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Chat Completions replay emits an invalid sequence for two messages in one tool-call turn

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Assessment

Difficulty
3/5
Estimated time
Half a day
Newbie friendliness
70/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
ai

Research direction

The merge logic lives in Converter.items_to_messages in agents/models/chatcmpl_converter.py, and the SDK comment the report quotes about the rejected sequence sits near it. Start by running the repro snippet from the issue to see the current [assistant(tool_calls), assistant, tool] output. Done means that consecutive same-turn messages fold into the tool-call assistant, the tool result follows immediately, and a later assistant turn stays separate. The existing single-message case should still convert as it does now.

Written by the indexing model from the issue text.

Description

Please read this first
  • Have you read the docs? Yes.
  • Have you searched for related issues? Yes. #4727 was fixed for a single message following a streamed tool call by #4728; this reports the remaining multi-message case.
Describe the bug

When replaying a Responses-format history ordered as [function_call, message, message, function_call_output], Converter.items_to_messages merges only the first message into the assistant with tool_calls. The second message becomes a separate assistant message before the tool result. The Chat Completions request therefore contains [assistant(tool_calls), assistant, tool]. The SDK's own comment says this sequence is rejected by the API. A message after a refusal in the same tool turn has the same problem.

The malformed payload was observed in a direct converter probe and in the payload passed by Runner.run to a local mocked chat.completions.create. A live HTTP 400 was not measured; the rejection is inferred from the documented Chat Completions tool-call ordering contract and the error shown in #4727.

Debug information
  • Agents SDK version: 0.23.1 (also reproduced on main at 125efa029b4bfd84238bd2c4fd69c3406f802663).
  • Related library versions: none required for the direct probe.
  • Python version: 3.12.
  • Operating system: Windows.
  • Model and model provider: Chat Completions adapter; local mock client, no paid API call.
  • Does the issue reproduce with the latest Agents SDK release? Yes, v0.23.1.
  • Does the issue occur consistently or intermittently? Consistently for this input order.

Relevant observed roles:

user
assistant(content='Let me check the weather.', tool_calls=[call_1])
assistant(content='One moment.')
tool(tool_call_id='call_1', content='sunny')
Repro steps
from agents.models.chatcmpl_converter import Converter

items = [
    {"role": "user", "content": "What is the weather?"},
    {"id": "fc_1", "call_id": "call_1", "name": "get_weather", "arguments": "{}", "type": "function_call"},
    {"id": "msg_1", "type": "message", "role": "assistant", "status": "completed", "content": [{"type": "output_text", "text": "Let me check the weather.", "annotations": []}]},
    {"id": "msg_2", "type": "message", "role": "assistant", "status": "completed", "content": [{"type": "output_text", "text": "One moment.", "annotations": []}]},
    {"type": "function_call_output", "call_id": "call_1", "output": "sunny"},
]
for message in Converter.items_to_messages(items):
    print(message)

On v0.23.1, the second assistant message separates the tool-call assistant from its tool result. A control history with just one assistant message between function_call and function_call_output converts correctly.

Expected behavior

The two same-turn text messages should be merged into a single assistant message with the declared tool_calls, immediately followed by the tool result. A later assistant turn after the tool result should stay separate.

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