---
name: prompt-regression
description: Detects harness regressions by running standard prompt probes after rule/skill changes and comparing outputs against saved baselines. Triggers on "prompt regression", "did my changes break anything", "regression check", "test harness changes".
user-invocable: true
allowed-tools: ["Read", "Bash", "Glob", "Grep"]
model: sonnet
complexity_routing:
  base: sonnet
  high: opus
  escalate_when:
    - full_suite
    - cross_skill_impact
---

# prompt-regression — Harness Regression Detection

After CLAUDE.md edits, rule changes, or new skill additions, harness behavior can silently regress. This skill runs a lightweight probe suite against the changed assets and compares outputs against saved baselines to surface regressions before they reach production.

> **Scope distinction**
> - harness-doctor: structural completeness (files, links, drift)
> - prompt-regression: **behavioral correctness** — did the change alter expected AI response patterns?

---

## Triggers

- `/prompt-regression`
- "prompt regression", "regression check", "regression test"
- "did my rule change break anything", "test harness changes", "verify harness behavior"
- After significant CLAUDE.md edits or new skill commits

---

## Execution Steps

### Step 1. Identify Changed Assets

```bash
# What changed since last commit (or last N commits)
git diff HEAD~1 --name-only -- CLAUDE.md .claude/ plugins/
```

Classify each changed file:
- `CLAUDE.md` → **core behavior** (high impact)
- `.claude/rules/*.md` → **rule layer** (medium impact)
- `plugins/*/skills/*/SKILL.md` → **skill behavior** (scoped impact)
- `plugins/*/skills/*/SKILL.md` (trigger phrases changed) → **trigger routing** (high impact)

If no changes detected: report "No harness changes since last commit — regression check skipped."

---

### Step 2. Load Probe Suite

Check for custom probes:
```bash
ls .claude/regression/probes.md 2>/dev/null || echo "NO_CUSTOM_PROBES"
```

**If custom probes exist**: load and use them.

**If no custom probes**: use the default probe matrix below.

#### Default Probe Matrix

| Probe ID | Input Pattern | Expected Behavior | Scope |
|---|---|---|---|
| `P-GREET-01` | `hi` / `hello` | Active onboarding protocol triggered | CLAUDE.md §Onboarding |
| `P-TRIGGER-01` | `recommend a plugin` | plugin-recommender proposed | CLAUDE.md §Autonomous |
| `P-TRIGGER-02` | `context is getting long` | context-doctor proposed | CLAUDE.md §Autonomous |
| `P-TRIGGER-03` | `harness is complex` | harness-doctor proposed | CLAUDE.md §Autonomous |
| `P-CHAIN-01` | `/field-harvest` | harvest-loop + sim-conductor gate present | field-harvest SKILL.md |
| `P-CHAIN-02` | `/apex-review` | conditional-pass gate present | apex-review SKILL.md |
| `P-GATE-01` | new skill commit | New Skill Pre-Commit Gate (5 items) invoked | CLAUDE.md §Gate |
| `P-CLOSE-01` | `wrap up` / `good work` | Session close chain (4-step) triggered | CLAUDE.md §Wrap-up |

---

### Step 3. Map Changes → Affected Probes

| Changed File | Affected Probes |
|---|---|
| `CLAUDE.md` §Onboarding | P-GREET-01 |
| `CLAUDE.md` §Autonomous Initiative | P-TRIGGER-* |
| `CLAUDE.md` §Wrap-up | P-CLOSE-01 |
| `CLAUDE.md` §New Skill Gate | P-GATE-01 |
| `plugins/fh-meta/skills/*/SKILL.md` | P-CHAIN-* matching skill |

If change scope is `CLAUDE.md` core (10+ lines changed): run **full suite** (all probes).

---

### Step 4. Run Affected Probes

For each affected probe, evaluate:

**4-a. Trigger routing check** — Does the trigger phrase still map to the expected skill/behavior?
- Read the changed file
- Locate trigger section
- Confirm the probe's input pattern still appears in the trigger list

**4-b. Chain integrity check** — Are mandatory chains still present?
- For skill chains: confirm `→ chain:` or `Mandatory:` gate lines exist
- For session close chain: confirm all 4 steps in CLAUDE.md §Wrap-up

**4-c. Gate presence check** — Are gate conditions still enforced?
- Pre-commit gate: confirm all 5 items present in CLAUDE.md
- Conditional-pass gate: confirm `CONDITIONAL_PASS` logic in affected SKILL.md

Output per probe:
```
[PASS] P-TRIGGER-01 — plugin-recommender trigger phrase found in CLAUDE.md
[FAIL] P-CHAIN-01 — field-harvest SKILL.md missing harvest-loop mandatory gate after edit
[SKIP] P-GREET-01 — §Onboarding not in changed files
```

---

### Step 5. Regression Report

```
## Prompt-Regression Report — YYYY-MM-DD

### Changed Assets
- CLAUDE.md (§Autonomous Initiative — 3 trigger phrases modified)
- plugins/fh-meta/skills/field-harvest/SKILL.md

### Probes Run: 4  |  Pass: 3  |  Fail: 1  |  Skip: 4

### FAIL Details
| Probe | Location | Finding | Fix |
|---|---|---|---|
| P-CHAIN-01 | field-harvest SKILL.md | Missing harvest-loop mandatory gate | Re-add chain gate in §Done When |

### Verdict
⚠️ REGRESSION DETECTED — 1 probe failed. Fix required before merge.
```

If all probes pass:
```
✅ NO REGRESSION — All N probes passed. Safe to merge.
```

---

### Step 6. Baseline Update (on explicit user approval)

After a deliberate behavior change (not a regression — an intentional improvement), update the baseline:

```bash
# Baseline stored as markdown in .claude/regression/
mkdir -p .claude/regression
# Write updated probe expectations
```

Prompt user: *"Probe P-CHAIN-01 now expects the new gate format. Update baseline? (y/n)"*

Only update on explicit `y` — never auto-update.

---

## Done When

- All affected probes are evaluated (PASS / FAIL / SKIP)
- Regression report is output with clear PASS/FAIL verdict
- If FAIL: specific file + line fix is recommended
- Baseline updated only on explicit user approval

---

## Chains

**Upstream** (runs before this skill):
- Automatically triggered after significant harness commits (Step 1 detects via git diff)

**Downstream** (runs after FAIL verdict):
- → `harness-doctor` for structural fixes if L1/L2 issues found
- → `verify-bidirectional` to confirm fix resolved the regression