# HARN-MAT-001 — match expression is not exhaustive
**Category:** Pattern match (MAT)
**Variant:** `Code::NonExhaustiveMatch` (non exhaustive match)
## What it means
A `match` expression is incomplete, ambiguous, or otherwise invalid. Harn
enforces exhaustive matching to keep agent decision logic auditable.
Specifically: match expression is not exhaustive.
## How to fix
- Add arms covering the remaining variants, or use a wildcard `_` arm as the explicit fallback.
- Remove duplicate arms — each variant should appear at most once.
## Stability
This code is stable. Its identifier, category, and meaning will not change
without a deprecation cycle. Cross-language tooling and IDE integrations can
dispatch on it directly.