docs: add guidance on using Protocol instead of Callable for extensible interfaces (#1637)

Author: dbschmigelski

docs/STYLE_GUIDE.md

@@ -57,3 +57,20 @@ logger.warning("Retry limit approaching! attempt=%d max_attempts=%d", attempt, m
 ```
 
 By following these log formatting guidelines, we ensure that logs are both human-readable and machine-parseable, making debugging and monitoring more efficient.
+
+## Type Annotations
+
+### Avoid `Callable` for Extensible Interfaces
+
+Do not use `Callable` for function type annotations that may need additional parameters in the future. `Callable` signatures are fixed and cannot be expanded without breaking existing implementations.
+
+```python
+# Bad: Cannot add parameters later without breaking all existing implementations
+EdgeCondition = Callable[[GraphState], bool]
+
+# Good: Protocol allows adding optional keyword arguments in the future
+class EdgeCondition(Protocol):
+    def __call__(self, state: GraphState, **kwargs: Any) -> bool: ...
+```
+
+Using `Protocol` with `**kwargs` allows the interface to evolve by adding new keyword arguments without breaking existing implementations that don't use them.