[Bugfix][Responses API] Order in-flight tool call tail before post-tool content in split_delta - #55371
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…ol content in split_delta In tool-heavy workloads with speculative decoding or multi-token output steps, an engine step frequently spans the boundary between an active tool call and subsequent content or reasoning. Previously, split_delta() decomposed compound deltas in fixed order reasoning -> content -> tool_calls. When the tool-call chunk was an argument continuation/tail (function name is None), emitting content first caused the state machine to close the tool call prematurely. The subsequent nameless tool call tail then attempted to open a new tool call with name=None, failing Pydantic validation on ResponseFunctionToolCallItem and aborting the streaming response mid-stream. This patch: 1. Orders in-flight tool call tails before subsequent reasoning/content in split_delta. 2. Preserves reasoning -> content -> tool_calls ordering when starting a new tool call. 3. Adds a defensive fallback for tool_name in SimpleStreamingEventProcessor.open. 4. Adds regression unit tests covering boundary-crossing steps and fallback handling. Fixes vllm-project#55284 Signed-off-by: SOUMYAJIT GHOSH <23051387@kiit.ac.in>
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WalkthroughThe Responses streaming path now orders tool-call continuations before reasoning and content, while newly named calls follow them. Tool-call closure clears stored metadata, and nameless opens fall back to ChangesResponses streaming tool-call handling
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The change updates Responses API streaming ordering and tool-call state handling to avoid stream termination at tool-call boundaries. No current merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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In `@vllm/entrypoints/openai/responses/streaming_events.py`:
- Around line 1149-1157: Update the streaming delta ordering around is_tool_tail
so tool_deltas are classified per grouped tool-call index rather than using
delta.tool_calls[0].function.name. Emit continuation groups first, then
reasoning/content, followed by newly named tool-call groups, preserving the
required order regardless of input order; add regression coverage for both
continuation-first and new-call-first cases.
- Line 1270: Update the tool-name fallback in emit_simple_tool_call_done so a
closed tool-call name is not reused after the state transitions to NONE; only
reuse state.tool_call_name while the matching tool-call state is active, or
clear it when the item closes. Add a regression test covering named tool call,
content, then nameless tool call, which must use "unknown".
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… call state on done Classify compound tool-call deltas per grouped index into continuations and new calls, emitting continuations before reasoning/content and new calls after. Clear tool_call_name and tool_call_index in emit_simple_tool_call_done so closed tool names never leak to subsequent isolated nameless calls. Signed-off-by: SOUMYAJIT GHOSH <23051387@kiit.ac.in>
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Verified locally (head de9c0f6 vs base 1ff5edb, CPU host, VLLM_TARGET_DEVICE=cpu).
Regression-proven. Transplanting the PR test file onto base: 5 tests FAIL on base / all 12 PASS at head:
test_tool_call_tail_emitted_before_content,test_mixed_continuation_and_new_tool_calls_ordering(split_delta reorder)test_speculative_boundary_crossing_tool_call_tail_and_content(the #55284 repro — pre-fix, the nameless tail arrives after content, so the processor opens TOOL_CALL withname=None)test_nameless_tool_call_open_fallback,test_closed_tool_call_does_not_leak_name_to_subsequent_nameless_tool_call(done-state clearing)
Positive controltest_new_tool_call_emitted_after_contentpasses on BOTH base and head — new-call ordering intentionally unchanged.
Crash class executed. Direct processor.open(TOOL_CALL, nameless tc) on a fresh processor: base raises ValidationError: 1 validation error ... ResponseFunctionToolCallItem name (the #55284 crash); head emits ResponseOutputItemAddedEvent with the unknown fallback name. Fail-soft confirmed.
