fix(bedrock): return string content in ChatBedrockConverse streaming

[SYED-M-HUSSAIN]

Fix ChatBedrockConverse streaming content format

Summary

Fixes #486 - ChatBedrockConverse.astream_events() was returning AIMessageChunk objects with content as a list instead of a string, which is inconsistent with other LangChain chat models and the expected interface.

Problem

When using astream_events() with ChatBedrockConverse, streaming chunks returned content in this format:

content=[{'type': 'text', 'text': 'Hello', 'index': 0}]

Instead of the expected string format used by other chat models:

content='Hello'

This inconsistency broke compatibility with downstream code expecting string content and violated the LangChain chat model interface.

Solution

Modified _parse_stream_event() to use the same content formatting logic as the non-streaming _parse_response() function:

• Applied _str_if_single_text_block() to content blocks in contentBlockStart and contentBlockDelta events
• Changed all empty content from [] to "" across all event types
• Preserved existing tool call functionality unchanged

Changes

Modified Files

• libs/partners/aws/langchain_aws/chat_models/bedrock_converse.py

Key Changes

1. contentBlockStart/contentBlockDelta events: Use _str_if_single_text_block([block]) instead of [block]
2. All other events: Change content=[] to content=""
3. Consistency: Streaming now uses identical content formatting as non-streaming responses

Testing

Before Fix

# Returns list content
{'event': 'on_chat_model_stream', 'data': {'chunk': AIMessageChunk(content=[{'type': 'text', 'text': 'Hello', 'index': 0}])}}

After Fix

# Returns string content
{'event': 'on_chat_model_stream', 'data': {'chunk': AIMessageChunk(content='Hello')}}

[SYED-M-HUSSAIN]

@michaelnchin please review this PR, thanks

[blanketspy99]

I guess this would break who is already consuming it as a list and we couldn't identify if the response chunk is part of text or tool_use or reasoning_content.

Probably it's better to check if content is list type then pick the text block else yield the str directly.

[SYED-M-HUSSAIN]

@blanketspy99 feedback incorporated

[SYED-M-HUSSAIN]

@michaelnchin please review this PR, thanks

[3coins]

I will take a look at this by tomorrow.

[3coins]

@SYED-M-HUSSAIN
These standard integration tests are failing.
https://github.com/langchain-ai/langchain-aws/actions/runs/15399473807/job/43328371357