_estimate_size accesses item["attributes"].values() unconditionally, but 'attributes' is NotRequired in SpanJSON. The new before_send_span feature allows users to return a modified dict. While the intent is that invalid returns should fall back to the original span, the validation at client.py:979 only checks isinstance(serialized, dict) and serialized (non-empty). This allows dicts like {"not_a_span": True} to pass validation. When such a dict reaches _span_batcher.add(), it will raise KeyError in _estimate_size at line 151, causing the span to be dropped and potentially propagating the exception.

Also found at:

• sentry_sdk/_span_batcher.py:165-170
• sentry_sdk/client.py:978-986