tasksource-decider-nano

A 150M-parameter typed-decision model for the Decision Index. It takes a state and typed questions (choice, noul) and returns a probability for every option. It does not generate text.

#1 among models under 300M parameters on the Decision Index 0.4 (Full 19.85, October 2026).

Architecture

A joint cross-encoder on cross-encoder/ettin-reranker-150m-v1 (ModernBERT-style encoder, 8,192-token context). Every option is scored against the full state, its question and the option text, like the reranker pair (state, "Question: <instructions>\nOption: <option>"), by a linear head on the mean of the option's hidden states. The scores are softmaxed over each question's options.

The state is encoded once per request. All questions and options of a request share it through a tree attention mask: the state attends to itself, a question to the state and itself, and an option to the state, its question and itself. Rotary positions are those of the single pair, so every option gets the score it would get on its own (up to float rounding), while a request with 50 options costs one state encoding instead of 50.

Inference reads every input in full. There is no truncation: a pair above 8,192 tokens raises Unsupported.

Usage

from decider import Decider
model = Decider.from_pretrained("tasksource/tasksource-decider-nano")
model.answer(state={"review": "The battery died after a week."},
             questions={"q": {"type": "choice", "instructions": "What is the sentiment?",
                              "criteria": {"neg": "negative", "pos": "positive"}}})

Decision Index run (kit 0.3, 9eb2dbe), from a local copy of this repository:

hf download tasksource/tasksource-decider-nano --local-dir tasksource-decider-nano
cd tasksource-decider-nano
python -m decision_index pipeline --engine decision_index_engine:DeciderEngine \
    --option repo=. --edition 0.3 --out runs/tasksource-decider-nano

decider.py and decision_index_engine.py are self-contained (PyTorch and transformers only).

Training

64,000 steps, batches of 8 requests, learning rate 2e-5, inputs up to 2,048 tokens, seed 1. The loss is soft cross-entropy over each question's options. 20% of the questions are reworded with task-specific prompt styles, and 5% of the questions get a "none of the above" option. The training mix has 480,000 requests:

No evaluation split is used, and benchmarks without a train split (for example RouterBench) are evaluation only. Every training request goes through a firewall against the Decision Index 0.3 suite: an exact match of the normalized state or of the (state, question) pair, and a template-insensitive word 6-gram content check, drop the request.

Evaluation

Raw index 50.38; kit 9eb2dbe, edition 0.3, 140,171 requests scored.