BUG: OpenAIModel raises AttributeError when gpt-5 returns tool-only response

[chahn]

Problem
OpenAIModel throws an error when gpt-5 responds with a tool-only step. The OpenAI API returns message.content = None, but remove_content_after_stop_sequences still tries to call .split() on that value, raising AttributeError: 'NoneType' object has no attribute 'split'.

Steps to reproduce

1. Save the script below as bug_stop_seq.py and add your OpenAI API Key: api_key="sk-..."
2. Run uv run bug_stop_seq.py.
3. Observe the stack trace showing the AttributeError.

# How to run with uv:
#   uv run bug_stop_seq.py
#

# /// script
# requires-python = ">=3.10"
# dependencies = [
#   "smolagents[openai,toolkit]",
# ]
# ///

from smolagents import ToolCallingAgent, OpenAIModel, VisitWebpageTool


def main():
    model = OpenAIModel(model_id="gpt-5", api_key="sk-...")
    visit_tool = VisitWebpageTool()
    agent = ToolCallingAgent(tools=[visit_tool], model=model)
    result = agent.run("Could you get me the title of the page at url 'https://huggingface.co/blog'?")
    print(f"\nAgent result:\n{result}")


if __name__ == "__main__":
    main()

Actual behavior and error logs

$ uv run bug_stop_seq.py
      Built smolagents @ file:///Users/chahn/tmp/smolagents
Uninstalled 47 packages in 384ms
Installed 47 packages in 71ms
╭───────────────────────────────────────────────────────────────────────── New run ──────────────────────────────────────────────────────────────────────────╮
│                                                                                                                                                            │
│ Could you get me the title of the page at url 'https://huggingface.co/blog'?                                                                               │
│                                                                                                                                                            │
╰─ OpenAIServerModel - gpt-5 ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Step 1 ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Error while generating output:
'NoneType' object has no attribute 'split'
[Step 1: Duration 5.04 seconds]
Traceback (most recent call last):
  File "/Users/chahn/.cache/uv/environments-v2/bug-30bad21716a0fa20/lib/python3.12/site-packages/smolagents/agents.py", line 1286, in _step_stream
    chat_message: ChatMessage = self.model.generate(
                                ^^^^^^^^^^^^^^^^^^^^
  File "/Users/chahn/.cache/uv/environments-v2/bug-30bad21716a0fa20/lib/python3.12/site-packages/smolagents/models.py", line 1680, in generate
    content = remove_content_after_stop_sequences(content, stop_sequences)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chahn/.cache/uv/environments-v2/bug-30bad21716a0fa20/lib/python3.12/site-packages/smolagents/models.py", line 78, in remove_content_after_stop_sequences
    split = content.split(stop_seq)
            ^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'split'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/chahn/tmp/smolagents/bug_stop_seq.py", line 35, in <module>
    main()
  File "/Users/chahn/tmp/smolagents/bug_stop_seq.py", line 29, in main
    result = agent.run("Could you get me the title of the page at url 'https://huggingface.co/blog'?")
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chahn/.cache/uv/environments-v2/bug-30bad21716a0fa20/lib/python3.12/site-packages/smolagents/agents.py", line 499, in run
    steps = list(self._run_stream(task=self.task, max_steps=max_steps, images=images))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chahn/.cache/uv/environments-v2/bug-30bad21716a0fa20/lib/python3.12/site-packages/smolagents/agents.py", line 596, in _run_stream
    raise e
  File "/Users/chahn/.cache/uv/environments-v2/bug-30bad21716a0fa20/lib/python3.12/site-packages/smolagents/agents.py", line 578, in _run_stream
    for output in self._step_stream(action_step):
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chahn/.cache/uv/environments-v2/bug-30bad21716a0fa20/lib/python3.12/site-packages/smolagents/agents.py", line 1302, in _step_stream
    raise AgentGenerationError(f"Error while generating output:\n{e}", self.logger) from e
smolagents.utils.AgentGenerationError: Error while generating output:
'NoneType' object has no attribute 'split'

Expected behavior
When the model responds via tool calls (returning content=None), remove_content_after_stop_sequences should short-circuit instead of calling .split(), allowing the agent run to continue without an exception.

Additional context

• The crash is specific to models that emit tool-only steps (e.g., gpt-5). gpt-4o returns content strings in the same scenario and therefore does not trigger the bug.
• Proposed fix: Guard remove_content_after_stop_sequences so it returns immediately when content=None, ignores empty stop strings, and tolerates stop_sequences=None. This keeps existing behavior for normal strings while preventing the crash.

Suggested fix
A fix for this issue could look like this PR: #1826

Environment

• OS: macOS 14.6.1 (Apple Silicon)
• Python version: 3.12.11
• Package version: 1.22.0

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Checklist

• I have searched the existing issues and have not found a similar bug report.
• I have provided a minimal, reproducible example.
• I have provided the full traceback of the error.
• I have provided my environment details.
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