---
status: draft
last_updated: 2026-06-13
---

# alknet-firewall — Architecture

## Current State

**Phase 0→1 (Exploration → Architecture)** — The project has a working PoC
demonstrating that behavioral signals from small language models can detect
adversarial inputs. The core detection logic (~1,745 lines) works reasonably
well but lacks tests, has excessive codebook size, and needs extraction from
the research codebase into a properly structured Python package.

This project extracts and productionizes the behavioral signal detection
approach from the metaspline research project. A ~125M parameter model
(SmolLM2-135M) processes untrusted inputs and produces hidden state
activations. SVD-based dimensionality reduction on these activations reveals
behavioral patterns — normal inputs cluster in expected regions while
adversarial inputs produce anomalous activation signatures. The system
raises "behavioral alarms" without needing to know specific attack types.

## Architecture Documents

| Document | Status | Description |
|----------|--------|-------------|
| [overview.md](overview.md) | Draft | Vision, scope, package structure, dependencies |
| [firewall.md](firewall.md) | Draft | Core firewall API, input screening, alarm protocol |
| [codebook.md](codebook.md) | Draft | SVD basis, detection parameters, codebook compilation |
| [model.md](model.md) | Draft | Model loading, activation extraction, model-agnostic design |
| [configuration.md](configuration.md) | Draft | Thresholds, model selection, detection tuning |
| [open-questions.md](open-questions.md) | Active | Unresolved questions tracker with OQ-IDs |

## ADR Table

| ADR | Title | Status |
|-----|-------|--------|
| [001](decisions/001-python-uv.md) | Python with uv | Accepted |
| [002](decisions/002-behavioral-signals.md) | Behavioral Signal Detection (Not Text Classification) | Accepted |
| [003](decisions/003-small-model-detector.md) | Small Model (~125M) as Detector | Accepted |
| [004](decisions/004-svd-based-detection.md) | SVD-Based Anomaly Detection | Accepted |
| [005](decisions/005-safetensors-only.md) | Safetensors-Only Model Loading | Accepted |
| [006](decisions/006-optional-pytorch.md) | PyTorch as Optional Dependency | Accepted |
| [007](decisions/007-runtime-model-download.md) | Runtime Model Download via HuggingFace Hub | Accepted |
| [008](decisions/008-three-level-alarm.md) | Three-Level Alarm System | Accepted |
| [009](decisions/009-last-token-extraction.md) | Last-Token Activation Extraction | Accepted |
| [010](decisions/010-monotonic-spline-distributions.md) | Monotonic Spline Distributions | Accepted |

## Open Questions

See [open-questions.md](open-questions.md) for the full tracker.

| OQ | Question | Priority | Status |
|----|----------|----------|--------|
| OQ-01 | Should ONNX Runtime be a supported inference backend in Phase 1? | medium | open |
| OQ-02 | What is the minimum viable codebook — can the 1,245-line codebook be compressed? | high | open |
| OQ-03 | Should the firewall support streaming/chunked input screening? | medium | open |
| ~~OQ-04~~ | ~~Should detection thresholds be per-model or globally configurable?~~ | ~~medium~~ | **resolved** (both: model-specific defaults, user-overridable) |
| OQ-05 | How should the firewall integrate with existing guardrail systems? | medium | open |
| OQ-06 | Should file-based configuration use TOML or YAML? | low | open |

## Document Lifecycle

| Status | Meaning | Transitions |
|--------|---------|-------------|
| `draft` | Under active development. May change significantly. | → `reviewed` when open questions are resolved |
| `reviewed` | Architecture is final. Implementation may begin. Changes require review. | → `stable` when implementation is complete |
| `stable` | Locked. Changes require review and may warrant an ADR. | → `deprecated` when superseded |
| `deprecated` | Superseded. Kept for reference. | Removed when no longer referenced |