---
pipeline_tag: text-generation
library_name: transformers
datasets:
- agentica-org/DeepScaleR-Preview-Dataset
base_model:
- deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B
base_model_relation: finetune
language:
- en
license: mit
tags:
- reasoning
- mathematical
- hybrid-reasoning
- adaptive-reasoning
- efficient-reasoning
- reinforcement-learning
- rlvr
---

# When2Think

<!-- Provide a quick summary of what the model is/does. -->

**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

<!-- Provide a longer summary of what this model is. -->

When2Think-1.5B is an RLVR-post-trained version of [`deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B`](https://huggingface.co/deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B).

The checkpoint learns two coupled decisions:

1. **Whether to reason**
   - **NOTHINK:** Answer directly without an extended explicit reasoning trace.
   - **THINK:** Generate explicit multi-step reasoning followed by a final answer.

2. **How much to reason**
   - Within THINK, adapt generated computation to the input rather than following a fixed or uniformly compressed length target.

The post-training framework combines:

- **Instance-level Difficulty-Aware Control (IDAC):** Uses cached reference success and token-cost statistics to modulate a correctness-gated efficiency bonus based on generated token count.
- **Batch-Wise Standardization (BWS):** Converts trajectory rewards into standardized advantages for critic-free policy optimization.
- **Importance-sampled THINK/NOTHINK exploration:** Adopts balanced mode exploration during post-training. The auxiliary exploration policy is removed at inference time.

These mechanisms are training-time components only. The released checkpoint runs as a standalone causal language model.

<!-- - **Developed by:** [More Information Needed]
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- **Finetuned from model [optional]:** [More Information Needed] -->

### Which Model Should I Use?


| Model | Whether to reason | How much to reason | Recommended use |
|---|---|---|---|
| **[When2Think-1.5B](https://huggingface.co/junshim/When2Think-1.5B)** | Learned THINK/NOTHINK selection | IDAC + BWS | Adaptive hybrid reasoning in a single checkpoint |
| **[When2Think-ThinkOnly-1.5B](https://huggingface.co/junshim/When2Think-ThinkOnly-1.5B)** | Always THINK, without hybrid importance sampling | IDAC + BWS | Explicit reasoning or analysis of within-THINK depth control |

The checkpoints represent different accuracy-computation operating points rather than a strict performance ordering.

### Model Sources

- **Paper:** [When2Think: Learning When and How Much to Reason](https://arxiv.org/abs/2609.19671)
- **Repository:** [JJunShim/When2Think](https://github.com/JJunShim/When2Think)
- **Collection:** [When2Think](https://huggingface.co/collections/junshim/when2think)

## Uses

<!-- Address questions around how the model is intended to be used, including the foreseeable users of the model and those affected by the model. -->

### Direct Use

<!-- This section is for the model use without fine-tuning or plugging into a larger ecosystem/app. -->

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

<!-- This section is for the model use when fine-tuned for a task, or when plugged into a larger ecosystem/app -->

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.

<!-- ### Out-of-Scope Use -->

<!-- This section addresses misuse, malicious use, and uses that the model will not work well for. -->

## How to Get Started with the Model

Use the code below to get started with the model.

```bash
pip install -U torch transformers accelerate
# optional, for fast serving
pip install vllm
```

### by Transformers Pipeline

- [TextGenerationPipeline](https://huggingface.co/docs/transformers/main_classes/pipelines#transformers.TextGenerationPipeline)

```py
from transformers import pipeline

model_path = "junshim/When2Think-ThinkOnly-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

- [AutoModelForCausalLM](https://huggingface.co/docs/transformers/model_doc/auto#transformers.AutoModelForCausalLM)
- [Text Generation](https://huggingface.co/docs/transformers/llm_tutorial)

```py
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)
```

```py
inputs = tokenizer.apply_chat_template(
    messages,
    tokenize=True,
    add_generation_prompt=True,
    return_dict=True,
    return_tensors="pt",
).to(model.device)

input_length = inputs["input_ids"].shape[1]
max_new_tokens = model.config.max_position_embeddings - input_length

with torch.inference_mode():
    outputs = model.generate(
        **inputs,
        max_new_tokens=max_new_tokens
    )

output_text = tokenizer.batch_decode(generated_ids, skip_special_tokens=True)[0]
```

### vLLM

