diff --git a/src/controlflow/tools/tools.py b/src/controlflow/tools/tools.py
index cce08fc4..ade8069d 100644
--- a/src/controlflow/tools/tools.py
+++ b/src/controlflow/tools/tools.py
@@ -118,19 +118,27 @@ def from_function(
     ):
         name = name or fn.__name__
         description = description or fn.__doc__ or ""
-
         signature = inspect.signature(fn)
-        try:
-            parameters = TypeAdapter(fn).json_schema()
-        except PydanticSchemaGenerationError:
-            raise ValueError(
-                f'Could not generate a schema for tool "{name}". '
-                "Tool functions must have type hints that are compatible with Pydantic."
-            )
+
+        # If parameters are provided in kwargs, use those instead of generating them
+        if "parameters" in kwargs:
+            parameters = kwargs.pop("parameters")  # Custom parameters are respected
+        else:
+            try:
+                parameters = TypeAdapter(fn).json_schema()
+            except PydanticSchemaGenerationError:
+                raise ValueError(
+                    f'Could not generate a schema for tool "{name}". '
+                    "Tool functions must have type hints that are compatible with Pydantic."
+                )
 
         # load parameter descriptions
         if include_param_descriptions:
             for param in signature.parameters.values():
+                # ensure we only try to add descriptions for parameters that exist in the schema
+                if param.name not in parameters.get("properties", {}):
+                    continue
+
                 # handle Annotated type hints
                 if typing.get_origin(param.annotation) is Annotated:
                     param_description = " ".join(
diff --git a/tests/tools/test_tools.py b/tests/tools/test_tools.py
index fc5efb38..31ca236c 100644
--- a/tests/tools/test_tools.py
+++ b/tests/tools/test_tools.py
@@ -7,11 +7,7 @@
 import controlflow
 from controlflow.agents.agent import Agent
 from controlflow.llm.messages import ToolMessage
-from controlflow.tools.tools import (
-    Tool,
-    handle_tool_call,
-    tool,
-)
+from controlflow.tools.tools import Tool, handle_tool_call, tool
 
 
 @pytest.mark.parametrize("style", ["decorator", "class"])
@@ -170,6 +166,77 @@ def add(a: int, b: float) -> float:
             elif style == "decorator":
                 tool(add)
 
+    def test_custom_parameters(self, style):
+        """Test that custom parameters override generated ones."""
+
+        def add(a: int, b: float):
+            return a + b
+
+        custom_params = {
+            "type": "object",
+            "properties": {
+                "x": {"type": "number", "description": "Custom parameter"},
+                "y": {"type": "string"},
+            },
+            "required": ["x"],
+        }
+
+        if style == "class":
+            tool_obj = Tool.from_function(add, parameters=custom_params)
+        elif style == "decorator":
+            tool_obj = tool(add, parameters=custom_params)
+
+        assert tool_obj.parameters == custom_params
+        assert "a" not in tool_obj.parameters["properties"]
+        assert "b" not in tool_obj.parameters["properties"]
+        assert (
+            tool_obj.parameters["properties"]["x"]["description"] == "Custom parameter"
+        )
+
+    def test_custom_parameters_with_annotations(self, style):
+        """Test that annotations still work with custom parameters if param names match."""
+
+        def process(x: Annotated[float, "The x value"], y: str):
+            return x
+
+        custom_params = {
+            "type": "object",
+            "properties": {"x": {"type": "number"}, "y": {"type": "string"}},
+            "required": ["x"],
+        }
+
+        if style == "class":
+            tool_obj = Tool.from_function(process, parameters=custom_params)
+        elif style == "decorator":
+            tool_obj = tool(process, parameters=custom_params)
+
+        assert tool_obj.parameters["properties"]["x"]["description"] == "The x value"
+        assert "description" not in tool_obj.parameters["properties"]["y"]
+
+    def test_custom_parameters_ignore_descriptions(self, style):
+        """Test that include_param_descriptions=False works with custom parameters."""
+
+        def process(x: Annotated[float, "The x value"], y: str):
+            return x
+
+        custom_params = {
+            "type": "object",
+            "properties": {"x": {"type": "number"}, "y": {"type": "string"}},
+            "required": ["x"],
+        }
+
+        if style == "class":
+            tool_obj = Tool.from_function(
+                process, parameters=custom_params, include_param_descriptions=False
+            )
+        elif style == "decorator":
+            tool_obj = tool(
+                process, parameters=custom_params, include_param_descriptions=False
+            )
+
+        assert "description" not in tool_obj.parameters["properties"]["x"]
+        assert "description" not in tool_obj.parameters["properties"]["y"]
+
 
 class TestToolFunctions:
     def test_non_serializable_return_value(self):