[Bugfix] Multiple fixes for gpt-oss Chat Completion prompting

[bbrowning]

Purpose

This fixes multiple issues with how we were prompting gpt-oss models for Chat Completion requests. A summary of the fixes:

• We were not setting the recipient to "assistant" on tool responses
• We were not mapping the tool_call_id to the proper function name on tool responses, resulting in the model getting confused about which tool response matched which tool call.
• When the model generates tool calls during a turn, we return any model output to the commentary channel as the content of that tool call response. We were not properly restoring this content as a commentary message when those tool calls were passed back in via Chat Completion messages.
• When the model generates reasoning messages during a tool call turn, we return that as a reasoning properly on the Chat Completion response. We were not properly restoring that reasoning content as a message to the analysis channel when subsequent turns passed that back in.
• When non-tool reasoning responses from the model were passed back in via the reasoning extra_body in subsequent turns, we were not properly turning those back into analysis messages. This is the same problem we had with reasoning in tool calls, but a slightly different fix for the non-tool call path.
• We were not properly marking the channel as final for non-tool, non-reasoning model output when generating the Harmony messages for subsequent turns.
• We were not dropping analysis messages that precede a model output to the final channel in all cases, resulting in us often showing the model analysis messages from previous turns where it has already generated a final output leading to increased token usage, context pollution, and not following the best practices for handling raw chain of thought outlined by OpenAI.

I added a series of tests that pass in Chat Completion requests and thoroughly validate the exact Harmony messages generated from those requests to test for all of the cases above.

Test Plan

Unit Tests

pytest tests/entrypoints/openai/test_serving_chat.py::TestServingChatMakeRequestWithHarmony pytest tests/entrypoints/test_harmony_utils.py 

Berkeley Function Calling Leaderboard v4 Results

Run vLLM built with this fix:

vllm serve openai/gpt-oss-120b \ --tool-call-parser openai \ --reasoning-parser openai_gptoss \ --enable-auto-tool-choice \ --tensor-parallel-size 4 

Run bfcl generate against that vLLM:

# Assumes you're inside a local clone of bfcl and adjusted it to use openai/gpt-oss-120b model cat <<EOF >> bfcl_eval/constants/model_config.py MODEL_CONFIG_MAPPING = { "openai/gpt-oss-120b": ModelConfig( model_name="openai/gpt-oss-120b", display_name="openai/gpt-oss-120b (FC) (vLLM)", url="https://huggingface.co/openai/gpt-oss-120b", org="OpenAI", license="apache-2.0", model_handler=OpenAICompletionsHandler, input_price=None, output_price=None, is_fc_model=True, underscore_to_dot=True, ), } EOF OPENAI_BASE_URL="http://localhost:8000/v1" \ OPENAI_API_KEY="fake" \ bfcl generate \ --model openai/gpt-oss-120b \ --test-category multi_turn \ --num-threads 8 

Test Result

Unit Tests

pytest -q --disable-warnings \ tests/entrypoints/openai/test_serving_chat.py::TestServingChatMakeRequestWithHarmony ....... [100%] 7 passed, 2 warnings in 2.75s pytest -q --disable-warnings \ tests/entrypoints/test_harmony_utils.py .............................. [100%] 30 passed, 2 warnings in 1.68s 

Berkeley Function Calling Leaderboard v4 Results

Baseline, from vLLM main as of Nov 12, 2025

commit 3044195

45.0% overall accuracy

🔍 Running test: multi_turn_base ✅ Test completed: multi_turn_base. 🎯 Accuracy: 50.00% 🔍 Running test: multi_turn_long_context ✅ Test completed: multi_turn_long_context. 🎯 Accuracy: 43.00% 🔍 Running test: multi_turn_miss_func ✅ Test completed: multi_turn_miss_func. 🎯 Accuracy: 45.50% 🔍 Running test: multi_turn_miss_param ✅ Test completed: multi_turn_miss_param. 🎯 Accuracy: 41.50% $ cat score-baseline/data_multi_turn.csv Rank,Model,Multi Turn Overall Acc,Base,Miss Func,Miss Param,Long Context 1,openai/gpt-oss-120b (FC) (vLLM),45.00%,50.00%,45.50%,41.50%,43.00% 

