harness

name	harness
description	Harness engineering guide — maps all sensors and feedforward guides, teaches self-correction protocol when a CI sensor fires. Use when a sensor fires, before making code changes, or when setting up agent context for a new task. Triggers: "harness", "sensor fire", "CI failure", "self-correction".
category	rust
license	MIT
metadata	{"author":"d-oit","version":"1.0","tags":"harness sensors ci quality self-correction"}

Skill: harness

What Is the Harness

Agent = Model + Harness. The harness is the system of feedforward guides (what to do before coding) and feedback sensors (what catches violations after coding). It has two axes:

• Feedforward (guides): Context, constraints, conventions that prevent errors before they happen.
• Feedback (sensors): Automated checks that fire after code changes, providing structured error output.

The harness has two modes:

• Computational: Deterministic checks (fmt, clippy, deny) — always trust the output.
• Inferential: LLM-based guidance (skill docs, agent context) — direction, not commands.

Sensor Response Protocol

When a computational sensor fires:

1. Read the full error message — it includes a fix hint.
2. Classify the error: fmt / lint / test / arch / security.
3. Apply the minimal fix — do not refactor unrelated code.
4. Re-run the specific sensor — cargo fmt, cargo clippy, etc.
5. Only commit when the sensor is green.
6. Write a metrics event to .agents/events/YYYY/MM/DD/ per the metrics-reporter skill.

Sensor Quick Reference

Sensor	Command	Config	Stage
fmt	cargo fmt --all -- --check	.pre-commit-config.yaml	pre-commit
clippy	cargo clippy --all-targets --all-features -- -D warnings	.clippy.toml, .pre-commit-config.yaml	pre-commit + CI
deny	cargo deny check	deny.toml	pre-commit + CI
nextest	cargo nextest run	Cargo.toml	CI
mutants	cargo mutants	[workspace.metadata.cargo-mutants] in Cargo.toml	CI weekly
arch_fitness	cargo test --test arch_fitness	tests/arch_fitness.rs	CI
insta snapshots	cargo insta review	tests/behaviour_harness.rs	CI
gitleaks	gitleaks detect	.gitleaks.toml	CI

Steering Loop

When any sensor fires repeatedly (>2 times in one sprint):

1. Identify the root cause category (maintainability / architecture / behaviour).
2. Update the corresponding feedforward guide to prevent recurrence.
3. If no guide exists, create one in .agents/skills/ using the skill-creator skill.
4. Document the update in CHANGELOG.md.

The steering loop closes the harness: sensors fire → humans and agents update guides → sensors fire less.

References

• Full harness map: HARNESS.md at repo root.
• Agent-optimised error output: scripts/harness-check.sh runs each sensor and emits structured error output with HARNESS VIOLATION prefix.