Code audit.
split_delta: groups classified by firstDeltaToolCallin each index group; continuation (function present,name is None) emitted first, reasoning/content in the middle, newly-starting calls (name present) last. Consistent withresolve_target_state's TOOL_CALL priority (function is not None) and withneeds_transition's same-index no-op — a tail dm arriving while state is TOOL_CALL with the same index correctly skips open/close and just appends argument deltas. Group insertion order preserved within each bucket.emit_simple_tool_call_doneclearstool_call_name/tool_call_index/tool_call_namespaceafter the event objects are constructed (strings captured immutably at construction), so emitted events keep the real name; clearing only affects later fallback resolution. Index-switch transitions (parallel calls) immediately re-set the name viaopen(), so the clear is safe there too.open()fallbackcall_name.name or state.tool_call_name or "unknown":state.tool_call_namewas previously never cleared on done, so a later isolated nameless open would inherit a stale completed tool's name (leak); clearing + the three-level fallback make the degenerate path honest. Only caller of the simple processor isserving.py:_process_simple_streaming_events; no other consumers of these functions exist.- ruff check + ruff format --check clean on the changed source.
Non-blocking notes:
- The reorder handles the in-flight-tail case per index group, but a compound delta carrying a tool OPEN (name present) plus a different index's continuation in one step classifies them into separate buckets (new_call vs continuation) and emits continuation first — correct only because the continuation's tool was opened in a prior step; worth a brief comment or test if multi-tool boundary steps become common.
unknownis a magic fallback literal (fine for a should-never-happen path); consider a named constant if more fallbacks appear.
No issues found — the fix is minimal, correctly ordered, and strictly widens the set of step-boundary shapes the simple streaming path can survive.
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Purpose
Fixes #55284
In agentic and tool-calling workloads using speculative decoding (such as DFlash or EAGLE) or fast multi-token output chunking, an engine step frequently spans the boundary between an active tool call and following content or reasoning.
Root Cause
split_delta(delta)decomposed compoundDeltaMessageobjects in a fixed order: reasoning -> content -> tool_calls.function.nameis None), emitting reasoning or content first causedSimpleStreamingEventProcessorto transition out of_StateType.TOOL_CALLintoCONTENT, prematurely closing the active tool call before its tail arguments were consumed.TOOL_CALLand invokedprocessor.open(TOOL_CALL, tool_call).tool_call.function.namewas None,emit_simple_tool_call_openattempted to constructResponseFunctionToolCallItem(name=None), failing Pydantic validation:pydantic_core._pydantic_core.ValidationError: 1 validation error for ResponseFunctionToolCallItemname: Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]RuntimeError: Caught handled exception, but response already started.Key Changes
split_delta:Classifies tool deltas grouped by
tool_call.indexinto in-flight continuations (continuation_deltas, wherefunction.name is None) and newly starting tool calls (new_call_deltas, wherefunction.name is not None).emit_simple_tool_call_done:Explicitly clears
state.tool_call_name = Noneandstate.tool_call_index = Noneupon item closure to prevent state leakage to subsequent nameless calls.Guarantees
ResponseFunctionToolCallItem.namedefaults safely to"unknown"if an isolated nameless tool call is ever opened unexpectedly.Review & Quality Checks
Test Plan
tests/entrypoints/openai/responses/test_streaming_events.py:test_tool_call_tail_emitted_before_content: verifies in-flight tool argument tails precede content and reasoning.test_new_tool_call_emitted_after_content: verifies newly starting tool calls follow content and reasoning.test_mixed_continuation_and_new_tool_calls_ordering: verifies mixed chunks correctly order continuations first, then content, then new tool calls.test_speculative_boundary_crossing_tool_call_tail_and_content: simulates speculative decoding step spanning tool completion and following content without Pydantic validation errors.test_closed_tool_call_does_not_leak_name_to_subsequent_nameless_tool_call: verifies closed tool name is not retained across subsequent tool calls.test_nameless_tool_call_open_fallback: verifies safe fallback when an unexpected nameless tool call reaches open().pytest tests/entrypoints/openai/responses/test_streaming_events.pyAll 12 unit tests pass 100% cleanly.
ruff checkandruff format --checkpass clean.