Workflow step that requests selected-document mutation through the Blue Contracts patch pipeline. Update Document is the controlled gateway for state changes in a sequential workflow. It takes a changeset produced by authoring, BEX, prior steps, or host bindings, validates it, previews it when needed, and submits it to the processor as ordered patch effects. Every patch is processed under Blue Contracts rules: runtime pointer validation, scope boundary checks, reserved-key protection, runtime insertion normalization, gas charging, post-patch type-soundness validation, dynamic type generalization when a valid more-general type is required, Type Generalization Policy enforcement, and Document Update cascade delivery. A patch that would leave the selected document inconsistent with its declared type must be repaired by permitted generalization or rejected; the processor must not commit a document that lies about its type. In sequential workflows, an implementation may maintain a frozen WorkingDocument. Applying an Update Document step to that working state gives later steps read-your-writes behavior before the final effect buffer is committed. The preview must use the same immutable patch planner and conformance/generalization logic as the final processor commit, otherwise workflows could observe a state that the processor could not legally produce.