[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

[sasmita016]

I took a closer look and this issue seems to be in the Bedrock Converse request path rather than anything related to middleware.

The key symptom is the Bedrock ValidationException saying messages.1 has empty content, and the repro notes that it happens with Claude 4.0 Sonnet when Extended Thinking is disabled, but not when it is enabled.

I’d like to investigate where an empty message block is being constructed for this model/config combination. @statefb does that sound like the right direction?