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preconditions

The step-level preconditions field declares tools that must already be on PATH for a step's work to be considered already satisfied — a lighter, inline, single-step-skip variant of the require step type's hard preconditions gate, for skipping one optional step rather than failing an entire run.

steps:
  - name: install-stringer
    type: shell
    command: go install golang.org/x/tools/cmd/stringer@latest
    preconditions:
      tools: ["stringer"]

go install writes the binary to GOBIN (or GOPATH/bin if GOBIN is unset) — that directory must already be on PATH for preconditions.tools to find it on a later run. If it isn't, stringer never resolves and the step runs every time.

With no explicit when, declaring preconditions alone is enough — the step implicitly means when: "!preconditions.success" (note the negation: preconditions.success means the tool is already there, i.e. skip; the run-signal is its negation — the opposite implicit polarity from inputs's checksum.changed, where the fact itself already means "needs to run"). Run this step when stringer is already installed and it's skipped; run it on a machine without stringer and it installs it.

preconditions is independent of inputs/artifacts and can be declared alongside them — a step with all three implicitly means when: "checksum.changed || !preconditions.success" (run if either signal says so).

Fields​

tools
List of executable names resolved via Go's exec.LookPath — no shell involved, so there's no which-vs-where cross-platform mismatch to work around. preconditions.success is true iff every entry resolves without error.

Facts exposed to when​

preconditions.success
true when every tool in preconditions.tools resolves on PATH. Reuses the same success/failure vocabulary the condition engine already uses for step lifecycle status, namespaced under preconditions — not a new, unrelated word for the same concept.

Power users can reference it directly, including negated, to combine it with anything else when already supports:

steps:
  - name: install-stringer
    type: shell
    command: go install golang.org/x/tools/cmd/stringer@latest
    preconditions:
      tools: ["stringer"]
    when: "!preconditions.success"

Storage​

preconditions.success is always evaluated live (exec.LookPath at run time) — there's no persisted state, unlike checksum.changed.

Custom commands

The same preconditions field works identically on custom command steps.