0033. Transitive skip: a guarded-off step skips its descendants

• Status: Accepted
• Date: 2026-07-13
• Deciders: thulasi.ram (architect)

Context

Step-level when: guards (0009) decide whether a step runs. The open question (0027 neighbourhood, deferred from slice 4) was what happens to a guarded-off step’s descendants.

The earlier pure-lowering select_steps removed a when:false step and dropped its dependents’ edges onto it, so a dependent became a root and ran anyway. That is surprising: a notify step that exists only to run after deploy would still run on a branch where deploy is guarded off. It also diverged from the runtime cascade the engine already had (dep_dead: a step with a terminal-non-success dependency is Skipped).

Decision

A when:false step is Skipped, not removed; the full DAG (edges intact) is preserved, and any step with a Skipped need is transitively skipped — the GitHub-Actions default success() join, consistent with Scarab’s stated GHA-ergonomics (0007).

• Run creation keeps every compiled step and marks the guarded-off ones Skipped (pure excluded_steps reports which; the server records the transition). Edges are not dropped.
• The existing scheduler cascade (dep_dead) does the propagation: a Pending step with any Skipped (or otherwise terminal-non-success) need becomes Skipped. No new cascade logic.
• A Skipped step is not a failure. Run settlement fails a run only on a Failed/Cancelled step; a run whose only non-successes are skips succeeds.
• An always()-style opt-out (run a descendant even when a need was skipped) is a later feature; the default is skip.

Consequences

• Conditional pipelines behave like GHA: guard off a deploy and its notify tail skips with it, the run still green.
• Compile-time when: and the runtime cascade are now one mechanism (mark Skipped), not two divergent ones.
• select_steps (remove + drop edges) is replaced by excluded_steps (report ids); pruning never drops edges, which is exactly what makes transitive skip work.

Alternatives considered

• Non-transitive (keep the old behaviour) — descendants run as roots. Simpler but surprising, and splits skip semantics between compile-time and runtime.
• Skip only when all needs were skipped — lets a step run if any need survived; rejected as more surprising than the GHA success() default and harder to reason about. Revisit alongside always()/if: conditions.