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fix(models/gemini): emit thought signature as its own reasoning delta #3306

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fix(models/gemini): emit thought signature as its own reasoning delta #3306

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29 changes: 20 additions & 9 deletions strands-py/src/strands/models/gemini.py
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Original file line number Diff line number Diff line change
Expand Up @@ -396,15 +396,17 @@ def _format_chunk(self, event: dict[str, Any]) -> StreamEvent:
"delta": {
"reasoningContent": {
"text": event["data"].text,
**(
{
"signature": base64.b64encode(event["data"].thought_signature).decode(
"ascii"
)
}
if event["data"].thought_signature
else {}
),
},
},
},
}

case "reasoning_signature":
return {
"contentBlockDelta": {
"delta": {
"reasoningContent": {
"signature": base64.b64encode(event["data"].thought_signature).decode("ascii"),
},
},
},
Expand Down Expand Up @@ -582,6 +584,15 @@ async def stream(
},
)

if data_type == "reasoning_content" and part.thought_signature:
yield self._format_chunk(
{
"chunk_type": "content_delta",
"data_type": "reasoning_signature",
"data": part,
},
)

if data_type is not None:
yield self._format_chunk({"chunk_type": "content_stop", "data_type": data_type})
yield self._format_chunk(
Expand Down
62 changes: 56 additions & 6 deletions strands-py/tests/strands/models/test_gemini.py
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Original file line number Diff line number Diff line change
Expand Up @@ -744,14 +744,67 @@ async def test_stream_response_reasoning(gemini_client, model, messages, agenera
exp_chunks = [
{"messageStart": {"role": "assistant"}},
{"contentBlockStart": {"start": {}}},
{"contentBlockDelta": {"delta": {"reasoningContent": {"signature": "YWJj", "text": "test reason"}}}},
{"contentBlockDelta": {"delta": {"reasoningContent": {"text": "test reason"}}}},
{"contentBlockDelta": {"delta": {"reasoningContent": {"signature": "YWJj"}}}},
{"contentBlockStop": {}},
{"messageStop": {"stopReason": "end_turn"}},
{"metadata": {"usage": {"inputTokens": 1, "outputTokens": 2, "totalTokens": 3}, "metrics": {"latencyMs": 0}}},
]
assert tru_chunks == exp_chunks


@pytest.mark.asyncio
async def test_stream_response_reasoning_signature_survives_aggregation(
gemini_client, model, messages, agenerator, alist
):
"""Test that a thought signature round-trips from the stream back into a request part.

Guarantees that a signed thought part keeps its signature through stream aggregation, so the
signature Gemini requires on a subsequent turn is the one it originally issued.
"""
gemini_client.aio.models.generate_content_stream.return_value = agenerator(
[
genai.types.GenerateContentResponse(
candidates=[
genai.types.Candidate(
content=genai.types.Content(
parts=[
genai.types.Part(
text="test reason",
thought=True,
thought_signature=b"abc",
),
],
),
finish_reason="STOP",
),
],
usage_metadata=genai.types.GenerateContentResponseUsageMetadata(
prompt_token_count=1,
total_token_count=3,
),
),
]
)

stream = strands.event_loop.streaming.process_stream(model.stream(messages))
events = await alist(stream)
message = events[-1]["stop"][1]

tru_reasoning = message["content"][0]["reasoningContent"]["reasoningText"]
exp_reasoning = {"text": "test reason", "signature": "YWJj"}
assert tru_reasoning == exp_reasoning

# The reasoning text must still reach consumers as its own stream event.
tru_reasoning_text = [event["reasoningText"] for event in events if "reasoningText" in event]
exp_reasoning_text = ["test reason"]
assert tru_reasoning_text == exp_reasoning_text

# Feeding the aggregated message back must reproduce the original signature bytes.
tru_part = model._format_request_content_part(message["content"][0], {})
assert tru_part.thought_signature == b"abc"


@pytest.mark.asyncio
async def test_stream_response_reasoning_and_text(gemini_client, model, messages, agenerator, alist):
"""Test that both reasoning and text content are captured in separate blocks."""
Expand Down Expand Up @@ -803,11 +856,8 @@ async def test_stream_response_reasoning_and_text(gemini_client, model, messages
exp_chunks = [
{"messageStart": {"role": "assistant"}},
{"contentBlockStart": {"start": {}}},
{
"contentBlockDelta": {
"delta": {"reasoningContent": {"signature": "c2lnMQ==", "text": "thinking about math"}}
}
},
{"contentBlockDelta": {"delta": {"reasoningContent": {"text": "thinking about math"}}}},
{"contentBlockDelta": {"delta": {"reasoningContent": {"signature": "c2lnMQ=="}}}},
{"contentBlockStop": {}},
{"contentBlockStart": {"start": {}}},
{"contentBlockDelta": {"delta": {"text": "2 + 2 = 4"}}},
Expand Down
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