[Bugfix] Count prompt-opened Poolside reasoning tokens

[BIT-Orange]

Count prompt-injected thinking spans through the first closing marker, including generations truncated before the marker, while preserving disabled-thinking accounting.

Purpose

Poolside Laguna's chat template can inject the opening <think> token into the generation prompt. Generated token IDs then begin inside the reasoning span and contain only </think>, or contain no marker at all when generation is truncated.

PoolsideV1ReasoningParser currently inherits the default token counter through DeepSeekV3ReasoningParser, so the Responses API reports reasoning_tokens=0 even when the parser emitted a reasoning item.

This change adds a Poolside-specific counter that:

• preserves zero reasoning tokens for the IdentityReasoningParser path;
• delegates generated <think>...</think> spans to the shared counter;
• counts prompt-opened reasoning up to the first </think>; and
• counts all generated tokens when generation truncates before </think>.

Fixes #49711.

This is not a duplicate of the related open PRs:

• [Bugfix] Count reasoning tokens when the start token is absent #49743 handles output containing </think> without <think>, but its current implementation still returns zero when generation truncates before the closing marker.
• [Bugfix] Fix GLM45 reasoning token counting #41077 adds DeepSeek V3 forwarding and GLM-specific absent-start handling; it likewise returns zero when no closing marker is generated.
• fix(reasoning): force-open poolside_v1 on bare </think> #49728 changes Poolside's reasoning-parser enablement semantics. This change does not force reasoning on and preserves the existing Identity path for no-thinking messages and native tool calls.

The follow-up also updates ParsableContext to buffer token IDs for the current generation and count reasoning at the RequestOutput.finished boundary. This preserves streaming deltas within a generation while counting built-in-tool follow-up generations independently.

Model evaluation is not applicable because this patch only corrects token accounting in the Responses usage metadata. It does not change model generation, reasoning/content extraction, tool calling, or request execution. No documentation update is required because this restores the intended behavior of an existing API field without introducing a new option or interface.

Test Plan

uv run --active --no-sync --with tblib python -m pytest \
  tests/reasoning/test_poolside_v1_reasoning_parser.py \
  tests/reasoning/test_base_thinking_reasoning_parser.py \
  tests/reasoning/test_deepseekv3_reasoning_parser.py \
  tests/reasoning/test_holo2_reasoning_parser.py -q

uv run --active --no-sync --with tblib python -m pytest \
  tests/reasoning/test_poolside_v1_reasoning_parser.py \
  tests/reasoning/test_deepseekr1_reasoning_parser.py \
  tests/entrypoints/openai/responses/test_serving_responses.py::test_reasoning_tokens_counted_for_text_reasoning_model \
  -q

uv run --active --no-sync pre-commit run \
  --from-ref origin/main --to-ref HEAD

git diff --check origin/main...HEAD

Test Result

Before the fix, the new regression cases failed with 3 failed, 3 passed.

After the fix:

• Targeted reasoning parser and Responses tests: 85 passed, 16 warnings.
• All pre-commit hooks passed.
• git diff --check origin/main...HEAD passed.

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[Copilot]

Pull request overview

Fixes Responses API reasoning_tokens accounting for Poolside Laguna outputs where the opening <think> marker is injected into the prompt (so generated tokens may contain only </think> or no marker when truncated). This aligns usage metadata with the parser’s extracted reasoning/channel split while preserving the disabled-thinking (IdentityReasoningParser) behavior.

Changes:

• Add a Poolside-specific count_reasoning_tokens() override that handles prompt-opened and truncated reasoning spans.
• Delegate to the shared DeepSeek R1 counting logic when an explicit <think> marker is present in generated tokens.
• Add targeted unit tests covering prompt-opened, truncated, explicit-marker, and disabled-thinking/tool-call cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
vllm/reasoning/poolside_v1_reasoning_parser.py	Implements correct reasoning token counting for prompt-opened <think> spans while preserving the Identity (disabled) path.
tests/reasoning/test_poolside_v1_reasoning_parser.py	Adds regression coverage for prompt-opened/truncated reasoning token counting and disabled-thinking behavior.

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