/docs_src/test_client.py

@@ -53,6 +53,7 @@ async def test_list_tools_returns_the_full_definition() -> None:
         assert tool.input_schema == snapshot(
             {
                 "type": "object",
+                "additionalProperties": False,
                 "properties": {
                     "query": {"title": "Query", "type": "string"},
                     "limit": {"default": 10, "title": "Limit", "type": "integer"},

tests/docs_src/test_context.py

@@ -20,6 +20,7 @@ async def test_the_context_parameter_is_not_in_the_input_schema() -> None:
         assert tool.input_schema == snapshot(
             {
                 "type": "object",
+                "additionalProperties": False,
                 "properties": {"query": {"title": "Query", "type": "string"}},
                 "required": ["query"],
                 "title": "search_booksArguments",
@@ -56,7 +57,7 @@ async def test_a_context_only_tool_takes_no_arguments() -> None:
     async with Client(tutorial002.mcp) as client:
         tools = {tool.name: tool for tool in (await client.list_tools()).tools}
         assert tools["describe_catalog"].input_schema == snapshot(
-            {"type": "object", "properties": {}, "title": "describe_catalogArguments"}
+            {"type": "object", "additionalProperties": False, "properties": {}, "title": "describe_catalogArguments"}
         )
 
 

tests/docs_src/test_dependencies.py

@@ -31,19 +31,23 @@ async def test_the_resolved_parameter_is_invisible_to_the_model() -> None:
     assert tool.input_schema == snapshot(
         {
             "type": "object",
+            "additionalProperties": False,
             "properties": {"title": {"title": "Title", "type": "string"}},
             "required": ["title"],
             "title": "reserve_bookArguments",
         }
     )
 
 
-async def test_a_client_supplied_value_for_a_resolved_parameter_is_ignored() -> None:
-    """tutorial001: the resolver's value is the only one the tool can receive."""
+async def test_a_client_supplied_value_for_a_resolved_parameter_is_rejected() -> None:
+    """tutorial001: resolved parameters are not in the schema, so extra arguments fail validation."""
     async with Client(tutorial001.mcp) as client:
         result = await client.call_tool("reserve_book", {"title": "Dune", "stock": {"title": "Dune", "copies": 999}})
 
-    assert result.content == [TextContent(type="text", text="Reserved 'Dune' (6 copies left).")]
+    assert result.is_error
+    assert isinstance(result.content[0], TextContent)
+    # pydantic's "Extra inputs are not permitted" wording changes across versions.
+    assert result.content[0].text.startswith("Error executing tool reserve_book:")
 
 
 async def test_a_resolver_can_depend_on_another_resolver() -> None:

tests/docs_src/test_first_steps.py

@@ -26,6 +26,7 @@ async def test_each_decorator_registers_one_primitive() -> None:
         assert tool.input_schema == snapshot(
             {
                 "type": "object",
+                "additionalProperties": False,
                 "properties": {
                     "a": {"title": "A", "type": "integer"},
                     "b": {"title": "B", "type": "integer"},

tests/docs_src/test_lifespan.py

@@ -29,6 +29,7 @@ async def test_context_parameter_never_reaches_the_input_schema() -> None:
         assert tool.input_schema == snapshot(
             {
                 "type": "object",
+                "additionalProperties": False,
                 "properties": {"genre": {"title": "Genre", "type": "string"}},
                 "required": ["genre"],
                 "title": "count_booksArguments",

tests/docs_src/test_real_host.py

@@ -20,6 +20,7 @@ async def test_the_host_sees_exactly_what_the_decorators_registered() -> None:
         assert search.input_schema == snapshot(
             {
                 "type": "object",
+                "additionalProperties": False,
                 "properties": {"query": {"title": "Query", "type": "string"}},
                 "required": ["query"],
                 "title": "search_booksArguments",

tests/docs_src/test_tools.py

@@ -20,6 +20,7 @@ async def test_signature_becomes_the_schema() -> None:
         assert tool.input_schema == snapshot(
             {
                 "type": "object",
+                "additionalProperties": False,
                 "properties": {
                     "query": {"title": "Query", "type": "string"},
                     "limit": {"title": "Limit", "type": "integer"},
@@ -49,6 +50,7 @@ async def test_default_value_makes_the_argument_optional() -> None:
         assert tool.input_schema == snapshot(
             {
                 "type": "object",
+                "additionalProperties": False,
                 "properties": {
                     "query": {"title": "Query", "type": "string"},
                     "limit": {"default": 10, "title": "Limit", "type": "integer"},

tests/server/mcpserver/test_func_metadata.py

@@ -11,7 +11,7 @@
 import pytest
 from dirty_equals import IsPartialDict
 from mcp_types import CallToolResult, InputRequiredResult
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, ValidationError
 
 from mcp.server.mcpserver.exceptions import InvalidSignature
 from mcp.server.mcpserver.utilities.func_metadata import func_metadata
@@ -275,6 +275,7 @@ async def test_lambda_function():
 
     # Test schema
     assert meta.arg_model.model_json_schema() == {
+        "additionalProperties": False,
         "properties": {
             "x": {"title": "x", "type": "string"},
             "y": {"default": 5, "title": "y", "type": "string"},
@@ -464,6 +465,7 @@ def test_complex_function_json_schema():
             "my_model_a_forward_ref",
             "my_model_b",
         ],
+        "additionalProperties": False,
         "title": "complex_arguments_fnArguments",
         "type": "object",
     }
@@ -1297,6 +1299,17 @@ def fn() -> CallToolResult | str | InputRequiredResult: ...  # pragma: no branch
         func_metadata(fn)
 
 
+def test_unknown_argument_raises_validation_error():
+    """SDK-defined: a tools/call argument name not on the tool schema fails validation."""
+
+    def read_doc(topic: str = "") -> str:
+        return topic
+
+    meta = func_metadata(read_doc)
+    with pytest.raises(ValidationError):
+        meta.arg_model.model_validate({"path": "something"})
+
+
 def test_union_of_only_input_required_subclasses_yields_no_output_schema():
     class StepA(InputRequiredResult):
         pass