With this fix on top of same commit from main

• 53.5% overall accuracy, an improvement of 8.5%
• 12% improvement in multi_turn_base
• 8.5% improvement in multi_turn_long_context
• 4.5% improvement in multi_turn_miss_func
• 8.0% improvement in multi_turn_miss_param

✅ Test completed: multi_turn_base. 🎯 Accuracy: 62.00% 🔍 Running test: multi_turn_long_context ✅ Test completed: multi_turn_long_context. 🎯 Accuracy: 51.50% 🔍 Running test: multi_turn_miss_func ✅ Test completed: multi_turn_miss_func. 🎯 Accuracy: 50.00% 🔍 Running test: multi_turn_miss_param ✅ Test completed: multi_turn_miss_param. 🎯 Accuracy: 49.50% $ cat score-harmony_fixes/data_multi_turn.csv Rank,Model,Multi Turn Overall Acc,Base,Miss Func,Miss Param,Long Context 1,openai/gpt-oss-120b (FC) (vLLM),53.25%,62.00%,50.00%,49.50%,51.50% 

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive set of fixes for prompting gpt-oss models for Chat Completion, addressing several issues related to tool calls, reasoning, and multi-turn conversations. The changes significantly improve the model's performance on function calling benchmarks. The addition of a thorough test suite to validate these fixes is a great contribution. The code is well-structured, and the fixes in vllm/entrypoints/harmony_utils.py are robust. I have one suggestion to improve the code by removing a side effect, which will enhance maintainability.

[bbrowning]

Before this fix, when running the bfcl multi_turn and single_turn suits (4,441 tests) I received 8 HarmonyErrors of unexpected tokens remaining in message header and 1 HarmonyError of Unknown role: final. After this fix, I received zero HarmonyErrors on the same test suites.

So, I can also confidently say that by fixing these issues in how we were prompting the gpt-oss-120b model, we are able to reduce the frequency that it does not follow its Harmony format in its responses.

[chatgpt-codex-connector]

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[ehfd]

Are we able to get this merged before v0.11.1? Looks very critical.

[ZJY0516]

CC @chaunceyjiang

[groundsada]

Can this be merged please 🙏?

[bbrowning]

I'm going to make some minor changes to address the review feedback from the two code review bots and to isolate this change entirely from the Responses API path so that we can merge this quicker without worry of it impacting Responses handling in any way. I'll get that done, re-tested, and push the updates today.

[bbrowning]

Are we able to get this merged before v0.11.1? Looks very critical.

Note that even without this change, the gpt-oss support on latest merged main branch is substantially better than what v0.11.0 or v0.10.2 shipped with. I don't know the timing or cutoff for fixes in v0.11.1, but even if this does not make that release there will still be measurably better gpt-oss support in v0.11.1. With that said, I support getting this in if the window for additional fixes is still open for v0.11.1.

[bbrowning]

Ok, from my perspective I've addressed the review bot comments, entirely isolated these changes to only impact the Chat Completions path, expanded the existing test_harmony_utils.py test suite so that it tests both Chat Completions and Responses paths with a common set of tests while moving logic specific to each into its own test class, and re-run my BFCL evaluation on gpt-oss-120b with the latest changes to ensure the numbers are approximately equal as before (approximately because there is a bit of run-to-run variance in this test suite as even though the inputs are identical in every test run the outputs are not entirely deterministic).

I'll keep an eye out for any other review feedback or comments required to get this in.

[ehfd]

While I agree that tool calling and various other quirks in GPT-OSS have improved from basically unusable to a usable state before this PR, this should be designated as a milestone for v0.11.1.

[heheda12345]

CC @chaunceyjiang @aarnphm @yeqcharlotte

[chaunceyjiang]

/cc @yeqcharlotte PTAL.

[alecsolder]

This is great! I think we're finally landing on an implementation that takes the learnings from all of us over the past few months :)

[alecsolder]

I don't think you should restrict for the role of assistant, they don't in harmony. Primarily this is to save context and remove tool calls and tool call outputs after there is a message to the final channel after them, and requiring the role to be assistant would prevent that

[alecsolder]

It's a little confusing but if we want to provide a reference for this, we can use this. It says the index of the "last assistant message", where message is the important word because the message type in responses API means final channel, so this is where our logic comes from.

