• Status: Accepted (replaces the rollback bullet of ADR-0019)
• Issue: #339 (the contract and the checks), #340 (the automatic undo)

ADR-0019 made rollback explicit: .onError(Err, toAction(UndoAction)…), no automatic compensation, no compensate metadata. That keeps every undo reviewed, typed and tested — but it leaves three gaps:

• Nothing ties an action to its undo, so a new .onError side path or an early failure finish added later can skip an undo silently — the external system (an ATS, a calendar, a payment) keeps a dangling record. agboss check and agboss describe --json cannot see which action undoes which.
• Some stops have no path at all: app.cancel, an uncaught step failure, an uncaught limit, a FailFast sibling child. No side path can be written for them.
• A failed join drops its finished branches' values, so an .onJoinError side path cannot undo a finished sibling by hand.

• An @Action declares its undo: @Action({ compensateWith: UndoAction, committedBy?: CommitAction }). The undo is an ordinary @Action (code, never a model call), not undone itself (no chains), and reads only what the path holds right after the action. committedBy names the action after which nothing undoes it; without it, a finish commits it.
• Explicit where a path can say it: an .onError / .onJoinError side path after an open undoable action that ends at a finish must run its undo actions, newest first — checked at assembly ([flow.uncompensated-action], [flow.compensation-out-of-order]); with committedBy, every path from the action runs the committer or the undo ([flow.uncommitted-action], an error: leaving committedBy out says "the finish commits").
• Automatic only where no path can say it (#340): the execution ends without a finish — a cancel, an uncaught failure or limit, an aborted child — or a join fails after some branches finished. Then the framework runs the open actions' undo actions itself, newest first, each bounded by compensationTimeout; a failing undo does not stop the rest, and the execution fails with CompensationFailedError (the original error as cause).
• A child workflow is the unit of work: its finish commits its own actions; an aborted child undoes its open ones before its error reaches the parent.
• Rule groups action.* and flow.*; no saga.*, no public name with "saga".

• Easier: an undo is declared once, next to the action, and every side path is proven to run it; agboss describe shows compensateWith … · committedBy … per action; a cancel or an uncaught failure no longer leaves side effects behind (#340); an undo interrupted by a stop, a crash or a lost lease is finished by the instance that takes the tree over, the undos done skipped (#340 PR 4: the reserved tree keys @agboss/unwinding, @agboss/compensated/<key>).
• Harder: one more contract to keep — an undo must be safe to run twice (it gets an idempotency key; a failed or interrupted undo runs again on the next holder) and read only what was on the path right after its action. A root's record is written only after its undo ended, and a cancel that ran the undo itself rejects with CompensationFailedError when an undo failed.
• Out of scope: the side effects of an agent's tool calls (agents get no compensateWith), and quorums (they launch agents only).
• Rejected: metadata only, without an automatic undo (leaves cancels and uncaught failures with side effects); an automatic undo everywhere (implicit, two ways to say one thing); records moving up to the root (a parent's side path would undo actions whose inputs are not in its scope).