diff --git a/src/mcp_runtime/fastmcp_output.py b/src/mcp_runtime/fastmcp_output.py
index e1cd1ba..5428a9b 100644
--- a/src/mcp_runtime/fastmcp_output.py
+++ b/src/mcp_runtime/fastmcp_output.py
@@ -11,6 +11,9 @@
 
 FastMCP validates every returned dict against the model; undeclared keys are
 dropped, so the annotation is the complete list of keys a client can see.
+
+Arguments are made to match: :func:`_forbid_undeclared_arguments` makes an
+undeclared *input* an error, where before it was accepted and silently dropped.
 """
 
 from types import UnionType
@@ -80,6 +83,35 @@ def _offers_message(schema: dict[str, Any]) -> bool:
     )
 
 
+def _forbid_undeclared_arguments(converted: FastMCPTool) -> None:
+    """Reject arguments a tool does not declare, rather than dropping them.
+
+    ``to_fastmcp`` upstream publishes the input schema from the LangChain tool's
+    ``tool_call_schema`` but validates against a *separate* model, built from
+    that schema's fields alone::
+
+        arg_model = create_model(..., **field_definitions, __base__=ArgModelBase)
+
+    ``ArgModelBase`` sets no extra policy, so pydantic's default applies: an
+    argument the tool does not declare is dropped, the tool runs without it, and
+    nothing anywhere says so.
+
+    That is a silent wrong answer rather than a loud failure: the call succeeds,
+    the result looks ordinary, and the one thing that would explain it — that an
+    argument went missing — is the one thing nobody is told. A validation error
+    naming the parameter is something a model reads and corrects instead.
+
+    Both halves are needed. Forbidding extras on the model is the enforcement;
+    ``additionalProperties: false`` on the published schema is what tells a
+    client the rule exists. Enforcing without publishing would refuse calls the
+    advertised schema allowed, which is its own kind of surprise.
+    """
+    arg_model = converted.fn_metadata.arg_model
+    arg_model.model_config["extra"] = "forbid"
+    arg_model.model_rebuild(force=True)
+    converted.parameters["additionalProperties"] = False
+
+
 def to_fastmcp(tool: BaseTool) -> FastMCPTool:
     """Convert a LangChain tool to FastMCP, deriving its output schema.
 
@@ -87,6 +119,7 @@ def to_fastmcp(tool: BaseTool) -> FastMCPTool:
     not follow the ToolResult contract — see the module docstring.
     """
     converted = _to_fastmcp(tool)
+    _forbid_undeclared_arguments(converted)
     annotation = _return_annotation(tool)
     if annotation is None or not all(
         _structured_dict(arm) for arm in _arms(annotation)
diff --git a/tests/mcp_runtime/test_fastmcp_output.py b/tests/mcp_runtime/test_fastmcp_output.py
index 01e2617..faa15bc 100644
--- a/tests/mcp_runtime/test_fastmcp_output.py
+++ b/tests/mcp_runtime/test_fastmcp_output.py
@@ -162,3 +162,33 @@ async def mixed(text: str) -> ToolResult | str:
 
     with pytest.raises(RuntimeError, match="mixed"):
         to_fastmcp(mixed)
+
+
+async def test_an_undeclared_argument_is_refused_not_dropped():
+    """Upstream builds the validation model from the schema's fields alone, so
+    ``model_config`` is lost and pydantic's default drops unknown arguments.
+
+    The caller is never told, which is the dangerous part: a parameter the model
+    meant to send simply is not there, and the tool runs as though it had never
+    been asked for.
+    """
+    converted = to_fastmcp(probe)
+
+    with pytest.raises(Exception, match="query_typo"):
+        await converted.run({"query": "hello", "query_typo": "hello"})
+
+
+async def test_a_declared_argument_still_gets_through():
+    converted = to_fastmcp(probe)
+
+    assert await run_structured(converted, {"query": "hello"}) == {
+        "message": "Found 1 for 'hello'.",
+        "items": [{"id": "hello"}],
+    }
+
+
+def test_the_input_schema_says_so_too():
+    """Upstream publishes no extra policy at all, so enforcing one on its own
+    would refuse calls the advertised schema allowed. The rule has to be in the
+    schema for a client to apply it, or to know why it was refused."""
+    assert to_fastmcp(probe).parameters["additionalProperties"] is False