AttributeError: 'NoneType' object has no attribute 'append' in accumulate_event when message_start.message.content is null

Error Observed

    File "/app/.venv/lib/python3.10/site-packages/anthropic/lib/streaming/_beta_messages.py", line 55, in __iter__
        for item in self._iterator:
    File "/app/.venv/lib/python3.10/site-packages/anthropic/lib/streaming/_beta_messages.py", line 116, in __stream__
        self.__final_message_snapshot = accumulate_event(
    File "/app/.venv/lib/python3.10/site-packages/anthropic/lib/streaming/_beta_messages.py", line 416, in accumulate_event
        current_snapshot.content.append(
    AttributeError: 'NoneType' object has no attribute 'append'

Bug

According to API docs and SDK types, content should always be List[ContentBlock]

anthropic-sdk-python/src/anthropic/types/message.py

Line 22 in e9f5e44

content: List[ContentBlock]

but there are places where content: null is geenrated in the SDK. Like:

anthropic-sdk-python/src/anthropic/lib/streaming/_messages.py

Line 452 in f5c30d0

    
           return cast(ParsedMessage[ResponseFormatT], ParsedMessage.construct(**cast(Any, event.message.to_dict())))

Environment

anthropic version: 0.52.0 (also reproduces against main as of this filing — same unguarded append)
Python: 3.10.20
Trigger:
- client.messages.stream
- client.beta.messages.stream

Hi, I'd like to work on this. I'll submit a PR shortly.

@tarunbatra

@tarunbatra sorry for the delay here. message_start.message.content is always an array on responses from our API (it starts as [] and blocks are appended as they stream), so a null there isn't a valid first-party event — I couldn't produce one across text, thinking, tool-use and server-tool streams today. The cases we've seen like this come from an intermediary that builds its own message_start (a gateway/proxy, a mock server, or an example payload copied from abbreviated docs). Closing for now, but if you're seeing it on a direct call to api.anthropic.com, feel free to reopen with a request-id from one of those responses and we'll take a look.

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`AttributeError: 'NoneType' object has no attribute 'append'` in `accumulate_event` when `message_start.message.content` is `null` #1799

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AttributeError: 'NoneType' object has no attribute 'append' in accumulate_event when message_start.message.content is null #1799

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nightcityblade commented on Aug 5, 2026

dtmeadows-ant commented on Aug 6, 2026

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`AttributeError: 'NoneType' object has no attribute 'append'` in `accumulate_event` when `message_start.message.content` is `null` #1799