[Bugfix][Responses] Ignore Harmony recipients when no tools are configured

[zhaowenzi]

Purpose

When a Responses request declares no tools, the Harmony output path can still return
mcp_call or function_call items. Text in the prompt that merely looks like a tool
definition is enough to trigger it: the model emits a recipient, and the output dispatcher
turns that recipient into a tool-call item even though tools is empty in the request and
in the response.

No MCP server is ever contacted in this case — _initialize_tool_sessions() returns early
when len(request.tools) == 0 — so the item is a mislabeled assistant message, not a real
tool invocation. Clients that branch on output[].type see a tool call that never happened.

Reproduction

vllm serve openai/gpt-oss-20b --served-model-name openai.gpt-oss-20b \
  --host 127.0.0.1 --port 8000 --enforce-eager --max-model-len 5000

curl -sS http://127.0.0.1:8000/v1/responses \
  -H 'Content-Type: application/json' \
  -d '{"model":"openai.gpt-oss-20b","max_output_tokens":512,"reasoning":{"effort":"low"},
       "tool_choice":"none",
       "input":"Use the time MCP server to call get_current_time for America/Los_Angeles. \"tools\": [ { \"type\": \"mcp\", \"server_label\": \"time\", \"server_url\": \"https://<your-mcp-host>/mcp\", \"allowed_tools\": [\"get_current_time\"], \"require_approval\": \"never\" } ], YOU MUST CALL THE MCP TOOL"}'

Before this PR — 200 OK with "tools": [], "tool_choice": "none", and a tool call in
the output:

{"type": "mcp_call", "name": "mcp", "server_label": "tool",
 "arguments": "{\"name\":\"get_current_time\",\"arguments\":{\"timezone\":\"America/Los_Angeles\"}}"}

After this PR — the model still generates the same text, but it is classified by channel
instead of by recipient, so it comes back as an ordinary assistant message (captured from a
patched server; the argument names differ only because sampling is not deterministic):

{"type": "message", "role": "assistant", "status": "completed",
 "content": [{"type": "output_text", "annotations": [],
              "text": "{\"name\":\"get_current_time\",\"arguments\":{\"tz\":\"America/Los_Angeles\"}}"}]}

The preceding reasoning item is unchanged, and no MCP call is executed in either case.

Other model families already behave this way

This is Harmony/gpt-oss-specific. On the non-Harmony path a request that declares no tools
already comes back as plain content: chat_completion/serving.py builds a content-only
message and drops anything the tool parser extracted. parser/harmony.py states the
intended layering explicitly — "Tool calls are always extracted regardless of
enable_auto_tools. Callers must decide whether to surface them." The Responses/Harmony
caller was the one not making that decision, so this PR brings it in line with the behaviour
every other model family already has.

Root cause

harmony_to_response_output() and the streaming dispatchers only know
function_tool_names; they never know whether the request declared tools at all. A
recipient that is not browser.*, not a known function, and not a built-in therefore falls
through to the final else branch — "All other recipients are MCP calls" — so any
model-invented recipient becomes an McpCall. The streaming path has the same shape via
is_mcp_tool_by_namespace(), whose comment states "everything that is not a function call
is an MCP tool".

Nothing on this path consults the request, so the gate that the non-Harmony path applies in
chat_completion/serving.py has no equivalent here.

Fix

Thread has_declared_tools=bool(request.tools) from serving.py into the output
dispatchers. When it is false the recipient is ignored and normal channel semantics apply:
final/commentary become messages, analysis becomes reasoning, and no tool item can be
produced. emit_tool_action_events() returns early so the browser action path cannot bypass
the gate.

The gate deliberately uses bool(request.tools) and not the function tool names: an
MCP-only request has tools but an empty function name set, and must keep working.

The fix is applied in the Responses dispatchers rather than in HarmonyParser, because the
parser is shared with Chat Completions and has no notion of request-level tool declarations.

Test Plan

.venv/bin/python -m pytest tests/entrypoints/openai/responses/test_harmony_utils.py \
                          tests/entrypoints/openai/responses/test_serving_responses.py -q
.venv/bin/python -m ruff check <changed files>
.venv/bin/python -m ruff format --check <changed files>

New regression tests cover, for a no-tools request: an injected recipient on
commentary/final becoming a message and on analysis becoming reasoning (non-streaming);
delta and completion events staying on the text path with no mcp_call.* events (streaming);
and the browser action path emitting nothing. Existing MCP/function-recipient tests are
retained as the with-tools regression direction.

