ADR-0007: A rule whose selection is empty fails by default

• Status: Accepted
• Date: 2026-09-20
• Derives from: design.md § Why (liveness), § The name, § The five stages (stage 4), § What agents actually do with rules
• Constrains: architecture.md § The rule engine, ADR-0008
• Implemented by: Wave 1 plan

Context

Four rules in the reference monorepo matched zero files for months and read as standing fences. ArchUnitNET fails an empty selection by default; dependency-cruiser does not, and heavy users bolt the check on. The product's name is this check.

Decision

• Every rule of every family (dependency, element, slice, diagram, ratchet) whose from, module or select side matches nothing is vacuous, is listed in summary.vacuousRules[], and makes the run exit with code 2 (ADR-0008).
• allowEmpty: true on a rule, or WithoutRequiringPositiveResults semantics in compatibility mode, turns the check off for that rule.
• In dependency-cruiser compatibility mode the check is still on, because it is a run-trust condition rather than a rule violation; --no-liveness disables it globally for a repository that has its own guard.
• rules --json reports fromMatches and toMatches so a guard can see the liveness of every rule without running the check.

Consequences

• A rule an agent writes that matches nothing cannot merge green.
• Conformance gate 1 must run the dependency-cruiser specs with liveness disabled, since those specs do not expect it; the harness passes --no-liveness and this is documented in conformance/README.md.

Alternatives considered

• Warn only. Rejected: the monorepo's four dead rules were warned about by nothing and noticed by no one.