fix: convert JSON content blocks to text for Amazon Nova models in tool results

[giulio-leone]

Summary

Fixes #1095

Amazon Nova models (Pro, Lite, Micro) hallucinate when tool results contain JSON content blocks ({"json": {...}}). The models produce incoherent responses, fabricate information, role-play both sides of conversations, and hallucinate fake XML tool calls. Claude models handle JSON content blocks correctly.

Root Cause

The _format_request_message_content() method passes JSON content blocks through to the Bedrock API as-is:

if "json" in tool_result_content:
    formatted_content.append({"json": tool_result_content["json"]})

While this is valid per the Bedrock API spec, Nova models cannot process JSON content blocks correctly and produce hallucinated output instead.

Fix

Added model-aware JSON-to-text conversion that automatically serializes JSON content blocks to their text representation (json.dumps()) when the model is an Amazon Nova variant. Non-Nova models continue to receive JSON blocks unchanged.

The implementation follows the existing pattern used for tool result status handling (_MODELS_INCLUDE_STATUS / _should_include_tool_result_status()), adding a parallel _MODELS_CONVERT_JSON_TO_TEXT list and _should_convert_json_to_text() method.

Before (Nova receives JSON block → hallucination)

{"toolResult": {"content": [{"json": {"key": "value"}}], "toolUseId": "..."}}

After (Nova receives text → correct behavior)

{"toolResult": {"content": [{"text": "{\"key\": \"value\"}"}], "toolUseId": "..."}}

Changes

• src/strands/models/bedrock.py:
  • Added _MODELS_CONVERT_JSON_TO_TEXT list with "amazon.nova" prefix
  • Added _should_convert_json_to_text() helper method
  • Updated JSON block handling in _format_request_message_content() to convert to text for Nova models

Testing

Added 5 new tests:

• test_nova_model_converts_json_to_text_in_tool_result — basic JSON-to-text conversion
• test_nova_model_converts_mixed_json_and_text_in_tool_result — mixed content preservation
• test_claude_model_preserves_json_in_tool_result — non-Nova models unaffected
• test_nova_model_handles_nested_json_in_tool_result — deeply nested structures
• test_should_convert_json_to_text_nova_variants — all Nova model ID patterns

All 1871 existing + new tests pass.

[giulio-leone]

Friendly ping — converts JSON content blocks to text format for Amazon Nova models in tool results, which don't support structured content in tool responses.