---
license: apache-2.0
library_name: onnx
pipeline_tag: text-classification
tags:
- modernbert
- onnx
- token-compression
- agentic-coding
language:
- en
model-index:
- name: compressor-reflex
  results:
  - task:
      type: text-classification
      name: Line-level keep/drop
    dataset:
      type: custom
      name: compressor-reflex held-out eval
      args: {split: eval, items: 340, must_keep_anchor_lines: 611, train_overlap_pct: 0.0}
    metrics:
    - name: Must-keep retention @ τ*=0.50
      type: accuracy
      value: 100.0
    - name: Downstream next-action fidelity
      type: accuracy
      value: 100.0
    - name: Tool-output compression ratio
      type: compression_ratio
      value: 86.73
---

# compressor-reflex

**Author:** Eric Maddox

## Model Description

`compressor-reflex` is a small (151M-parameter) extractive line-level compressor for tool output in agentic coding loops (IDEs such as Cursor and Antigravity). It scores every line of a tool result for keep/drop and runs post-tool, pre-context — cutting the tokens that reach the next model call while guaranteeing safety-critical lines are never dropped.

- **Architecture:** ModernBERT encoder (`heman10x/rlcd-modernbert-151m`) with a per-line keep/drop head.
- **Format:** INT8 ONNX (`model_int8.onnx`, 150MB). CPU-only via ONNX Runtime. Zero API tokens to run.
- **Decision rule:** keep line iff P(keep) ≥ τ\*, with τ\*=0.50 calibrated on a held-out must-keep anchor set.
- **Fail-open policy:** outputs of ≤5 physical lines or ≤64 tokens bypass the compressor and pass through verbatim; if scoring ever yields zero kept lines, the first and last lines are kept. The model never silently drops everything.

### MCP server

The companion MCP server packages this model for IDE deployment (Cursor, Antigravity, Claude Desktop): install with `pip install compressor-reflex-mcp` ([PyPI](https://pypi.org/project/compressor-reflex-mcp/)). It downloads these weights from this repo on first run and exposes `compress_tool_output` / `compress_file` tools plus a transparent proxy mode.

## Intended Uses & Limitations

**Intended use:** compressing tool outputs (build logs, test results, grep/file reads, command output) before they enter an LLM's context window, in agentic coding workflows. Designed as a local MCP-side layer.

**Not intended for:** anything where a dropped line is catastrophic and unverifiable — the fail-open policy mitigates this, but the 100% retention figure is measured on the eval distribution, not a formal guarantee. Not a general text summarizer: it is extractive (keep/drop), never generative.

## Training and Evaluation Data

- **Train:** 2,200 items (400 must-keep anchors at 3× emphasis) — tool outputs across 6 projects (build logs, pytest output, grep results, file reads, error traces).
- **Validation:** 250 items.
- **Eval (held-out):** 340 items, 611 must-keep anchor lines across 244 items, including 172 deep anchor lines at line index ≥50. **0% verbatim item, intent, and anchor overlap with train/val** (independently re-verified). Eval and training sets are never mixed; the eval set is not distributed with this release.

## Training Procedure

Single-epoch fine-tune of the 151M base (per delivered `training_history.json`: loss 0.5681). Training logs are thin (one epoch of metrics; the log covers data loading only) — disclosed here because the weights are judged on the held-out eval, not on training claims. No training data, eval data, or weights were modified after the independent validation.

## Evaluation

Held-out eval results below were **independently reproduced**: the shipped `replay_harness.py` was re-run on Linux against the shipped INT8 weights and eval set, reproducing `eval_report.json` bit-exactly.

| Metric | Result |
|---|---|
| Must-keep retention @ τ\*=0.50 | 611/611 = **100%** |
| Downstream next-action fidelity | 100% (50/50 replayed steps) |
| Tool-output compression ratio | **86.73%** (163,716 → 21,726 tokens) |
| CPU latency, INT8, per chunk | p50 ~0.3–0.8s, p99 ~0.9–5.7s (hardware- and length-dependent) |

Real coding sessions (builder-measured, n=12 across 4 repositories — methodology reviewed, not independently re-run):

| Metric | Result |
|---|---|
| Tool-output compression | **~90%** |
| Total session token savings | **~48% median** (range 28–61%; 5/12 sessions ≥50%) |

The 50% session-savings bar was narrowly missed on the median (48.30%) due to invariant per-turn system-prefix dilution (~3.5K tokens/turn the compressor cannot touch). Reported as measured, not rounded up.

## Limitations

- **Closest call:** the lowest-scoring must-keep anchor in eval scores 0.5449 — retained at τ\*=0.50 but dropped at τ≥0.55. The calibrated threshold carries a thin margin on this anchor; disclosed.
- **Latency:** 150ms/chunk-class latency is not achievable with this model size on CPU (p99 ~0.9–5.7s). Very long outputs should be chunk-capped in deployment. Latency scales with output length — which is also where the token savings are largest.
- **Distribution:** retention figures hold for the tool-output distribution it was trained and evaluated on. Unusual output formats may behave differently; the fail-open policy bounds the downside.

## How to Get Started

```python
import numpy as np
import onnxruntime as ort
from transformers import AutoTokenizer
from serializer import CompressorSerializer  # shipped in this repo

tok = AutoTokenizer.from_pretrained("aialchemist-dev/compressor-reflex")
ser = CompressorSerializer(tok)
sess = ort.InferenceSession("model_int8.onnx", providers=["CPUExecutionProvider"])

TAU = 0.50  # calibrated keep threshold — do not raise without re-validating retention

def compress(intent: str, raw_text: str) -> list[str]:
    lines = raw_text.splitlines()
    if len(lines) <= 5 or len(tok.encode(raw_text, add_special_tokens=False)) <= 64:
        return lines  # fail-open bypass
    scores: list[float] = []
    for ch in ser.chunk_tool_output(intent, raw_text):
        ids = np.asarray(ch["input_ids"])
        mask = np.asarray(ch["attention_mask"])
        seq, nl = ids.shape[0], ch["num_lines"]
        lm = np.zeros((nl, seq), dtype=np.float32)
        for i, (s, e) in enumerate(ch["line_spans"]):
            s, e = min(s, seq - 1), min(max(e, s + 1), seq)
            if e > s:
                lm[i, s:e] = 1.0 / (e - s)
        probs = sess.run(None, {"input_ids": ids[None],
                                "attention_mask": mask[None],
                                "line_mask": lm})[0]
        scores.extend(np.asarray(probs).ravel().tolist())
    kept = [l for l, p in zip(lines, scores) if p >= TAU]
    return kept if kept else [lines[0], lines[-1]]  # fail-open: never return empty
```

See `INSERTION.md` for the full deployment spec and `replay_harness.py` for the reference evaluation.

## Citation

```bibtex
@misc{maddox2026compressorreflex,
  author = {Eric Maddox},
  title = {compressor-reflex: a small local line-level compressor for tool output},
  year = {2026},
  publisher = {Hugging Face},
  howpublished = {\url{https://huggingface.co/aialchemist-dev/compressor-reflex}}
}
```

## License

Apache-2.0