diff --git a/CHANGELOG.md b/CHANGELOG.md
index 222389b3..8186f4fb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,7 +16,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### New features
 
 * `ChatAuto()`'s new `provider_model` takes both provider and model in a single string in the format `"{provider}/{model}"`, e.g. `"openai/gpt-5"`. If not provided, `ChatAuto()` looks for the `CHATLAS_CHAT_PROVIDER_MODEL` environment variable, defaulting to `"openai"` if neither are provided. Unlike previous versions of `ChatAuto()`, the environment variables are now used *only if function arguments are not provided*. In other words, if `provider_model` is given, the `CHATLAS_CHAT_PROVIDER_MODEL` environment variable is ignored. Similarly, `CHATLAS_CHAT_ARGS` are only used if no `kwargs` are provided. This improves interactive use cases, makes it easier to introduce application-specific environment variables, and puts more control in the hands of the developer. (#159)
-* The `.register_tool()` method now accepts a `Tool` instance as input. This is primarily useful for binding things like `annotations` to the `Tool` in one place, and registering it in another. (#172)
+* The `.register_tool()` method now: 
+  * Accepts a `Tool` instance as input. This is primarily useful for binding things like `annotations` to the `Tool` in one place, and registering it in another. (#172)
+  * Supports function parameter names that start with an underscore. (#174)
 * The `ToolAnnotations` type gains an `extra` key field -- providing a place for providing additional information that other consumers of tool annotations (e.g., [shinychat](https://posit-dev.github.io/shinychat/)) may make use of.
 
 ### Bug fixes
diff --git a/chatlas/_tools.py b/chatlas/_tools.py
index dd125eb3..b71ddf09 100644
--- a/chatlas/_tools.py
+++ b/chatlas/_tools.py
@@ -326,13 +326,21 @@ def func_to_basemodel(func: Callable) -> type[BaseModel]:
             )
             annotation = Any
 
+        # create_model() will error if the field name starts with `_` (since Pydantic
+        # uses this to indicate private fields). We can work around this by using an alias.
+        alias = None
+        if name.startswith("_"):
+            field_name, alias = (name.lstrip("_"), name)
+        else:
+            field_name, alias = (name, None)
+
         if param.default != inspect.Parameter.empty:
-            field = Field(default=param.default)
+            field = Field(default=param.default, alias=alias)
         else:
-            field = Field()
+            field = Field(alias=alias)
 
         # Add the field to our fields dict
-        fields[name] = (annotation, field)
+        fields[field_name] = (annotation, field)
 
     return create_model(func.__name__, **fields)
 
diff --git a/tests/conftest.py b/tests/conftest.py
index 984bb4a8..fa9443ce 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -162,9 +162,9 @@ def assert_tools_parallel(
 ):
     chat = chat_fun(system_prompt="Be very terse, not even punctuation.")
 
-    def favorite_color(person: str):
+    def favorite_color(_person: str):
         """Returns a person's favourite colour"""
-        return "sage green" if person == "Joe" else "red"
+        return "sage green" if _person == "Joe" else "red"
 
     chat.register_tool(favorite_color)