Jeeves. Reasoning improves Jev-like decision models

A reasoning Jev-style classifier with a diffusion drafter, trained with SFT and CISPO.

Acknowledgements

Inspired by Kev.

Highlights

• A 9B Jev-like model (Qwen3.5-9B, LoRA, pointer head) that thinks before it decides, with a block-4 diffusion drafter and the full training code and train/dev/test data.
• Beats Kev-9B and Jev on test data it was never trained on (0.889 vs 0.822 and 0.857) and on JevBench's public tiers (0.935 vs 0.866 for Jev).
• Supports yes/no (noul), multiple-choice (choice), and rating (score) questions in the same request, through a Jev-compatible API.
• About 0.3 s per request without thinking and a 3.3 s median with it on one H100. Can be sped up by truncating chain length.
• Runs on CUDA (Hopper for the FP8 kernel).

Problem

Jev-like models give calibrated decision probabilities, but at low accuracy. A lot of pipelines therefore rely on a reasoning model as a fallback. Jeeves trains a Jev-like Qwen3.5-9B (LoRA and a pointer head) using CISPO to reason before it decides.

This results in better performance on out of domain tasks, and outperforms Jev in JevBench hard (public).

Results

Accuracy with thinking, greedy, 2,560-token cap. The Kev-9B and Jev columns are the numbers Kev publishes.

* No Kev-9B JevBench result is published. These are Kev-8B (Qwen3).

All JevBench numbers are on the public easy, standard and hard tiers (231 items). The sealed judge tier is not included, and the Jev and Kev numbers are restricted to the same public items.

Without thinking the same checkpoint scores 0.804 on our test split (2,962 items), against 0.840 with it.

Quickstart

Requirements: Python 3.12 and a CUDA GPU.

pip install -r requirements.txt

Download the released weights and serve them:

hf download PostHog/jeeves --local-dir jeeves-weights
python -m inference.serve --model jeeves-weights --drafter jeeves-weights/drafter_k4.safetensors --port 8009

Or fuse your own trained checkpoint into a standalone model and serve it with a drafter:

python export.py runs/cispo/final --out runs/fused
python -m inference.serve --model runs/fused --drafter runs/drafter_k4/drafter.safetensors --port 8009

Then send a request in Jev's format:

curl -s localhost:8009/v1/systemone -H 'content-type: application/json' -d '{
  "state": "Shoes arrived two weeks late and in the wrong size. Also I see two charges on my card.",
  "questions": {
    "department":  {"type": "choice", "instructions": "Which team should handle this?",
                    "criteria": {"returns": "Exchanges, refunds, wrong or damaged items",
                                 "shipping": "Delivery status, delays, lost packages",
                                 "billing": "Charges, invoices, payment problems"}},
    "escalate":    {"type": "noul", "instructions": "Does this need urgent human attention?"},
    "frustration": {"type": "score", "instructions": "How frustrated is the customer?",
                    "criteria": ["Calm", "Frustrated", "Very angry"]}
  },
  "options": {"max_think": 512}}'

Response on one H100 (FP8), with the three questions thinking in parallel:

{
    "model": "jeeves-latest",
    "answers": {
        "department": {
            "type": "choice",
            "choice": "billing",
            "confidence": 0.19,
            "probabilities": { "returns": 0.4, "shipping": 0.14, "billing": 0.46 }
        },
        "escalate": { "type": "noul", "noul": 0.72 },
        "frustration": {
            "type": "score",
            "score": 1.5,
            "legend": { "0": "Calm", "1": "Frustrated", "2": "Very angry" },
            "probabilities": { "0": 0.04, "1": 0.43, "2": 0.54 },
            "confidence": 0.75
        }
    },
    "usage": { "input_tokens": 129, "output_tokens": 160, "reasoning_tokens": 1536 },
    "latency_ms": 8141.6
}

Python

sdk/ is a drop-in replacement for Jev's Python SDK (typesafe-sdk):

from jeeves_sdk import Choice, Noul, Score, TypeSafeClient

with TypeSafeClient() as client:
    result = client.system_one(
        state="I was charged twice. Please help.",
        questions={
            "billing": Noul(instructions="Is this about billing?"),
            "tone": Choice(instructions="What is the tone?", criteria={"calm": None, "angry": None}),
            "urgency": Score(instructions="How urgent is this?", criteria=["can wait", "this week", "today"]),
        },
        max_think=768,
        return_reasoning=True,
    )
    print(result.nouls["billing"].noul, result.choices["tone"].choice, result.scores["urgency"].score)
    print(result.reasoning["tone"].text)

The client connects to http://127.0.0.1:8009 by default (or JEEVES_BASE_URL), needs no API key, and waits up to 120s.

