/bidirectional-differential

name: bidirectional-differential
description: Audit coherence across an arrow of intent by running two parallel fresh Claude sessions — one reconstructs code from a single EARS, the other reconstructs the EARS from stripped code — then classifies the drift between them. Use when the user invokes /differential-audit, asks to audit EARS-to-code drift for a feature or segment, wants a differential round-trip on a specific spec, or reaches Phase 6 code-complete in linked-intent-dev with arrow-maintenance overlay present and a touched-EARS set to consider. Surfaces intent that the code encodes but the EARS doesn't state, and requirements the EARS states but the code under-pins. Requires docs/arrows/ overlay. Heavy per-EARS cost in subprocess spawns and Anthropic API spend; scope via the opening conversation before running.

Bidirectional Differential

This skill runs a bidirectional differential audit on EARS↔code pairs. The audit is **advisory** — findings concentrate human review on specific cases, and acting on findings is always user-judged. When the audit surfaces drift, the recommended repair path walks the whole arrow: validate intent with the user, then cascade EARS → Tests → Code.

When to act

**Command mode** — user invokes `/differential-audit`:

• No arguments → open the scoping conversation (see §Scoping).
• One or more EARS IDs as arguments → audit them directly with configured defaults; skip the scoping conversation.

**Ambient mode** — at linked-intent-dev's Phase 6 boundary (code is complete for a change) in a project where arrow-maintenance's overlay exists and ambient triggering is not disabled. Emit **one batched prompt** listing every EARS the change touched, offering `all`, `none`, a comma-separated subset, or `skip-arrow`. If ambient is disabled in the project's `CLAUDE.md` (see §Configuration), do not fire.

Ambient mode is advisory: declining, skipping, or any classification outcome MUST NOT block Phase 6 completion.

Hard precondition — arrow-maintenance overlay

Before spawning any blind sessions in either mode, verify the arrow-maintenance overlay exists:

• `docs/arrows/index.yaml` present, and
• at least one per-arrow overlay file under `docs/arrows/`.

If absent, abort with:

> *Bidirectional differential audits attach to the arrow-maintenance overlay. Run /update-lid and then /arrow-maintenance first to establish the arrow surface this skill extends.*

Do not spawn any `claude -p` sessions and do not write any files when the overlay is absent. This skill is heavier maintenance than arrow-maintenance; a project without the lighter layer in place will not act on this skill's findings either.

Scoping

The scoping conversation is the first user-facing moment. The audit itself runs without further input once scope is fixed. Full script in `references/scoping-conversation.md`.

Users describe what they want audited in natural terms — "the login flow", "the billing pipeline", "the scoring rules" — more often than in arrow or LLD names. The scoping conversation interprets those descriptions, maps them to arrows/LLDs in the overlay, and confirms the mapping with the user before moving to EARS-level scope. Then it captures the final EARS set and the runs-per-direction (default 3) and shows a cost estimate before spawning anything.

**Do not auto-select EARS.** The skill does not have a reliable heuristic for picking which EARS to audit within a chosen arrow — that choice is the user's, and the scoping conversation exists precisely to surface it.

Audit protocol

For each scoped EARS, execute the six-step protocol in `references/audit-protocol.md`. Summary:

1. **Resolve inputs**. EARS text resolved by grepping the ID across the project's `*-specs.md` files; implementing code from regions annotated with `@spec {EARS-ID}`. If no `@spec` points at the EARS, surface a coverage-gap entry and skip this EARS. 2. **Strip leaky identifiers** from the code before the B-direction receives it — `@spec` annotations, EARS ID mentions, vocabulary-echoing identifiers, comments paraphrasing the EARS, test describe/it strings that echo EARS phrasing. See `references/audit-protocol.md §Stripping rules`. The B-direction session must not be able to reconstruct the EARS by reading it back out of the code. 3. **Spawn N A-direction sessions** in parallel via `claude -p`. Each gets only the EARS text + a one-line codebase description. Task: produce naive implementation. 4. **Spawn N B-direction sessions** in parallel via `claude -p`, concurrent with A-direction. Each gets only the stripped code + a one-line EARS-syntax reminder. Task: reconstruct the EARS. 5. **Compare and classify**. Within-direction variance first (do A-runs agree with each other; do B-runs agree with each other); between-direction alignment second (does A's diff against real code correspond to B's diff against real EARS). Pick one of the six codes:

• `BD-COHERENT`, `A-ONLY-DRIFT`, `B-ONLY-DRIFT`, `BIDIRECTIONAL-DRIFT`, `INCONSISTENT-BLIND` — see `references/classification-codes.md` for decision rules and worked examples.
• `UNANNOTATABLE` — signpost for negative requirements with no production sink (see §Unwanted below).

6. **Write the per-EARS audit record** to `docs/arrows/_experiments/bidirectional-differential/{segment-name}/{EARS-ID}.md` using the template in `references/audit-report-template.md`. Re-running replaces the file (mutation, not accumulation — commit the old audit before re-running if before/after comparison matters).

**Default N=3.** If within-direction runs split 2-vs-1 on the classification-relevant dimension, re-run the affected direction at N=5 and classify on the majority. If the 5-run result still splits or the split shape changes between runs, classify `INCONSISTENT-BLIND` — don't force a code.

After per-EARS records are written, produce a **user summary** with per-arrow classification counts, top-priority drift findings across the audited scope, and recommended next ste