When2Think-1.5B

When2Think

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When2Think-1.5B is a post-trained hybrid reasoning model that learns both whether to reason explicitly and how much reasoning to allocate to each problem.

The model encourages direct answering on easier instances while preserving extended reasoning on harder ones. Unlike uniform length-compression methods, When2Think treats reasoning depth as an instance-adaptive resource.

Highlights

• Adaptive Think/NoThink Behavior: Learns when to answer directly and when to invoke explicit multi-step reasoning.
• Accuracy-Efficiency Trade-off: Reduces unnecessary reasoning without uniformly suppressing useful reasoning on difficult problems.
• Standalone Deployment: Requires only the released checkpoint for generation.

Model Details

Model Description

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When2Think-1.5B is an RLVR-post-trained version of deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B The model learns two reasoning behaviors:

• NoThink: Direct answering without an extended explicit reasoning trace
• Think: Explicit multi-step reasoning followed by a final answer

When2Think is trained with Instance-level Difficulty-Aware Control (IDAC), which regulates reasoning depth using pre-computed reference accuracy and token-usage statistics. Batch-Wise Standardization (BWS) converts the resulting trajectory rewards into standardized advantages, enabling stable critic-free PPO-style optimization.

Importance sampling (IS) is used during post-training to balance exploration between Think and NoThink. These training components are not required at inference time. The released model generates its learned hybrid reasoning behavior as a standalone causal language model.

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Which Model Should I Use?

• Use When2Think-1.5B for adaptive hybrid reasoning. It learns when direct answering is sufficient and when explicit reasoning is needed, providing the intended balance between accuracy and token efficiency.
• Use ThinkOnly-1.5B when explicit reasoning should always be used, or when studying difficulty-aware reasoning-depth control without hybrid Think/NoThink mode selection.

Model Sources

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• Paper: When2Think: Learning Difficulty-Aware Length Control for Efficient Hybrid Reasoning Models

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Uses

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Direct Use

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When2Think-1.5B is intended for:

• Mathematical problem solving
• Adaptive direct-answer and explicit-reasoning generation
• Research on efficient reasoning models
• Analysis of Think/NoThink mode selection

The model can be loaded as a standard causal language model using Hugging Face Transformers. No separate router, verifier, critic, difficulty estimator, reward model, or reference policy is required for inference.  

Downstream Use

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The checkpoint may be used as a starting point for:

• Continued post-training on verifiable reasoning tasks
• Adaptation to mathematical, symbolic, coding, or scientific reasoning
• Research on adaptive reasoning policies
• Studies of direct-answer and explicit-reasoning behavior   Additional fine-tuning may alter the learned Think/NoThink balance, response length, and reasoning-depth behavior.

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How to Get Started with the Model

Use the code below to get started with the model.

by Transformers Pipeline

TextGenerationPipeline

from transformers import pipeline

model_path = "junshim/When2Think-1.5B"

prompt = "Find the value of $x$ that satisfies the equation $4x+5 = 6x+7$."
messages = [
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": prompt}
]

generator = pipeline(
    "text-generation",
    model=model_path,
    device_map="auto",
    dtype="auto"
)
outputs = generator(
    messages,
    max_new_tokens=512,
    clean_up_tokenization_spaces=False
)

by Transformers

from accelerate import Accelerator
from transformers import AutoModelForCausalLM, AutoTokenizer

accelerator = Accelerator()
device = accelerator.device

model = AutoModelForCausalLM.from_pretrained(
    model_path,
    torch_dtype="auto",
    device_map=device
)
tokenizer = AutoTokenizer.from_pretrained(model_path)

print(model.generation_config, device)

text = tokenizer.apply_chat_template(
    messages,
    tokenize=False,
    add_generation_prompt=True
)
inputs = tokenizer([text], return_tensors="pt").to(device)

generated_ids = model.generate(
    **inputs,
    max_new_tokens=512
)
output_text = tokenizer.batch_decode(generated_ids, skip_special_tokens=True)[0]

Output Parsing

import re

ASSISTANT_RE = re.compile(
    r"<｜Assistant｜>(.*?)(?=<｜User｜>|<｜end of sentence｜>|$)",
    re.DOTALL,
)
THINK_RE = re.compile(r"<think>(.*?)</think>", re.DOTALL)


def parse_deepseek_r1(text: str) -> list[dict]:
    dialogue = []

    for match in ASSISTANT_RE.finditer(text):
        assistant = match.group(1).strip()
        think = THINK_RE.search(assistant)

        if think:
            reasoning = think.group(1).strip()
            content = (
                assistant[:think.start()] + assistant[think.end():]
            ).strip()
        else:
            reasoning = None
            content = assistant

        dialogue.append({
            "reasoning": reasoning,
            "content": content,
        })

    return dialogue


parse_deepseek_r1(output_text)

Evaluation

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Testing Data, Factors & Metrics

Testing Data

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Factors

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Metrics

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Results

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Summary

Citation

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BibTeX:

@misc{shim2026when2think,
  title         = {When2Think: Learning Difficulty-Aware Length Control for Efficient Hybrid Reasoning Models},
  author        = {Jaejun Shim and HyunJin Kim and Young Jin Kim and JinYeong Bak},
  year          = {2026},
  eprint        = {2609.19671},
  url           = {https://arxiv.org/abs/2609.19671}
}

More Information

• Begin of Sentence: <｜begin▁of▁sentence｜>
• End of Sentence: <｜end▁of▁sentence｜>
• Pad: <｜end▁of▁sentence｜>
• Begin of Thinking: <think>
• End of Thinking: </think>
• User Role: <｜User｜>
• Assistant: <｜Assistant｜>
• Max position embeddings (length): 131072

Model Card Contact

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