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.

[giulio-leone]

Rebased this branch onto the latest main and force-pushed it.

Local validation I ran:

• .venv/bin/python -m pytest tests/strands/models/test_bedrock.py -k 'nova_model_converts_json_to_text_in_tool_result or nova_model_converts_mixed_json_and_text_in_tool_result or claude_model_preserves_json_in_tool_result or nova_model_handles_nested_json_in_tool_result or should_convert_json_to_text_nova_variants' -q -> 5 passed\n\nReal branch-vs-main proof (repo venv + explicit PYTHONPATH=<checkout>/src to verify which checkout was imported):\n- main imported sdk-python-main/src/strands/models/bedrock.py\n - Nova: has_convert_helper=False, content={"json": {"key": "value", "nested": {"n": 42}}}\n - Claude: has_convert_helper=False, content={"json": {"key": "value", "nested": {"n": 42}}}\n- this branch imported sdk-python/src/strands/models/bedrock.py\n - Nova: has_convert_helper=True, convert_json_to_text=True, content={"text": "{\\"key\\": \\"value\\", \\"nested\\": {\\"n\\": 42}}"}\n - Claude: has_convert_helper=True, convert_json_to_text=False, content={"json": {"key": "value", "nested": {"n": 42}}}\n\nSo the rebased branch keeps the existing JSON behavior for non-Nova models while fixing the Nova-specific request-shaping bug that was sending structured JSON blocks instead of their text representation.

[giulio-leone]

Refreshed this branch onto current main and revalidated it on the rebased head.

Repo-native gate (run twice with no code changes between passes):

• uv run hatch run test-format
• uv run hatch run test-lint
• uv run hatch run test -- tests/strands/models/test_bedrock.py -q

Results on refreshed head 2dc7e3f847d79c7482cbc344e34485da41175f09:

• ruff check: pass
• mypy ./src: pass (Success: no issues found in 140 source files)
• tests/strands/models/test_bedrock.py: pass (128 passed)

Direct branch-vs-main proof on exact source trees used the same temporary test file in both checkouts and asserted three things:

1. Nova models serialize JSON-only tool results to text.
2. Nova models serialize mixed text+JSON tool results so the JSON portion becomes text.
3. Non-Nova models still preserve raw JSON tool-result blocks.

Proof result:

• Refreshed branch: 3 passed
• Current main (fd8168a53... at verification time): 2 failed, 1 passed

The current main failures are the exact regression this PR fixes:

• Nova JSON-only tool results still come through as raw {"json": ...} blocks instead of {"text": json.dumps(...)}.
• Mixed Nova tool results still leave the JSON block untouched instead of serializing it to text.
• The non-Nova control still passes on both trees, so the fix remains scoped to Nova behavior.