diff --git a/.changeset/sku-owners-index-scope.md b/.changeset/sku-owners-index-scope.md
new file mode 100644
index 0000000..c72ab29
--- /dev/null
+++ b/.changeset/sku-owners-index-scope.md
@@ -0,0 +1,9 @@
+---
+"@otta-sh/store-emdash": patch
+---
+
+Fix add-to-cart failing with a D1 unique-constraint error on the second request for any given SKU.
+
+`sku_owners` and `customer_emails` declared their natural key as a `uniqueIndexes` entry, documented as "a lookup plan, never the enforcement — no physical index exists in any tier." On the host's current release that is no longer true: a declared unique index materializes as one physical index per plugin, keyed on `(plugin_id, collection, <field>)` with no per-collection scoping. Any other collection under the same plugin that also carries a same-named top-level field collides on that same index — `reservation_keys` and `reservation_index` both carry `sku` (one row per reserve attempt, many rows legitimately sharing a sku), so the first reserve for a SKU succeeded and every later one for that SKU threw a raw `SQLITE_CONSTRAINT_UNIQUE`, surfacing to shoppers as "Something went wrong" on Add to cart.
+
+Both collections now declare their natural key as a plain (non-unique) `indexes` entry. The create-if-absent compare-and-set against the sku/email-as-document-id was always the real enforcement; the index was redundant for its own collection and actively unsafe for its neighbors.
diff --git a/packages/store-emdash/src/identity-documents.ts b/packages/store-emdash/src/identity-documents.ts
index b528e76..a588aae 100644
--- a/packages/store-emdash/src/identity-documents.ts
+++ b/packages/store-emdash/src/identity-documents.ts
@@ -86,9 +86,14 @@ export interface IdentityCollectionIndexDeclaration {
  *   cross-cutting rule (b)). Nothing filters on the embedded addresses — every
  *   address method is given its owning customer id, so the document is reached by
  *   id and the address is found inside it.
- * - `customer_emails` declares its own doc id as a unique index, exactly as
- *   `sku_owners` does. It enforces nothing (no physical index exists in any tier);
- *   the claim document and its create-if-absent write are the enforcement.
+ * - `customer_emails` declares its own doc id as a lookup index, exactly as
+ *   `sku_owners` does — see that collection's declaration
+ *   (`product-commerce-documents.ts`) for why it is a plain `indexes` entry,
+ *   never `uniqueIndexes`: the host materializes a unique index as one
+ *   physical, plugin-wide index with no per-collection `WHERE` clause, so a
+ *   `uniqueIndexes` entry here would equally risk colliding with `customers`'
+ *   own declared `emailLower` index above. The claim document and its
+ *   create-if-absent write are the real enforcement.
  * - `sessions` declares `customerId` alone — the only filter the port asks for.
  *   The history's `createdAt DESC, id DESC` ordering is applied **in code** after a
  *   bounded paged read, so no ordering index is declared: a session's `id` is a
@@ -106,7 +111,7 @@ export interface IdentityCollectionIndexDeclaration {
  */
 export const IDENTITY_COLLECTIONS: Readonly<Record<string, IdentityCollectionIndexDeclaration>> = {
 	[CUSTOMERS_COLLECTION]: { indexes: ["emailLower"] },
-	[CUSTOMER_EMAILS_COLLECTION]: { uniqueIndexes: ["emailLower"] },
+	[CUSTOMER_EMAILS_COLLECTION]: { indexes: ["emailLower"] },
 	[SESSIONS_COLLECTION]: { indexes: ["customerId"] },
 	[LOGIN_CHALLENGES_COLLECTION]: { indexes: ["consumed", "expiresAt"] },
 	[LOGIN_CHALLENGE_CLAIMS_COLLECTION]: {},
diff --git a/packages/store-emdash/src/product-commerce-documents.ts b/packages/store-emdash/src/product-commerce-documents.ts
index d069017..37fbef1 100644
--- a/packages/store-emdash/src/product-commerce-documents.ts
+++ b/packages/store-emdash/src/product-commerce-documents.ts
@@ -115,14 +115,25 @@ export interface CollectionIndexDeclaration {
  * `EmdashProductCommerceStore.listProducts`.
  *
  * `sku_owners` declares its natural key, which IS its document id — a lookup
- * plan, never the enforcement (ADR-0019 §4's `uniqueIndexes` rule). The store
- * reaches it by id alone.
+ * plan, never the enforcement. The store reaches it by id alone.
+ *
+ * Declared as a plain (non-unique) `indexes` entry, NOT `uniqueIndexes`: the
+ * host materializes a declared unique index as ONE PHYSICAL SQLITE INDEX per
+ * plugin, keyed on `(plugin_id, collection, <field>)` with no `WHERE`
+ * clause — it is not scoped to this collection alone. Any OTHER collection
+ * under this plugin that also carries a top-level `sku` field (`inventory`'s
+ * `reservation_keys` and `reservation_index` both do, one row per reserve
+ * attempt, many rows legitimately sharing one sku) collides on that same
+ * physical index the moment a second such row exists — a production
+ * incident (2026-09-22), not a hypothetical. The create-if-absent CAS
+ * against the sku-as-id is the real enforcement; the index is a lookup plan
+ * only and must stay non-unique.
  */
 export const PRODUCT_COMMERCE_COLLECTIONS: Readonly<Record<string, CollectionIndexDeclaration>> = {
 	[PRODUCT_COMMERCE_COLLECTION]: {
 		indexes: ["productId", "lifecycle", "publishKey", "productKind", "taxClass", "createdAt"],
 	},
-	[SKU_OWNERS_COLLECTION]: { uniqueIndexes: ["sku"] },
+	[SKU_OWNERS_COLLECTION]: { indexes: ["sku"] },
 };
 
 /**