Add EXAONE 4.0 reasoning parser

[nuxlear]

• Add EXAONE 4.0 reasoning parser
• Add request parameter for ReasoningParser.extract_reasoning_content_streaming()

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

EXAONE 4.0 currently uses reasoning_parser=deepseek_r1 (Please refer #21718).
However, it incorrectly treats all output as reasoning content instead of normal content when no <think> or </think> tags are found and enable_thinking=False.

While this was easily fixed by modifying the output of extract_reasoning_content(), the issue persists in streaming mode.
By adding a request parameter to extract_reasoning_content_streaming(), we can figure out whether the streamed token is reasoning content or normal content.

Test Plan

You can change the "stream" option for testing.

• Run server

vllm serve LGAI-EXAONE/EXAONE-4.0.1-32B --tool-call-parser hermes --reasoning-parser exaone4

1. Test normal request (all outputs should be content)

curl -X POST http://localhost:8850/v1/chat/completions \
    -H "Content-Type: application/json" \
    -d '{
        "model": "LGAI-EXAONE/EXAONE-4.0.1-32B",
        "messages": [
            {"role": "user", "content": "Which is bigger, 3.7 or 3.11?"}
        ],
        "max_tokens": 1024,
        "stream": false
    }'

2. Test reasoning request (should starts with reasoning_content)

curl -X POST http://localhost:8850/v1/chat/completions \
    -H "Content-Type: application/json" \
    -d '{
        "model": "LGAI-EXAONE/EXAONE-4.0.1-32B",
        "messages": [
            {"role": "user", "content": "Which is bigger, 3.7 or 3.11?"}
        ],
        "chat_template_kwargs": {"enable_thinking": true},
        "max_tokens": 4096,
        "stream": false
    }'

Test Result

{"id":"chatcmpl-717caf53062e4b06ba51ad9e71c9512b","object":"chat.completion","created":1754885297,"model":"LGAI-EXAONE/EXAONE-4.0.1-32B","choices":[{"index":0,"message":{"role":"assistant","content":"To determine which number is bigger between **3.7** and **3.11**, follow these steps:\n\n1. **Compare the Whole Number Parts:**\n   - Both numbers have the same whole number part: **3**.\n\n2. **Compare the Decimal Parts:**\n   - **3.7** can be written as **3.70** to make the comparison easier.\n   - Now, compare **0.70** and **0.11**:\n     - **70** (from 0.70) is greater than **11** (from 0.11).\n\n3. **Conclusion:**\n   - Since **0.70 > 0.11**, it follows that **3.7 > 3.11**.\n\n\\[\n\\boxed{3.7}\n\\]","refusal":null,"annotations":null,"audio":null,"function_call":null,"tool_calls":[],"reasoning_content":null},"logprobs":null,"finish_reason":"stop","stop_reason":null}],"service_tier":null,"system_fingerprint":null,"usage":{"prompt_tokens":27,"total_tokens":219,"completion_tokens":192,"prompt_tokens_details":null},"prompt_logprobs":null,"kv_transfer_params":null}

