Sanity Library Reference Docs
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    StartBlock: { filter?: string; requirements?: StartRequirement[] } & {
        kind: StartKind;
    }

    How standalone runs of this workflow begin. filter is a READ-SIDE visibility predicate — "should a start surface offer this workflow for this document?" — evaluated by definitionsForDocument/applicability in the browse-time-pure start-filter context ($tag/$definition/$now bound; $fields cannot exist before inputs do, so a $fields read here is deploy-rejected). It is NOT a startInstance gate; the verb never reads it. requirements are named readiness checks evaluated in author order in the start-time context (GROQ nodes add $fields; singleSubject is the one-in-flight-run-per-subject rule) — every node must pass before startInstance commits. Both are advisory like every engine-side check; the Content Lake remains the only enforcement point.