Loop blocks (until)

Warden sagas support a single-level loop block: a bounded do-while over nested reason / commit steps. After each successful body pass, the engine evaluates until.cel. Loops are always bounded by required max_iterations.

Authoring

steps:
  - id: refine
    kind: loop
    max_iterations: 5
    until:
      cel: "steps.validate.facts.ok == true"
    steps:
      - id: attempt
        kind: reason
        # ...
      - id: validate
        kind: commit
        # ...

  - id: finalize
    kind: commit
    # ...

Rules:

• max_iterations is required and must be >= 1.
• until.cel is required and compile-checked at deploy.
• Loop bodies may contain reason / commit only (no nested loops in v1).
• Multiple sibling loops in one saga are allowed; each has an isolated context.loops.<id> bucket.
• Step ids must be unique across the whole blueprint, including every loop body.
• when.cel inside a body is allowed; SKIPPED counts as a clean step completion and clears that step’s latest-wins context.steps.<id> entry for the current body pass (so until.cel cannot see a prior iteration’s output).

Runtime model

1. Delay-tail materialization: saga start creates only [prefix] → [first loop iteration 1 body]. Steps after the active loop are minted when until.cel becomes true (or when entering the next sibling loop).
2. forward_seq: every forward row gets a monotonic execution sequence. Scheduling walks forward_seq ASC; compensation walks forward_seq DESC.
3. Latest-wins context: context.steps.<id> reflects the latest iteration’s outcome (including an empty shell after SKIPPED). History lives on SagaStepInstance rows (loop_id, iteration, forward_seq).
4. Hard fail: any body step failure or policy/HITL reject aborts the saga (no further iterations).
5. Exhaustion: if until stays false after max_iterations, the saga fails with LOOP_EXHAUSTED and compensates.

Compensation

Undo resolves compensation with paths against the forward row being compensated (output_payload / resolved_arguments), not live context.steps, so iteration N cannot bleed into iteration 1's undo.

CEL bindings

until.cel (and when.cel) see input, steps, loops, saga, plus loop: { id, iteration, max_iterations } when evaluating inside a loop.

Smoke path: max_iterations: 1 with until.cel: "true" exits after one body pass and materializes the tail.