A Bedrock message content block is missing a required field

WHAT IT MEANS

The path points to the malformed block

ValidationException: The model returned the following errors: messages.1.content.0.thinking.signature: Field required

The indexed path is zero-based: it names a conversation turn, its content block, and the nested field that the selected model expects. The same shape can identify fields such as text, image.source, document.source.type, or a thinking-block signature.

NEXT CHECK

Repair the exact content block

1. Read the indexes. Find the corresponding turn and content block in the serialized request; do not assume the first message or first block is the one named by the error.
2. Match the block type. Text blocks need their text, thinking blocks need the signed fields required for replay, and image or document blocks need the required source object and type.
3. Check adapters. If middleware converts Anthropic Messages to Bedrock InvokeModel or Converse, log only sanitized content-block types and keys to find a dropped or renamed field.
4. Retry after the schema is valid. Investigate model access or IAM only if a separate service-side error remains.

AUTOMATE THE TRIAGE

Keep the check in your normal workflow

SAM Doctor recognizes indexed Bedrock message-content field failures with medium confidence and points at the exact nested request path. Runs locally; no AWS access, no log upload.

python -m pip install sam-doctor
sam-doctor diagnose deployment.log --format markdown

RELATED

Related errors

• Bedrock messages field required - add the top-level Messages API array before fixing a nested block.
• Bedrock empty modelId - pass the required model identifier before validating the body.
• Bedrock empty system prompt - omit an empty Converse system content block.

OFFICIAL REFERENCE

Anthropic Claude Messages API

Request and response parameters - use the model-specific content-block schema for the selected Claude model.