Options

options is optional and ignored by Jev clients that don't send it. Server-wide defaults are set with the matching serve flags.

On 325 dev questions:

How it works

Questions, states and answers are loaded into the Qwen chat template like

<state> …state…
<q> instructions <opt> option 1 </opt> <opt> option 2 </opt> …
<think>

The model then rolls out its reasoning chain, and after the </think> token we append

</think>

<q> instructions <opt> option 1 </opt> <opt> option 2 </opt> …
<decide>

A pointer head scores each option with a scaled dot product between a query projection of the hidden state at <decide> and a key projection of the hidden state at that option's </opt>, where

<state>, <q>, <opt>, </opt>, <decide> = "<|fim_prefix|>", "<|fim_middle|>", "<|box_start|>", "<|box_end|>", "<|fim_suffix|>"

These are rare, largely unused tokens in the Qwen tokenizer. Ablations found that using plain text like "State" in the prompt instead worsened performance. Likewise, not repeating the questions after the reasoning block also decreases performance. The final probabilities are a softmax over the option scores, divided by a temperature fitted on the dev set.

Training

1. SFT (2 epochs, 596 steps on 8 GPUs). LoRA r=16 on all projections of Qwen3.5-9B plus the pointer head, trained on 19,126 questions from 12 public datasets and synthetic policy data. Half the questions carry a reasoning chain sampled from the base model.
2. CISPO (a 624-step schedule stopped at step 402). 9,992 RL questions, 8 rollouts each at temperature 1, capped at 2,560 thinking tokens.
3. Calibration. A single temperature fitted on dev, stored with the checkpoint.

Stopping at step 402 keeps the best calibration and dev score. Past it, the head over-sharpens on the saturated RL pool.

Diffusion drafter

A diffusion view of the frozen model (drafter/), inspired by Orthrus.

Unlike Orthrus, which supports attention-only models, it supports Qwen3.5's Gated DeltaNet layers by letting mask tokens cross-attend to those layers' post-convolution keys and values.

Block 4 is the default because it stays cheap when several questions are batched.

Reproduce

Data

You can build the datasets locally using the prep scripts. This downloads the public datasets from Hugging Face at the revisions pinned in prep/public.py:

Each public dataset stays under its own license.

Training

On 8 GPUs, with the data in data/, bash run.sh runs the whole pipeline:

torchrun --nproc_per_node 8 train.py sft --run-dir runs/sft
torchrun --nproc_per_node 8 train.py cispo --run-dir runs/cispo --init runs/sft/final
torchrun --nproc_per_node 8 test.py runs/cispo/final
torchrun --nproc_per_node 8 jevbench.py runs/cispo/final
python export.py runs/cispo/final --out runs/fused
torchrun --nproc_per_node 8 -m drafter.gen --model runs/fused
torchrun --nproc_per_node 8 train.py drafter --model runs/fused --block 4 --run-dir runs/drafter_k4

Repository

Limitations

• Knowledge questions trail Jev (MMLU 0.793 vs 0.900, MMLU-Pro 0.739 vs 0.840).
• Thinking is slow at the tail: 17 s at p90 with full chains. Use max_think and nothink_threshold when latency matters.
• The Kev and Jev comparisons outside JevBench use different items from the same sources.
• No language consistency reward was included so thinking chains are not well interpretable.

Quote this

If you use Jeeves, its training recipe or its drafter, please cite:

@software{waltz2026jeeves,
  author = {Waltz, Nicholas P.},
  title  = {Jeeves: Reasoning Improves Jev-like Decisions},
  year   = {2026},
  url    = {https://github.com/PostHog/jeeves},
  note   = {Qwen3.5-9B decision model trained with SFT and CISPO, with a block-4 diffusion drafter}
}

References

• Jev's Architecture Unmasked, the Jev design that Kev and Jeeves follow.
• Qwen Team. Qwen3.5-9B, the base model.
• Yang, Kautz, Hatamizadeh. Gated Delta Networks: Improving Mamba2 with Delta Rule. ICLR 2025.
• Hu et al. LoRA: Low-Rank Adaptation of Large Language Models. ICLR 2022.
• MiniMax. MiniMax-M1: Scaling Test-Time Compute Efficiently with Lightning Attention. 2025. Introduces CISPO.
• Guo, Pleiss, Sun, Weinberger. On Calibration of Modern Neural Networks. ICML 2017. Temperature scaling.
• Orthrus, arXiv 2605.12825. The diffusion drafter ours adapts to Gated DeltaNet.
• Leviathan, Kalman, Matias. Fast Inference from Transformers via Speculative Decoding. ICML 2023.