[alecsolder]

what is this case for? If tool calls is populated, I can understand reasoning being populated, but not this (which would imply it would be more like a "final" message)

[bbrowning]

I think you're right that this content closer matches the final content, but now you made me double-check how we generate the outputs and we have some issues in mapping of reasoning and tool call outputs when generating Chat Completion responses. In my head, the content here matched commentary channel responses from tool calls while the reasoning key matches analysis channel responses. However, that's not necessarily the case and it looks like there are some edge cases in our reasoning and tool call parsers for gpt-oss models that can sometimes end up with this, sometimes end up with analysis or commentary messages missed entirely, or most often end up with analyis messages as reasoning and final messages as the tool call content.

In practice, this still seems to work quite well as-is. But, we can likely pick up a bit more accuracy by doing a once-over of Chat Completions reasoning/tool parsers and chat_completion_*_generator paths for harmony models to ensure streaming/non-streaming cases are all aligned with exactly how we parse input. I probably need a new test suite here that starts with Harmony messages, goes through our response parsing to Chat Completion responses, sends those back in as Chat Completion requests, and ensures the Harmony messages generated from that match what we originally sent.

[bbrowning]

Part of what worries me here is on the response generation side, where it makes assumptions about which messages to map as reasoning vs content solely based on order of the output messages without checking of any channels:

vllm/vllm/entrypoints/harmony_utils.py

Lines 522 to 534 in 63fed55

	if len(output_msgs) == 0:
	# The generation has stopped during reasoning.
	reasoning = parser.current_content
	final_content = None
	elif len(output_msgs) == 1:
	# The generation has stopped during final message.
	reasoning = output_msgs[0].content[0].text
	final_content = parser.current_content
	else:
	reasoning_msg = output_msgs[:-1]
	final_msg = output_msgs[-1]
	reasoning = "\n".join([msg.content[0].text for msg in reasoning_msg])
	final_content = final_msg.content[0].text

Then, we overwrite he content from that if using a tool call parser at

vllm/vllm/entrypoints/openai/serving_chat.py

Lines 1340 to 1351 in 63fed55

	tool_call_info = tool_parser.extract_tool_calls(
	"",
	request=request,
	token_ids=token_ids, # type: ignore
	)
	content = tool_call_info.content
	message = ChatMessage(
	role=role,
	reasoning=reasoning,
	content=content,
	tool_calls=tool_call_info.tool_calls,
	)

The tool call parser at https://github.com/vllm-project/vllm/blob/63fed5550609b96b578d2512aefced09efe76e1e/vllm/entrypoints/openai/tool_parsers/openai_tool_parser.py only looks at harmony messages with function recipients or to the final channel.

So, we may end up mixing analysis and commentary channel messages in reasoning or even dropping one of those entirely when making the Chat Completion response. To get this request mapping to Harmony messages exactly right, I'll need to clean up and pair this with some adjustments to the Harmony message to response mapping path so I can do this deterministically.

That will take a bit of time to match up, add some round-trip tests to ensure it doesn't deviate in the future, and re-run the eval. I can tackle this next week, either as part of this or a folllow-up based on timing preferences.

[alecsolder]

This runs counter to how I feel like it should work, but I think is correct.

Trying to understand OpenAI's implementation, when reasoning is output to the commentary channel, that information is meant to signal that this reasoning should be shown to the user as-is (vs needing a summarization) i.e they want to provide the actual reasoning for the tool call so the user can approve it or not vs only a summarization. But then when it is provided as input again, it doesn't matter so they just always change it back to analysis so that it can be dropped.

[alecsolder]

nit: this could be pulled out because it is repeated multiple times

[alecsolder]

when would this if case not trigger? I think from_role_and_contents always returns None channel

[alecsolder]

I think this could use a comment as it differs from the OpenAI implementation, however I think there has been enough proof at this point that it increases benchmark scores so should be kept

[alecsolder]

nit: previous?

[alecsolder]

I think system and developer cases here need more complex logic.

I think from a high level:

• We should never have more than 1 system message
• We should never have more than 1 developer message

(Can this validation be added to a test?)

Since we create system message and developer message every time in _make_request_with_harmony I'm worried about what happens with system messages here.

Since there is no "instructions" field like in responses, we need to merge the developer message created with the included tools with the developer message here, but not duplicating the tools for example. Complicated and definitely worth the tests I think.