fix: Span Creation Fails on Messages with Empty Content Field

[kianjones9]

Description

Seems there's a bug in version 4.1.0 on the Anthropic client with beta endpoint, where the ddtrace throws if the message content field is empty. I tried using the latest release candidate on PyPI, and tried using the tagged release version of 4.1.0 from source.

Testing

Added regression test.

Wasn't working at all and now I see the traces show up.

Risks

None

Additional Notes

In case this is relevant, here's the original Traceback:

Error processing streamed completion/chat response.
Traceback (most recent call last):
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/contrib/internal/anthropic/_streaming.py", line 80, in _process_finished_stream
    resp_message = _construct_message(streamed_chunks)
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/contrib/internal/anthropic/_streaming.py", line 94, in _construct_message
    message = _extract_from_chunk(chunk, message)
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/contrib/internal/anthropic/_streaming.py", line 111, in _extract_from_chunk
    message = transformation(chunk, message)
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/contrib/internal/anthropic/_streaming.py", line 168, in _on_content_block_stop_chunk
    content_type = _get_attr(message["content"][-1], "type", "")
                             ~~~~~~~~~~~~~~~~~~^^^^
IndexError: list index out of range
2025-12-18 16:27:55,199 - ddtrace.contrib.internal.anthropic._streaming - WARNING - [dd.env=development dd.service=letta-server] - Error processing streamed completion/chat response.
Traceback (most recent call last):
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/contrib/internal/anthropic/_streaming.py", line 80, in _process_finished_stream
    resp_message = _construct_message(streamed_chunks)
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/contrib/internal/anthropic/_streaming.py", line 94, in _construct_message
    message = _extract_from_chunk(chunk, message)
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/contrib/internal/anthropic/_streaming.py", line 111, in _extract_from_chunk
    message = transformation(chunk, message)
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/contrib/internal/anthropic/_streaming.py", line 168, in _on_content_block_stop_chunk
    content_type = _get_attr(message["content"][-1], "type", "")
                             ~~~~~~~~~~~~~~~~~~^^^^
IndexError: list index out of range
Error generating LLMObs span event for span <Span(id=8004297588287725996,trace_id=139924324531680363119519793493218318237,parent_id=16512148357580560487,name=anthropic.request)>, likely due to malformed span
Traceback (most recent call last):
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/llmobs/_llmobs.py", line 283, in _submit_llmobs_span
    span_event = self._llmobs_span_event(span)
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/llmobs/_llmobs.py", line 302, in _llmobs_span_event
    raise KeyError("Span kind not found in span context")
KeyError: 'Span kind not found in span context'
2025-12-18 16:27:55,202 - ddtrace.llmobs._llmobs - ERROR - [dd.env=development dd.service=letta-server] - Error generating LLMObs span event for span <Span(id=8004297588287725996,trace_id=139924324531680363119519793493218318237,parent_id=16512148357580560487,name=anthropic.request)>, likely due to malformed span
Traceback (most recent call last):
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/llmobs/_llmobs.py", line 283, in _submit_llmobs_span
    span_event = self._llmobs_span_event(span)
  File "/Users/kian/Developer/letta-cloud/apps/core/.venv/lib/python3.13/site-packages/ddtrace/llmobs/_llmobs.py", line 302, in _llmobs_span_event
    raise KeyError("Span kind not found in span context")
KeyError: 'Span kind not found in span context'