{"id":"chatcmpl-fbaf462004774a73a947f9ff99d1cfb3","object":"chat.completion","created":1754885833,"model":"LGAI-EXAONE/EXAONE-4.0.1-32B","choices":[{"index":0,"message":{"role":"assistant","content":"\n\nTo determine which is bigger between 3.7 and 3.11, compare the decimal values digit by digit, starting from the left.\n\n- Both numbers have the same whole number part (3).\n- In the tenths place, 3.7 has a 7, while 3.11 has a 1. Since 7 is greater than 1, 3.7 is larger at this point.\n- Even without further digits, the tenths place comparison is sufficient to conclude that 3.7 is bigger than 3.11.\n\nTo verify:\n- Rewrite 3.7 as 3.70 for easier comparison: 3.70 vs. 3.11.\n- Compare place values:\n  - Units: 3 = 3\n  - Tenths: 7 > 1\n  - Since the tenths differ, no need to check hundredths.\n- Alternatively, convert to fractions:\n  - 3.7 = 37/10 = 370/100\n  - 3.11 = 311/100\n  - Since 370/100 > 311/100, 3.7 is larger.\n\nSubtraction also confirms:  \n3.70 - 3.11 = 0.59, which is positive, so 3.7 is bigger.\n\nThus, **3.7 is bigger than 3.11**.","refusal":null,"annotations":null,"audio":null,"function_call":null,"tool_calls":[],"reasoning_content":"I need to determine which is bigger between 3.7 and 3.11. Both are decimals, so I should compare them digit by digit.\n\nFirst, look at the whole number part. Both have 3, so they're equal up to the units place. Now, I need to look at the tenths place.\n\nFor 3.7, the tenths digit is 7. For 3.11, the tenths digit is 1. Since 7 is greater than 1, 3.7 should be larger than 3.11.\n\nBut let me write them with the same number of decimal places to make it easier. 3.7 can be written as 3.70, and 3.11 is already 3.11. So, comparing 3.70 and 3.11:\n\n- Units: both 3\n\n- Tenths: 7 vs 1 → 7 > 1\n\nSince the tenths digit is higher for 3.7, it doesn't matter what comes after; 3.70 is greater than 3.11.\n\nI could also think in terms of fractions. 3.7 is 37/10, which is 370/100. And 3.11 is 311/100. Comparing 370/100 and 311/100, clearly 370 > 311, so 370/100 > 311/100, meaning 3.7 > 3.11.\n\n370/100 is 3.70, and 311/100 is 3.11, yes. So, 3.70 is indeed greater than 3.11.\n\nAnother way: 3.11 is three and eleven hundredths, while 3.7 is three and seven tenths. Seven tenths is seventy hundredths, and seventy hundredths is greater than eleven hundredths, so 3.7 > 3.11.\n\nI think I'm overcomplicating it. The simple comparison shows that since the tenths place is higher in 3.7, it's bigger.\n\nBut just to be thorough, let's consider if there's any trick here. Sometimes people might misread 3.7 as 3.07 or something, but no, 3.7 is clearly three point seven, which is 3.70.\n\nIn some contexts, decimals might be written differently, but standardly, 3.7 means 3.70.\n\nPerhaps the question is about numerical values only, so no tricks.\n\nSo, I think 3.7 is bigger than 3.11.\n\nBut let me confirm with subtraction: 3.7 - 3.11 = ? 3.7 minus 3.11.\n\nTo subtract, align decimals:\n\n  3.70\n\n- 3.11\n\n______\n\nStart from right: 0 - 1, can't do, borrow. 10 - 1 = 9, but since we borrowed, the 7 becomes 6 (because 70 becomes 69? Let's think carefully.\n\nActually, 3.70 minus 3.11:\n\nHundredths place: 0 < 1, so need to borrow from tenths. But tenths place has 7, which is 70 hundredths. So, take one from tenths, so tenths become 6, and hundredths become 10. Then 10 - 1 = 9.\n\nNow tenths place: 6 (after borrowing) minus 1 = 5.\n\nUnits place: 3 - 3 = 0.\n\nSo, 0.59, which is positive, meaning 3.70 > 3.11.\n\nYes, difference is 0.59, so 3.7 is bigger.\n\nIf I think on a number line, 3.11 is to the left of 3.7, so smaller.\n\nTherefore, 3.7 is bigger.\n\nThe question is \"which is bigger, 3.7 or 3.11?\" So, answer should be 3.7.\n\nBut just to be complete, is there any context where 3.11 could be larger? I don't think so. Unless it's a different base or something, but it's standard decimal.\n\nPerhaps someone might confuse it with fractions, but 3.7 is 37/10 = 3.7, 3.11 is 311/100 = 3.11, and 37/10 = 370/100 > 311/100.\n\n370/100 vs 311/100, yes.\n\nOr as mixed numbers: 3 7/10 vs 3 11/100. 7/10 = 70/100 > 11/100, so 3 70/100 > 3 11/100.\n\nAll ways confirm.\n\nSo, I think it's clear.\n"},"logprobs":null,"finish_reason":"stop","stop_reason":null}],"service_tier":null,"system_fingerprint":null,"usage":{"prompt_tokens":23,"total_tokens":1572,"completion_tokens":1549,"prompt_tokens_details":null},"prompt_logprobs":null,"kv_transfer_params":null}

(Optional) Documentation Update

[gemini-code-assist]

Code Review

This pull request introduces a new reasoning parser for the EXAONE 4.0 model and refactors the ReasoningParser interface to support it. The core change is the addition of the request parameter to extract_reasoning_content_streaming to allow parsers to access request-specific information, like enable_thinking. While the changes are generally well-implemented and include comprehensive tests, I've found a critical issue in the non-streaming implementation of the new parser where it fails to use this new request parameter, leading to behavior that is inconsistent with its streaming counterpart and incorrect under certain conditions.

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[nuxlear]

Sorry for the delay. I’ve updated my branch with upstream/main and resolved the conflicts.

Thank you for reviewing my PR. I’m ready to address your feedback 😄

[nuxlear]

@aarnphm @chaunceyjiang Could you review this PR? Thank you for your attention!

[nuxlear]

@chaunceyjiang thank you for your quick response. I've addressed your feedback :)

[nuxlear]

@chaunceyjiang @aarnphm Could you please review this PR again? Thanks a lot!