[BUG]Claude 4.0 (Sonnet) unresponsive due to ValidationException: empty message content

[yukauchiyama-prog]

🐞Describe the bug

When using Claude 4.0 (Sonnet) in BedrockChat, the system freezes during AI response generation and no response is returned.

Confirmed facts:

• Lambda runtime errors occur when Claude 4.0 (Sonnet) becomes unresponsive
• Claude 4.5 (Haiku) works normally (returns responses without delay)
• Unresponsive behavior occurs when "Extended Thinking" is disabled
• Normal operation occurs when "Extended Thinking" is enabled
• Reproduced with identical input content, only difference being Extended Thinking enabled/disabled

🔄 To Reproduce

Steps to reproduce:

1. Open BedrockChat
2. Select Claude 4.0 (Sonnet)
3. Disable the "Extended Thinking" feature
4. Enter a question in the chat
5. System freezes during AI response generation and no response is returned

Confirmed behaviors:

• Issue persists after clearing browser cache
• Issue persists in different browsers
• Claude 4.5 (Haiku) works without issues

📷 Screenshots

🔎 Logs for Chat Issues

Occurrence pattern:
Occurs randomly. The timing of unresponsive behavior matches the Lambda error log timestamps.

Lambda Error Logs:

The following errors were detected in CloudWatch Logs (Occurrence time: 2025-12-17 11:15:36):

2025-12-17 11:15:36.229 | ERROR:app.main - An error occurred (ValidationException) when calling the Converse operation: The content field in the Message object at messages.1 is empty. Add a ContentBlock object to the content field and try again.

2025-12-17 11:15:36.247 | ERROR:app.main -   File "/var/task/starlette/middleware/errors.py", line 165, in __call__

2025-12-17 11:15:36.338 | ERROR:    Exception in ASGI application

2025-12-17 11:15:36.339 | ERROR Lambda runtime invoke{requestId="abe07513-00b9-440c-806c-7d1eae8cb92d" xrayTraceId="Root=1-694290d8-6a300bca05b0c92744b6bfec;Parent=58b0f731932f9c78;Sampled=0;Lineage=1:ffe5f926:0"}: lambda_runtime: Error { kind: SendRequest, source: Some(hyper::Error(Io, Os { code: 104, kind: ConnectionReset, message: "Connection reset by peer" })) }

CloudWatch Logs Query:

fields @timestamp, @message
| filter @message like /ERROR/
| sort @timestamp desc
| limit 10000

🔎 Logs for Bot Creation/Update Issues

N/A - This is a chat-related issue

📝 Additional context

Environment Information

• Deployment Version: v3.15.1

Summary of Behavior Testing

Model	Extended Thinking	Result
Claude 4.0 (Sonnet)	Disabled	❌ Unresponsive/Freeze (Lambda errors occur)
Claude 4.0 (Sonnet)	Enabled	✅ Normal operation
Claude 4.5 (Haiku)	Disabled	✅ Normal operation (no delay)

Expected Behavior

Claude 4.0 (Sonnet) should operate normally regardless of whether Extended Thinking is enabled or disabled