[Misc] Rename think_start_str/think_end_str to reasoning_start_str/reasoning_end_str

[chaunceyjiang]

Purpose

follow up #20859
Rename think_start_str/think_end_str to reasoning_start_str/reasoning_end_str.

Avoid being too tightly coupled to <think>. Some models experiment with fields like <seed:think>, or patterns such as </think>\n<answer>\n.

Test Plan

Test Result

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[claude]

Claude Code Review

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[mergify]

Documentation preview: https://vllm--38242.org.readthedocs.build/en/38242/

[gemini-code-assist]

Code Review

This pull request renames think_start_str and think_end_str to reasoning_start_str and reasoning_end_str across the codebase, including documentation, tests, and core configuration files, to improve naming consistency. However, an offline inference example in docs/features/reasoning_outputs.md was not updated and still uses the old parameter names, which will cause a TypeError. Additionally, it is suggested to consider renaming other related terms like thinking_token_budget and ThinkingTokenBudgetLogitsProcessor for further consistency.

[sfeng33]

I noticed a couple of things:

1. Missing rename in tests/v1/logits_processors/test_correctness.py

This file still references the old property names and would break after this PR merges:

tests/v1/logits_processors/test_correctness.py:106:    think_start_token_ids = [THINK_START_TOKEN_ID]
tests/v1/logits_processors/test_correctness.py:107:    think_end_token_ids = [THINK_END_TOKEN_ID]
tests/v1/logits_processors/test_correctness.py:494:        start_tokens = tb_processor.think_start_token_ids
tests/v1/logits_processors/test_correctness.py:521:        end_tokens = tb_processor.think_end_token_ids

2. Breaking change for --reasoning-config JSON input

Since the @config decorator uses extra="forbid", existing deployments passing {"think_start_str": "<think>", "think_end_str": "</think>"} via --reasoning-config will get a validation error after upgrading.

[chaunceyjiang]

@sfeng33 Thanks for the review. Regarding the second issue, since the preceding PR was merged not long ago and hasn’t been included in any released version yet, I’d like to merge this PR as soon as possible.

[njhill]

Thanks @chaunceyjiang