---
name: paired-probe
description: Refuse a verdict a probe did not earn. Runs a check where the fault IS present and where it is NOT, and blocks the answer when both arms print the same thing, because a check that cannot disagree with you has measured nothing. Also catches the zero-sample sweep that reads as "clean" and the swallowed error that reads as success. Use before reporting any status, audit, sweep, or "nothing found" result, and whenever a check surprises you by passing.
version: 1.0.0
author: yonatangross
user-invocable: false
disable-model-invocation: false
complexity: low
tags: [verification, debugging, quality-gates]
compatibility: "Claude Code 2.1.220+."
---

# paired-probe

**A check that prints the same thing whether or not the fault is present has
measured nothing.** It still returns an answer, that answer looks like evidence,
and it gets acted on. This skill makes the blindness fail loudly instead.

## When to reach for it

Before reporting any of these, because all of them are verdicts:

- "nothing found", "all clean", "no failures", "safe to delete"
- a sweep, audit, or status roll-up over N items
- a security or CI gate that just went green
- any check that passed when you expected it to fail

And immediately whenever a result *surprises* you by passing. Surprise is the
cheapest available signal that the instrument, not the world, is what changed.

## The three gates

| Gate | Question | Failure it catches |
|---|---|---|
| **Differential** | What does this print when the fault is ABSENT? | A probe that answers identically either way |
| **Non-empty** | How many items did it actually examine? | A sweep that measured zero and reported clean |
| **Exit-aware** | Did the probe itself run? | A swallowed error printing success |

Could-not-observe is a **third outcome**, never folded into either verdict.

## Usage

```bash
# Differential: stage the fault, then remove it. Both arms must differ.
scripts/paired-probe.sh --name "retired model pin fails the gate" \
  --present "printf 'model: claude-opus-4\n' > wf.yml && bash tests/ci/lint.sh" \
  --absent  "rm -f wf.yml && bash tests/ci/lint.sh"

# Single-shot, when the fault cannot be staged (a live sweep):
scripts/paired-probe.sh --name "worktrees examined" \
  --measure "git worktree list --porcelain | awk '/^worktree /{print \$2}'" \
  --min-count 1
```

Exit codes: `0` discriminates or met the count, `1` **BLIND**, `2` usage,
`3` could-not-observe.

## Why this exists

Four probes from a single session, 2026-08-21, each confidently wrong and none
failing loudly. Three were caught by other people rather than by the check:

| The probe | What it asked | Why it lied |
|---|---|---|
| "is this branch pushed?" | the local ref cache | unfetched and never-pushed print identically |
| "is the branch on origin?" | the remote branch list | a squash-merge DELETES the head branch, so landed work reads as lost |
| "does this worktree hold unique work?" | `diff main HEAD` | symmetric, so a stale tree flags `main` against itself |
| "any worktree at risk?" | a loop over a blocked temp file | the write failed, `\|\| true` swallowed it, the loop read zero items and printed "safe to prune" |

Every one dies at gate 1 or 2 in seconds.

## The rule that generalises

Ask what the instrument **structurally cannot observe** before trusting its
silence. A tool reports on the channel it queried, not on reality: the local
cache instead of the remote, the whole file instead of the frontmatter, the
proxy instead of the origin. When the answer is a zero or an empty set, that is
exactly when to check the channel, because zero is what a broken instrument
returns too.

## Related Skills

- `ork:verify` grades finished work; this grades the *check itself*.
- `ork:quality-gates` for escalation once a real defect is confirmed.