diff --git a/tests/test_stateful_tool_env.py b/tests/test_stateful_tool_env.py
index 532ca6913..9ce0b1bfb 100644
--- a/tests/test_stateful_tool_env.py
+++ b/tests/test_stateful_tool_env.py
@@ -85,6 +85,77 @@ def test_stateful_tool_env_add_tool_skips_args(self, mock_stateful_tool_env):
         assert mock_stateful_tool_env.skipped_args["secret_tool"] == ["secret"]
         assert "secret_tool" in mock_stateful_tool_env.tool_map
 
+    def test_add_tool_skips_dict_type_args(self, mock_stateful_tool_env):
+        def tool_with_dict(command: str, state: dict | None = None) -> str:
+            return command
+
+        mock_stateful_tool_env.add_tool(tool_with_dict, args_to_skip=["state"])
+
+        schema = next(
+            t
+            for t in mock_stateful_tool_env.oai_tools
+            if t["function"]["name"] == "tool_with_dict"
+        )
+        assert "state" not in schema["function"]["parameters"]["properties"]
+
+    def test_add_tool_does_not_mutate_original_signature(self, mock_stateful_tool_env):
+        """Verify that add_tool with args_to_skip doesn't mutate the original function."""
+        import inspect
+
+        def my_tool(command: str, hidden: int, visible: bool = True) -> str:
+            """A tool with multiple parameters."""
+            return command
+
+        original_params = list(inspect.signature(my_tool).parameters.keys())
+        original_annotations = dict(my_tool.__annotations__)
+
+        mock_stateful_tool_env.add_tool(my_tool, args_to_skip=["hidden"])
+
+        # Original function signature should be unchanged
+        assert list(inspect.signature(my_tool).parameters.keys()) == original_params
+        assert my_tool.__annotations__ == original_annotations
+        assert "hidden" in inspect.signature(my_tool).parameters
+
+        # But schema should have hidden removed
+        schema = next(
+            t
+            for t in mock_stateful_tool_env.oai_tools
+            if t["function"]["name"] == "my_tool"
+        )
+        assert "hidden" not in schema["function"]["parameters"]["properties"]
+        assert "command" in schema["function"]["parameters"]["properties"]
+
+    def test_add_tool_does_not_mutate_bound_method_signature(
+        self, mock_stateful_tool_env
+    ):
+        """Verify that add_tool with args_to_skip doesn't mutate bound method signatures."""
+        import inspect
+
+        class ToolProvider:
+            def my_tool(self, command: str, hidden: int, visible: bool = True) -> str:
+                """A tool with multiple parameters."""
+                return command
+
+        bound_method = ToolProvider().my_tool
+        original_params = list(inspect.signature(bound_method).parameters.keys())
+
+        mock_stateful_tool_env.add_tool(bound_method, args_to_skip=["hidden"])
+
+        # Original bound method signature should be unchanged
+        assert (
+            list(inspect.signature(bound_method).parameters.keys()) == original_params
+        )
+        assert "hidden" in inspect.signature(bound_method).parameters
+
+        # But schema should have hidden removed
+        schema = next(
+            t
+            for t in mock_stateful_tool_env.oai_tools
+            if t["function"]["name"] == "my_tool"
+        )
+        assert "hidden" not in schema["function"]["parameters"]["properties"]
+        assert "command" in schema["function"]["parameters"]["properties"]
+
     @pytest.mark.asyncio
     async def test_tool_env_tool_invalid_json_arguments(
         self, mock_openai_client, sample_chat_dataset
diff --git a/verifiers/envs/stateful_tool_env.py b/verifiers/envs/stateful_tool_env.py
index 8dbccbd1b..90381bc8d 100644
--- a/verifiers/envs/stateful_tool_env.py
+++ b/verifiers/envs/stateful_tool_env.py
@@ -1,3 +1,4 @@
+import inspect
 import json
 from abc import abstractmethod
 from typing import Callable, cast
@@ -11,6 +12,37 @@
 from verifiers.utils.tool_utils import convert_func_to_oai_tool
 
 
+def filter_signature(func, args_to_skip):
+    """Return a wrapper with filtered signature for schema generation.
+
+    Does not mutate the original function.
+    """
+    if not args_to_skip:
+        return func
+    sig = inspect.signature(func)
+    filtered_sig = sig.replace(
+        parameters=[
+            p
+            for n, p in sig.parameters.items()
+            if n not in args_to_skip and n != "self"
+        ]
+    )
+    filtered_annotations = {
+        k: v
+        for k, v in getattr(func, "__annotations__", {}).items()
+        if k not in args_to_skip
+    }
+
+    def wrapper(*args, **kwargs):
+        return func(*args, **kwargs)
+
+    setattr(wrapper, "__name__", getattr(func, "__name__", "unknown"))
+    setattr(wrapper, "__doc__", getattr(func, "__doc__", None))
+    setattr(wrapper, "__signature__", filtered_sig)
+    setattr(wrapper, "__annotations__", filtered_annotations)
+    return wrapper
+
+
 class StatefulToolEnv(vf.ToolEnv):
     def __init__(
         self,
@@ -48,7 +80,7 @@ def add_tool(self, tool: Callable, args_to_skip: list[str] = []):
         Assumes all non-skipped args use standard JSON types (no remaining $ref/$defs).
         """
         self.tools.append(tool)
-        oai_tool = convert_func_to_oai_tool(tool)
+        oai_tool = convert_func_to_oai_tool(filter_signature(tool, args_to_skip))
         assert "function" in oai_tool
         assert "parameters" in oai_tool["function"]
         params = oai_tool["function"]["parameters"]