```bash
vllm serve "junshim/When2Think-ThinkOnly-1.5B" --reasoning-parser deepseek_r1
```

### Output Parsing

For decoded outputs that use `<think>...</think>`, the following helper separates the reasoning trace from the final response. Treat this as a convenience parser for that output format, not as a format guarantee across all serving stacks.

- [reasoning.py](https://github.com/JJunShim/When2Think/blob/main/reasoning.py)

## Evaluation

<!-- This section describes the evaluation protocols and provides the results. -->

### Testing Data, Factors & Metrics

See the [paper](https://arxiv.org/abs/2609.19671) for dataset versions, prompts, answer extraction, and the complete evaluation protocol.

#### Testing Data

<!-- This should link to a Dataset Card if possible. -->

The reported evaluation covers verifiable mathematical reasoning and cross-domain transfer:

- GSM-Plus
- OlympiadBench-Math
- AIME24
- AIME25
- Minerva
- MATH-500
- Stratified MMLU-Pro transfer

#### Factors

<!-- These are the things the evaluation is disaggregating by, e.g., subpopulations or domains. -->

The analysis considers:

- Benchmark and task domain
- Problem difficulty where difficulty labels are available
- THINK/NOTHINK selection behavior
- Generated token usage
- Model scale and training variant

#### Metrics

<!-- These are the evaluation metrics being used, ideally with a description of why. -->

- **Pass@3:** Sampling-based Pass@k accuracy with `k = 3`, following the protocol in the paper.
- **Pass@1:** Used for difficulty-stratified MATH-500 analysis.
- **Average generated tokens:** Mean generated tokens per response, reported independently of the Pass@k sampling budget.
- **THINK ratio:** Fraction of responses using explicit reasoning.

### Key Results

| Model | MATH-500 L1 Pass@1 ↑ | Tokens ↓ | GSM-Plus Pass@3 ↑ | Tokens ↓ | AIME24 Pass@3 ↑ | Tokens ↓ | AIME25 Pass@3 ↑ | Tokens ↓ |
|---|---:|---:|---:|---:|---:|---:|---:|---:|
| R1-Distill-Qwen backbone | 92.1 | 1,199 | 79.4 | **590** | 46.0 | 14,195 | 32.0 | 12,616 |
| **When2Think-1.5B** | **95.8** | **619** | 85.7 | 1,052 | 56.0 | 10,236 | **40.0** | **9,549** |
| **When2Think-ThinkOnly-1.5B** | N/R | N/R | **86.8** | 1,652 | **57.3** | **10,046** | **40.0** | 9,846 |

> `When2Think-ThinkOnly-1.5B` corresponds to the paper's
> **w/o IS (IDAC + BWS)** variant. Its MATH-500 Level 1 results are not
> separately reported in the paper and are therefore marked as `N/R`.

### Summary

Key observations:

- **Easy-instance allocation:** On MATH-500 Level 1, When2Think improves Pass@1 from **92.1% to 95.8%** while reducing average generated tokens from **1,199 to 619**, a reduction of **580 tokens**, or approximately **48.4%**. This demonstrates that the hybrid checkpoint can favor direct answering when extended deliberation provides limited benefit.
- **Task-dependent allocation:** On GSM-Plus, When2Think uses more tokens than the backbone while improving Pass@3 from **79.4% to 85.7%**. Together with the MATH-500 Level 1 result, this shows that adaptive allocation may reduce, preserve, or increase computation depending on the input.
- **When2Think-1.5B on AIME24:** Pass@3 improves by **10.0 percentage points**, while average generated tokens decrease by **27.9%** relative to the backbone.
- **When2Think-1.5B on AIME25:** Pass@3 improves by **8.0 percentage points**, while average generated tokens decrease by **24.3%** relative to the backbone.
- **When2Think-ThinkOnly-1.5B:** Achieves **57.3% Pass@3** on AIME24 and **40.0% Pass@3** on AIME25 while always using explicit reasoning.
- **Within-THINK control:** The strong THINK-only results show that difficulty-aware computation control contributes independently of THINK/NOTHINK mode selection.
- **Different operating points:** The hybrid and THINK-only checkpoints represent different accuracy-computation operating points rather than a strict performance ordering.

For complete baselines, standard deviations, difficulty-stratified analysis, and ablations, see the [paper](https://arxiv.org/abs/2609.19671).

## Citation

<!-- If there is a paper or blog post introducing the model, the APA and Bibtex information for that should go in this section. -->

**BibTeX:**

```bibtex
@misc{shim2026when2think,
  title         = {When2Think: Learning When and How Much to Reason},
  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

For questions, open an issue in [JJunShim/When2Think](https://github.com/JJunShim/When2Think/issues).