[BUG] Agent cannot run when final_answer_checks is provided

[Zoe14]

Describe the bug
final_answer_checks are being called after every action step instead of just final_answer_step
When final_answer_checks is not none, the agent run would fail with Check check_final_answer failed with error: None is not of type 'object'
Looking at the log, it is running the check_final_answer before final_answer step.

Code to reproduce the error
create any agent that has final_answer_checks

Error logs (if any)
Check check_final_answer failed with error: None is not of type 'object'

Expected behavior
should only run check_final_answer when there is a final_answer

Packages version:
version = "1.18.0"

Additional context
It is introduced by this change.
when token streaming is not enabled, _step_stream always returns a FinalOutput. when it is not final answer, it is a FinalOutput(output=None)

[aymeric-roucher]

@Zoe14 can you provide a reproducible example? I tried to repro it but below works fine:

from smolagents import CodeAgent, InferenceClientModel, WebSearchTool


model = InferenceClientModel(
    model_id="Qwen/Qwen3-32B",
    provider="nebius",
    token="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",
    bill_to="agents-hack",
)

web_search_tool = WebSearchTool()
code_agent = CodeAgent(tools=[web_search_tool], model=model, final_answer_checks=[lambda x, y: True])


answer = code_agent.run("What's the address of the Hugging Face HQ in Paris?")
print(answer)

[Zoe14]

@aymeric-roucher

from smolagents import CodeAgent, InferenceClientModel, WebSearchTool


model = InferenceClientModel(
    model_id="Qwen/Qwen3-32B",
    provider="nebius",
    bill_to="agents-hack",
)

web_search_tool = WebSearchTool()
code_agent = CodeAgent(tools=[web_search_tool], model=model, final_answer_checks=[lambda x, y: print("checking final answer is a string ", x)])


answer = code_agent.run("What's the address of the Hugging Face HQ in Paris?")
print(answer)

if you don't do anything with the X (the final answer), then of course it doesn't matter if it is None.