Cart Merge Contract Repair
Repair guest-to-account cart merge bugs including duplicate lines, stale prices, coupon leakage, replay races, and checkout state drift.
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You say
Our Next.js storefront merges guest carts into account carts after login. Identical SKUs sometimes duplicate, personalized lines can collapse together, guest coupons survive when the account is ineligible, prices are copied without revalidation, and repeated auth callbacks can run the merge twice. Repair the repository locally and add regression tests without accessing production carts or payment systems.
Your agent does
Repair decision: CROSS-LAYER CART-MERGE CONTRACT DRIFT FOUND — SAFE LOCAL PATCH AVAILABLE.
Findings:
- Unstable line identity can create duplicate or collapsed items.
- Personalized lines are not consistently distinguished.
- Guest promotions can survive without account revalidation.
- Guest price snapshots can become stale after login.
- Repeated authentication callbacks can replay the same merge.
- Checkout can read a cart version created before recalculation.
Repair:
- Added canonical line identity.
- Added stable merge operation IDs and source/target version guards.
- Separated active and saved-item state.
- Added post-merge pricing, promotion, quantity, and availability validation.
- Added checkout version protection.
- Added deterministic replay and collision tests.
Production carts, live pricing, promotions, inventory, and payment systems were not accessed.
What you get
About this skill
Guest-to-account cart merges can silently corrupt state: duplicate lines, collapsed personalization, stale prices, invalid coupons, quantity-limit bypasses, saved-item leakage, repeated merges, and checkout totals based on an obsolete cart version.
Cart Merge Contract Repair gives an AI coding agent a structured workflow for tracing and repairing these failures inside an existing repository.
It analyzes cart ownership, deterministic line identity, variants, modifiers, personalization, bundles, quantities, saved items, pricing, promotions, inventory boundaries, currency and shipping context, authentication callbacks, merge tokens, retries, recalculation, and checkout handoff.
The skill helps implement bounded fixes such as canonical line fingerprints, stable merge operation IDs, source/target version guards, idempotent retries, post-merge validation, truthful lifecycle states, and deterministic regression tests.
It deliberately separates structural cart merging from business-policy decisions. It will not invent coupon precedence, pricing policy, inventory rules, bundle equivalence, or customer-facing behavior where the repository does not establish them.
Ideal for ecommerce, marketplaces, ordering systems, subscription platforms, retail applications, and other products that merge anonymous carts into authenticated accounts.
How to install
Drop the file into your AI Agent. Works with Claude, Cursor, ChatGPT, and 20+ more.
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