Summary

Adds built-in reasoning and tool parsers for naver-hyperclovax/HyperCLOVAX-SEED-Think-14B,
registered as hyperclovax_seed_think_14b for both --reasoning-parser and
--tool-call-parser:

vllm serve naver-hyperclovax/HyperCLOVAX-SEED-Think-14B \
  --reasoning-parser hyperclovax_seed_think_14b \
  --tool-call-parser hyperclovax_seed_think_14b \
  --enable-auto-tool-choice

The model emits tool calls as <|im_start|>assistant -> tool/function_call\n[{...}] and
reasoning after a /think prompt; this parser turns that into structured reasoning_content
and tool_calls. (It is a separate parser from the 32B variant, whose chat template and
tool-call format differ.)

Attribution & test versions

The parser logic is adapted from the official HyperCLOVA X vLLM plugin
(https://github.com/NAVER-Cloud-HyperCLOVA-X/hcx-vllm-plugin) — the plugin linked from the
HuggingFace model card — and ported to vLLM's built-in conventions (single self-contained
module, lazy registration, modernized imports).

The reference plugin's last tool-parser commit is ~10 months old, so to preserve its import
compatibility it is tested on vllm/vllm-openai:v0.9.2, while this PR (modernized imports)
is tested on v0.21.0.

The problem and the fix (tool_choice="required")

The reference plugin's reasoning-end boundary detection (is_reasoning_end /
extract_content_ids) is not tuned for the structured-output path, so vLLM's grammar engages
at the wrong token for tool_choice="required". This PR tightens that boundary so the grammar
applies from the correct token.

Test matrix — 54 cases (each axis varied independently):

Each matrix was run with and without the model card's recommended tool-use system prompt:

Two independent axes:

• required is decided by the parser (system-prompt-independent). The reference plugin
  fails 18/18 regardless of prompt — finish_reason=stop instead of tool_calls, HTTP 400
  with force_reasoning, or no tool call when streaming. This PR passes 18/18 regardless.
• auto is decided by the system prompt (parser-independent). With the recommended
  tool-use prompt both reach 18/18; without it both reach 0/18 (see model-format limitation
  below).
• reasoner is correct in all four conditions.

Model-format limitation under auto (not a parser bug)

Without the model card's recommended tool-use system prompt, the model follows the prompt
format it was trained on less reliably, and tool_choice="auto" produces two off-spec cases.
Ideal output (per the chat template):

<|im_start|>assistant -> tool/function_call
[{"name": "get_current_weather", "arguments": {"location": "Seoul", "unit": "celsius"}}]<|im_end|>

Observed off-spec cases (auto, empty system prompt, input "서울 날씨 알려줘." + a
get_current_weather tool; temperature=0, seed=42):

# Case 1 — real function name instead of "function_call", bare object instead of array.
#          (The tool intent is present, just off-format.)
-> tool/get_current_weather
{"location": "Seoul", "unit": "celsius"}

# Case 2 — plain text, no tool call at all. (The model chose not to call a tool.)
현재 서울의 날씨 정보를 가져올 수 없습니다. 하지만, 아래 링크를 통해 ...

• Under tool_choice="required" the boundary fix lets vLLM's grammar force the canonical
  [{...}] form, so both cases come out as valid tool calls (the 18/18 above).
• Under auto there is no grammar, so neither is recovered: Case 1 is off-format and
  Case 2 has no tool intent to recover from. The parser returns the text as content rather
  than fabricating a call.

This is a model-format limitation (the model adheres to a specific system-prompt format),
not a parser defect: supplying the recommended tool-use system prompt makes auto produce
valid calls too (18/18 in the matrix). What the parser does add on top of the required-path
fix is robust handling of canonical-format variants (bare-array hand-off, arguments↔
parameters, dash-prefixed arrays, multiple calls).

Tests run

Unit (v0.21.0, mock tokenizer, no GPU) — 26/26 passed

python -m pytest tests/reasoning/test_hyperclovax_seed_think_14b_reasoning_parser.py \
                 tests/tool_parsers/test_hyperclovax_seed_think_14b_tool_parser.py -v

Covers the 9-combination force_reasoning × skip_reasoning matrix, reasoning↔content boundary
split, streaming, is_reasoning_end boundary detection, and the tool-call forms above, in both
non-streaming and streaming paths.

End-to-end: the 54-case matrix above (14B served with the built-in parser).
Lint: ruff check and ruff format --check pass.

AI assistance

This PR was prepared with AI assistance (Claude Code). The human submitter reviewed every
changed line and ran the tests above.

Naming note

Uses the _14b suffix to distinguish it from the 32B variant, whose chat template and
tool-call format differ. Happy to rename per maintainer preference.