[BUG] only set signature in message if signature was provided by the model

[howardzhou]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.7.0

Python Version

3.12

Operating System

ubuntu 22.04

Installation Method

pip

Steps to Reproduce

Run gpt oss through bedrock

Expected Behavior

Remove this line,
https://github.com/strands-agents/sdk-python/blob/main/src/strands/event_loop/streaming.py#L292

it breaks the fix in previous PR which fixed the model not having signature
https://github.com/strands-agents/sdk-python/pull/682/files

Actual Behavior

GPT oss doesn't have signature, this makes the context messages having 'signature': '', which breaks the bedorck inference

Additional Context

No response

Possible Solution

Remove this line,
https://github.com/strands-agents/sdk-python/blob/main/src/strands/event_loop/streaming.py#L292

See details in PR https://github.com/strands-agents/sdk-python/pull/682/files

Related Issues

No response

[howardzhou]

@clareliguori

[zastrowm]

Confirmed the problem & working on a fix. Adding unit test to catch future regressions

[zastrowm]

Per #799, this can be reproduced with:

from strands import Agent, tool
from strands.models import BedrockModel

@tool
def calculator(expression: str):
    """Calculate the expression"""
    return "not supported"

model_id = "openai.gpt-oss-120b-1:0"
model = BedrockModel(
    model_id=model_id,
)
agent = Agent(
    model=model,
    # tools=[calculator],
    tools=[calculator],
    system_prompt="You're a helpful assistant.",
)

# Async function that iterators over streamed agent events
async def process_streaming_response():
    agent_stream = agent.stream_async("Calculate 2+2 using a tool")
    async for event in agent_stream:
        print(event)

# Run the agent
await process_streaming_response()

[zastrowm]

Fix is merged via #806

Until we release the fix to pypi, a hook can be used to workaround the issue:

from typing import Any

from strands.experimental.hooks import AfterModelInvocationEvent
from strands.hooks import HookProvider, HookRegistry
from strands import Agent, tool
from strands.models import BedrockModel

class OpenAiSignatureRemoverHook(HookProvider):
    """
    Workaround for https://github.com/strands-agents/sdk-python/issues/790: openai.gpt doesn't support
    empty signatures in reasoning but strands defaults to an empty signature, so strip all empty 
    signatures from blocks.
    """

    def register_hooks(self, registry: HookRegistry, **kwargs: Any) -> None:
        registry.add_callback(AfterModelInvocationEvent, self.remove_empty_signatures)

    def remove_empty_signatures(self, event: AfterModelInvocationEvent):
        # If we have a message returned from the LLM
        if (response := event.stop_response) and (message := response.message):
            # For each content block, remove any empty signatures until 
            for content in message.get("content") or []:
                if (
                    (reasoningContent := content.get("reasoningContent"))
                    and (reasoningText := reasoningContent.get("reasoningText"))
                    and (reasoningText.get('signature') == "")
                ):
                    del reasoningText['signature']

Usage is as follows:

from typing import Any

from strands.experimental.hooks import AfterModelInvocationEvent
from strands.hooks import HookProvider, HookRegistry
from strands import Agent, tool
from strands.models import BedrockModel

@tool
def calculator(expression: str):
    """Calculate the expression"""
    return "not supported"

model_id = "openai.gpt-oss-120b-1:0"
model = BedrockModel(
    model_id=model_id,
)

agent = Agent(
    model=model,
    tools=[calculator],
    hooks=[OpenAiSignatureRemoverHook()],
    system_prompt="You're a helpful assistant.",
)

await agent.invoke_async("Calculate 2+2 using a tool")
await agent.invoke_async("Summerize that into a single response")

[dbschmigelski]

Hi, this should be resolved with the 1.7.1 release https://github.com/strands-agents/sdk-python/releases/tag/v1.7.1