# Diagnostic Loop Catalog

Read this when the exact distinguishing loop is not obvious or several loop shapes remain plausible.

Use it to select the smallest observation whose possible outcomes differ between meaningful hypotheses at the required boundary.

## Common Loops

- **Failing test:** use when an existing stable interface or test boundary reaches the reported behavior.
- **Reproduction command:** one CLI, package task, or script with representative fixture input.
- **HTTP or protocol script:** the smallest real request sequence that crosses the affected boundary.
- **Browser path:** the user flow with relevant DOM, console, network, and visual observations.
- **Captured trace:** a sanitized log window, HAR, event payload, seed, timestamped screenshot, dump, or production sample replayed safely.
- **Throwaway harness:** a minimal script around the real module when no test framework reaches it.
- **Differential loop:** the same input through old and new code, configuration, dataset, dependency, or environment.
- **Instrumented run:** targeted probes at boundaries that distinguish competing hypotheses.

## Predict Before Running

Record only outcomes capable of distinguishing the viable hypotheses:

```text
Observation outcome | Effect on leading hypothesis | Effect on alternative
```

If every likely outcome leaves the same hypotheses plausible, the loop is not distinguishing enough. Sharpen the observer or choose another loop before generating evidence.

## Improve The Loop

Prefer a loop that is:

- **Sharper:** observes the reported symptom or hypothesis rather than generic failure.
- **More representative:** preserves the relevant input, state, environment, and observing boundary.
- **More distinguishing:** can support one plausible cause while contradicting another.
- **More deterministic:** pins time, randomness, dependencies, filesystem, or network only when doing so preserves the failure.
- **Faster:** removes unrelated setup without narrowing the claim.
- **Safer:** isolates mutation, sanitizes sensitive evidence, and has bounded cleanup or recovery.

An observer must sit on the real path and cover the complete operation. A manual counter beside the path, a helper call, or a nearby mocked flow does not prove the reported boundary ran.

## Boundary Examples

- A UI save remains pending: exercise the browser path and inspect the request, console, and rendered state; a reducer unit test does not prove the reported boundary.
- A registered callback allegedly never runs: invoke or observe the registered entrypoint; calling its helper directly proves only helper behavior.
- A failure appears only in CI: compare the same input across local and CI configuration before changing timing or adding retries.
- Two causes fit one symptom: choose an observation whose outcomes differ between them; another generic failure is not distinguishing evidence.

Stop once one loop is selected with predicted distinguishing outcomes. Return it to the main skill; do not execute it from this reference.