spec-backprop

name	spec-backprop
description	Walk-back algorithm for the Spec-Auditor subagent. Given a failure at phase N, identifies the earliest phase whose spec, if tightened, would have prevented the failure. Proposes a surgical amendment.
when_to_use	["eval ratchet failed at phase >= 5","the same feature has failed >= 2 retries on the same phase","a /spec-audit slash command was invoked manually"]
brd_ref	§4.7

Spec-gap backpropagation

The harness's tendency on a phase-N failure is to re-run phase N. But the root cause is usually upstream — the spec at phase K (K<N) was loose enough that the failure could occur. Without backprop, the harness grinds on symptoms.

Algorithm

let gap = describe failure at phase N
for K in (N-1, N-2, ..., 1):
  let spec_K = read the spec / contract / step list for phase K
  if spec_K, if tightened, could prevent gap:
    return amendment for phase K
return "no upstream spec gap — failure is genuinely at phase N"

What "tightened spec prevents the failure" means

The amendment must be:

• Local — one paragraph or 2-3 step additions.
• Specific — names concrete files, fields, or invariants.
• Mechanically checkable — a follow-up phase can verify the spec was met without re-interpreting the prose.

If you can only describe the tightening abstractly ("the spec should be clearer"), keep walking back; you haven't found the gap.

What does NOT trigger backprop

• A failing test caused by a bug in the production code. That's a phase-N failure with a code fix — no spec amendment needed.
• A failing test caused by a missing dependency. That's a phase-1 environmental fix — init.sh is the artifact to amend, not the BRD.
• A flake. Re-run before backpropagating.

Anti-patterns

• Walking all the way to phase 1. If the failure could only have been prevented by amending phase 1, the architecture is wrong — escalate to HITL, don't propose a BRD §1 amendment.
• Proposing rewrites. Surgical diffs only. Replacing whole paragraphs hides the gap; the diff should make the gap visible in 1-3 added lines.
• Cascading amendments in one audit. One gap per audit. If two exist, the second audit runs after the first amendment lands.

Source

Pattern from kingofevil/forge (MIT). See agents/spec-auditor.md for the subagent definition and BRD §10.3 §4.7.