add info about non-retryable constraint to string matching

Signed-off-by: Filinto Duran <[email protected]>

Author: filintod

README.md

@@ -182,6 +182,41 @@ def my_activity(ctx: task.ActivityContext, input):
 
 Even with a retry policy configured, `NonRetryableError` will fail immediately without retrying.
 
+#### Error type matching behavior
+
+**Important:** Error type matching uses **exact class name comparison**, not `isinstance()` checks. This is because exception objects are serialized to gRPC protobuf messages, where only the class name (as a string) survives serialization.
+
+**Key implications:**
+
+- **Not inheritance-aware**: If you specify `ValueError` in `non_retryable_error_types`, it will only match exceptions with the exact class name `"ValueError"`. A custom subclass like `CustomValueError(ValueError)` will NOT match.
+- **Workaround**: List all exception types explicitly, including subclasses you want to handle.
+- **Built-in exception**: `NonRetryableError` is always treated as non-retryable, matched by the name `"NonRetryableError"`.
+
+**Example:**
+
+```python
+from datetime import timedelta
+from durabletask import task
+
+# Custom exception hierarchy
+class ValidationError(ValueError):
+    pass
+
+#  This policy ONLY matches exact "ValueError" by name
+retry_policy = task.RetryPolicy(
+    first_retry_interval=timedelta(seconds=1),
+    max_number_of_attempts=3,
+    non_retryable_error_types=[ValueError]  # Won't match ValidationError subclass!
+)
+
+#  To handle both, list them explicitly:
+retry_policy = task.RetryPolicy(
+    first_retry_interval=timedelta(seconds=1),
+    max_number_of_attempts=3,
+    non_retryable_error_types=[ValueError, ValidationError]  # Both converted to name strings
+)
+```
+
 ## Getting Started
 
 ### Prerequisites