[codex] clarify UDF signature annotations

CHANGELOG.md

@@ -21,6 +21,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Clarify UDF documentation on required function type annotations. ([#757](https://github.com/Open-EO/openeo-python-client/issues/757))
+
 ## [0.50.0] - 2026-05-18
 
 ### Added

docs/udf.rst

@@ -152,6 +152,26 @@ as defined in the :py:mod:`openeo.udf.udf_signatures` module,
 so that the back-end knows what the *entrypoint* function is
 of your UDF implementation.
 
+The function name and Python type annotations are part of that signature:
+they are used to detect how the UDF should be invoked.
+Do not omit the type annotations on the UDF entrypoint, even if your editor
+or linter would otherwise consider them optional.
+For example, this is not enough:
+
+.. code-block:: python
+
+    def apply_datacube(cube, context):
+        ...
+
+Instead, write the full annotated signature:
+
+.. code-block:: python
+
+    import xarray
+
+    def apply_datacube(cube: xarray.DataArray, context: dict) -> xarray.DataArray:
+        ...
+
 
 Module ``openeo.udf.udf_signatures``
 -------------------------------------