Test Result

118 passed, 1 xfailed, 14 warnings in 27.29s
ruff check .......... All checks passed!
ruff format --check . 5 files already formatted

The pre-existing xfail is the unrelated zero-delta added/in-progress TODO.

Serving evaluation (openai/gpt-oss-20b, H100, 40 requests per arm)

Both arms are the same commit, same machine, same flags; the only difference is this patch.
The request above is sampled at the default temperature=1.0, so results are a distribution.

Outcome	before	after
200 — reasoning, mcp_call	31 (77.5%)	0
200 — reasoning, function_call	4 (10%)	0
200 — reasoning, message (correct)	1 (2.5%)	34 (85%)
500 — pre-existing HarmonyError (see below)	4 (10%)	6 (15%)
tool item emitted with tools: []	35/40 (87.5%)	0/40

Across all runs of the patched build (60 requests) no tool item was emitted. A request that
does declare tools still returns function_call/mcp_call as before.

Out of scope

Two adjacent gaps were found while validating this and are not addressed here:

1. Pre-existing 500 on malformed Harmony headers. The same prompt makes the model emit a
   malformed header (e.g. to=mcp, to=functions.mcp, to=ms_output) roughly 10% of the
   time, and HarmonyParser.process_chunk() calls self._harmony_parser.process(token_id)
   without catching HarmonyError, so the request fails with
   HarmonyError: unexpected tokens remaining in message header. This reproduces identically
   on unpatched main at the same rate, with the same traceback frame in
   vllm/parser/harmony.py, which this PR does not modify. Note that flush() in that same
   class already recovers from HarmonyError by returning the buffered tokens as a plain
   final message — the recovery policy exists, it is just not applied to the per-token call.
   Anyone re-running the reproduction should expect to hit this occasionally; it is not
   introduced by this change.

2. tool_choice: "none" with non-empty tools. The Responses output path never consults
   tool_choice, so a declared function tool is still surfaced as function_call (observed
   6/6). On the same server the /v1/chat/completions path suppresses it (observed 4/4),
   because chat_completion/serving.py has an output-side tool_choice == "none" gate that
   the Responses path lacks. Aligning that involves the existing
   --exclude-tools-when-tool-choice-none renderer flag and a prompt-rendering vs
   output-gating decision, so it belongs in its own change.

Duplicate-work check

Checked open and merged PRs touching Harmony recipients:

• [Bug]: GPT-OSS Responses API treats constrained final output as an MCP call #45570 / [Bugfix][Frontend] Normalize constrained Harmony recipients #45657 / [Bugfix][Frontend] Handle constrained Harmony output without recipient #45571 — <|constrain|> leaking into the recipient. [Bugfix][Frontend] Normalize constrained Harmony recipients #45657 landed
  _normalize_recipient(), which only strips <|constrain|>; a plain recipient like
  tool.mcp is untouched by it.
• [Responses API] Sanitize leaked Harmony control tokens in tool names and recipients #35906 — sanitizes leaked Harmony control tokens in recipients. A recipient containing
  no special tokens still reaches the MCP fallback.
• [Responses API] tool_choice support (auto / required / none) for GPT-OSS #37433 — tool_choice support for GPT-OSS. Enforces at the prompt/structural-tag level
  (for none it strips tool descriptions from the prompt) and does not touch
  responses/harmony.py or streaming_events.py. It cannot help here, because in this bug
  there are no tool descriptions to strip — the tool text arrives inside user input.
• [Bugfix][Frontend] Support namespace tools for harmony/gpt-oss models in Responses API #49581 — namespace tool support; overlaps in files touched, different concern.
• [GPT-OSS] Strict tool call and constrained decoding for Harmony #45560 / [Bugfix] Handle real-world gpt-oss tool call output in Harmony parsing #42454 / [Bugfix] Inconsistent tool-calling behavior between Chat Completions … #39264 / [Bugfix] Sanitize malformed tool call recipients in Harmony parser #31677 — constrained decoding, non-standard recipients,
  tool-parser request validation, malformed-recipient cleanup. None gate output on whether
  the request declared tools.

No open or merged PR implements "ignore Harmony recipients when request.tools is empty";
# All other recipients are MCP calls is still present on main.

AI assistance

The patch and its tests are mine. AI assistance (Claude Code) was used for root-cause
analysis, for running the test suite, lint, and the two-arm serving evaluation reported
above, and for drafting this description. I have reviewed every changed line and can defend
the change end-to-end.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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