Drop pydantic's auto-derived field titles from tool input schemas

Pydantic titles every field by title-casing its name, so a parameter
named exercise_id gains "title": "Exercise Id" - a restatement of the
key it already sits under. Tool schemas go to the model on every
request, so that repetition is paid for in context on every turn and
carries nothing the field name does not.

Measured against a real server: the wger MCP server's tools/list is
43,710 bytes across 49 tools, of which 8,333 bytes - 19% - is 297
auto-derived title keys. For an agent granted 43 of those tools on a
32k-context local model, dropping them returns about 2,000 tokens per
request.

NoAutoTitleJsonSchema suppresses only the automatic titles. A title set
explicitly through Field(title=...) is the author's choice and is left
alone, as are descriptions and every constraint.

Scoped to tool input schemas. Output schemas, prompts and resource
templates generate titles the same way and are deliberately left for a
separate change.

Author: wromansky

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

@@ -21,7 +21,7 @@
     returns_input_required,
 )
 from mcp.server.mcpserver.utilities.context_injection import find_context_parameter
-from mcp.server.mcpserver.utilities.func_metadata import FuncMetadata, func_metadata
+from mcp.server.mcpserver.utilities.func_metadata import FuncMetadata, NoAutoTitleJsonSchema, func_metadata
 from mcp.shared._callable_inspection import is_async_callable
 from mcp.shared.exceptions import MCPError
 from mcp.shared.tool_name_validation import validate_and_warn_tool_name
@@ -103,7 +103,9 @@ def from_function(
             skip_names=skip_names,
             structured_output=structured_output,
         )
-        parameters = func_arg_metadata.arg_model.model_json_schema(by_alias=True)
+        parameters = func_arg_metadata.arg_model.model_json_schema(
+            by_alias=True, schema_generator=NoAutoTitleJsonSchema
+        )
 
         # Match `model_dump_one_level`'s kwarg keys (alias when present, else field name)
         # so a by-name resolver param resolves to a key that exists at call time.

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

@@ -74,6 +74,22 @@ def emit_warning(self, kind: JsonSchemaWarningKind, detail: str) -> None:
         raise ValueError(f"JSON schema warning: {kind} - {detail}")
 
 
+class NoAutoTitleJsonSchema(GenerateJsonSchema):
+    """A JSON schema generator that omits pydantic's auto-derived field titles.
+
+    Pydantic titles every field by title-casing its name, so ``exercise_id``
+    gains ``"title": "Exercise Id"`` - a restatement of the key it sits under.
+    Tool schemas are sent to a model on every request, where that repetition is
+    paid for in context and carries nothing the field name does not.
+
+    Titles set explicitly via ``Field(title=...)`` are untouched: this only
+    suppresses the automatic ones.
+    """
+
+    def field_title_should_be_set(self, schema: Any) -> bool:
+        return False
+
+
 _LOCAL_DEFS_PREFIX = "#/$defs/"
 
 

tests/client/test_client.py

@@ -159,7 +159,7 @@ async def test_client_list_tools(app: MCPServer):
                         name="greet",
                         description="Greet someone by name.",
                         input_schema={
-                            "properties": {"name": {"title": "Name", "type": "string"}},
+                            "properties": {"name": {"type": "string"}},
                             "required": ["name"],
                             "title": "greetArguments",
                             "type": "object",

tests/docs_src/test_client.py

@@ -56,8 +56,8 @@ async def test_list_tools_returns_the_full_definition() -> None:
             {
                 "type": "object",
                 "properties": {
-                    "query": {"title": "Query", "type": "string"},
-                    "limit": {"default": 10, "title": "Limit", "type": "integer"},
+                    "query": {"type": "string"},
+                    "limit": {"default": 10, "type": "integer"},
                 },
                 "required": ["query"],
                 "title": "search_booksArguments",

tests/docs_src/test_context.py

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

tests/docs_src/test_dependencies.py

@@ -31,7 +31,7 @@ async def test_the_resolved_parameter_is_invisible_to_the_model() -> None:
     assert tool.input_schema == snapshot(
         {
             "type": "object",
-            "properties": {"title": {"title": "Title", "type": "string"}},
+            "properties": {"title": {"type": "string"}},
             "required": ["title"],
             "title": "reserve_bookArguments",
         }

tests/docs_src/test_first_steps.py

@@ -27,8 +27,8 @@ async def test_each_decorator_registers_one_primitive() -> None:
             {
                 "type": "object",
                 "properties": {
-                    "a": {"title": "A", "type": "integer"},
-                    "b": {"title": "B", "type": "integer"},
+                    "a": {"type": "integer"},
+                    "b": {"type": "integer"},
                 },
                 "required": ["a", "b"],
                 "title": "addArguments",

tests/docs_src/test_lifespan.py

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

tests/docs_src/test_progress.py

@@ -19,9 +19,7 @@ async def test_context_parameter_is_invisible_to_the_model() -> None:
     """tutorial001: `ctx` comes from the type hint and never reaches the input schema."""
     async with Client(tutorial001.mcp) as client:
         (tool,) = (await client.list_tools()).tools
-        assert tool.input_schema["properties"] == {
-            "urls": {"items": {"type": "string"}, "title": "Urls", "type": "array"}
-        }
+        assert tool.input_schema["properties"] == {"urls": {"items": {"type": "string"}, "type": "array"}}
         assert tool.input_schema["required"] == ["urls"]
 
 

tests/docs_src/test_real_host.py

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