> Canonical page: https://posttrainllm.com/artifacts/qwen3-4b-multibackend-distilled

Negative-transfer case study Report artifact 2026-07-03

# Qwen3-4B Multibackend Distilled

Adding multibackend teacher data did not create a broader agent. It preserved the saturated file-ops score while producing the worst breadth result in the recorded Qwen3-4B lineage. The public weights are useful as a reproducible warning: breadth gates must be frozen before specialist distillation.

- distillation
- tool calling
- negative transfer
- rejected

## Headline Numbers

Evidence class applies to each value: measured = retained evaluation or runtime result; historical = recorded result not freshly reproduced; derived = calculated or decided from records; observed = current artifact state; not-measured = an explicit evidence gap.

File-ops depth

 100% saturated narrow gate historical

Breadth

 31% down from 59.6% stock historical

Breadth delta

 -28.6pp recorded stock-to-candidate change derived

Decision

 Reject failed negative-transfer artifact derived

## Competitive Context

**System**

**Metric**

**Score**

**Size / Class**

**Comparable?**

**Readout**

 Multibackend-distilled 4B recorded depth / breadth 100% / 31% 4B Direct The target gate stayed saturated while breadth collapsed.

 Stock Qwen3-4B same recorded depth / breadth family 58% / 59.6% 4B Direct The candidate gained depth but lost 28.6 breadth points.

 File-ops-distilled 4B same recorded depth / breadth family 100% / 42.3% 4B Direct Already showed negative transfer; multibackend distillation made breadth worse.

 ReST-fused 4B same recorded depth / breadth family 100% / 65% 4B Direct The later teacher-free ReST attempt is the breadth-preserving research candidate.

Direct rows share this artifact's eval setup. Directional rows are useful market context but should not be read as leaderboard claims.

## Attempts and decisions

**Attempt**

**Method**

**Depth**

**Breadth**

**Decision**

 A0 Stock Qwen3-4B 58% 59.6% Baseline

 A1 File-ops distillation 100% 42.3% Route only

 A2 Multibackend distillation 100% 31% Reject

 A3 Teacher-free ReST 100% 65% Research-only routed ship

## Release Blockers

### Breadth collapsed

The recorded 31% breadth result is 28.6 points below stock and 11.3 points below the narrower file-ops distillation.

 Unblock: Do not promote this checkpoint. Start from a breadth-preserving recipe with a frozen regression gate.

### No committed machine-readable eval package

The exact historical result is preserved in the attempt ledger and session record, but raw predictions and a package-level eval report were not committed.

 Unblock: Retain the result as historical evidence; only rerun if a new recipe needs the same checkpoint as a controlled baseline.

## Evidence

- [Hugging Face model](https://huggingface.co/posttrainllm/qwen3-4b-multibackend-distilled)
- [Attempt ledger](https://posttrainllm.com/docs/attempt-ledger/)
- [Historical model inventory](https://posttrainllm.com/docs/sessions/06-17-stepback-inventory-roi/)
- [Public artifact registry](https://posttrainllm.com/docs/factory/public-artifacts/)

## Next Release Action

Keep the weights public as a failed comparison artifact. Do not spend compute revalidating them unless a new breadth-preserving recipe explicitly needs this checkpoint as its baseline.