IndexError when streaming

[palako]

https://github.com/anthropics/anthropic-sdk-python/blob/main/src/anthropic/lib/streaming/_messages.py line 465 raises an IndexError, it seems that ither current_snapshot.content is empty or doesn't have at least event.index+1 elements.

Unfortunatelly I can't reproduce the problem consistently, so I can't provide more info, but an author more familiar with that code should be able to identify where to add the correct boundaries validation.

[xXMrNidaXx]

We've seen sporadic IndexError issues like this at RevolutionAI when streaming under high concurrency. Based on the location (line 465 in _messages.py), this appears to be a race condition in how content blocks are indexed during streaming.

Likely cause:

The streaming handler expects content blocks to arrive in order, but under certain conditions (network jitter, provider-side batching, or concurrent streams), the event.index can reference a content block that hasn't been created yet:

# The problematic pattern:
content_block = current_snapshot.content[event.index]  # IndexError if index > len

Workaround until fixed:

Wrap your streaming loop in a try/except to handle the edge case:

try:
    async for event in stream:
        # process event
except IndexError as e:
    # Log for diagnostics, retry, or graceful degradation
    logger.warning(f"Stream index mismatch: {e}, retrying...")
    # Retry logic or fallback to non-streaming

Suggested fix:

The SDK should bounds-check before accessing the array:

# Defensive approach
if event.index >= len(current_snapshot.content):
    # Either expand the array or skip this event
    pass
content_block = current_snapshot.content[event.index]

Debugging questions:

1. Are you using stream_mode="text" or stream_mode="raw"?
2. Any tool calls involved in the response?
3. Concurrent streams from the same client instance?

Tool calls especially seem to cause index misalignment since they can insert multiple content blocks in rapid succession.

Would be helpful to capture the exact event.index value and len(current_snapshot.content) when this happens — even intermittent repros help narrow it down.