diff --git a/src/core/enums.ts b/src/core/enums.ts
index a428d16..e64daca 100644
--- a/src/core/enums.ts
+++ b/src/core/enums.ts
@@ -162,7 +162,9 @@ export const CanonicalDomainBoundary = {
     ] as const,
   },
   transportBoundaries: {
-    sharedDomain: ["state", "category", "terminal-reason", "paymentId ownership"] as const,
+    // paymentIdentifier is the shared idempotency input across both transports:
+    // HTTP uses payment-identifier extension; RPC uses paymentIdentifier field.
+    sharedDomain: ["state", "category", "terminal-reason", "paymentId ownership", "paymentIdentifier idempotency"] as const,
     rpc: ["service-binding request/response shapes", "internal relay error codes"] as const,
     http: ["x402 request/response shapes", "polling and error envelopes"] as const,
   },
diff --git a/src/core/primitives.ts b/src/core/primitives.ts
index 87a6b3c..9f1f75a 100644
--- a/src/core/primitives.ts
+++ b/src/core/primitives.ts
@@ -35,3 +35,13 @@ export const NonNegativeIntegerSchema = z
   .nonnegative();
 
 export const UrlSchema = z.string().url();
+
+// Caller-controlled idempotency key for x402 V2 payment-identifier extension.
+// Charset and length match the V2 spec: [a-zA-Z0-9_-]{16,128}.
+// Distinct from PaymentIdSchema (relay-assigned, pay_ prefix); this is caller-provided.
+export const PaymentIdentifierSchema = z
+  .string()
+  .regex(
+    /^[a-zA-Z0-9_-]{16,128}$/,
+    "Expected a caller-provided payment identifier: [a-zA-Z0-9_-]{16,128}",
+  );
diff --git a/src/http/schemas.ts b/src/http/schemas.ts
index da370a1..21deb19 100644
--- a/src/http/schemas.ts
+++ b/src/http/schemas.ts
@@ -5,6 +5,7 @@ import {
   AmountStringSchema,
   IsoDateTimeSchema,
   NonNegativeIntegerSchema,
+  PaymentIdentifierSchema,
   PaymentIdSchema,
   PositiveIntegerSchema,
   StacksAddressSchema,
@@ -45,9 +46,11 @@ export const HTTP_VERIFY_INVALID_REASONS = [
 
 export const HttpVerifyInvalidReasonSchema = z.enum(HTTP_VERIFY_INVALID_REASONS);
 
+// Uses PaymentIdentifierSchema rather than PaymentIdSchema: extension ids are caller-provided,
+// not relay-assigned, so the pay_ prefix requirement was inappropriate.
 export const HttpPaymentIdentifierExtensionSchema = z.object({
   info: z.object({
-    id: PaymentIdSchema,
+    id: PaymentIdentifierSchema,
   }),
 });
 
diff --git a/src/rpc/schemas.ts b/src/rpc/schemas.ts
index 58b3074..d14cf69 100644
--- a/src/rpc/schemas.ts
+++ b/src/rpc/schemas.ts
@@ -5,6 +5,7 @@ import {
   AmountStringSchema,
   IsoDateTimeSchema,
   NonNegativeIntegerSchema,
+  PaymentIdentifierSchema,
   PaymentIdSchema,
   PositiveIntegerSchema,
   StacksAddressSchema,
@@ -37,6 +38,7 @@ export const RPC_ERROR_CODES = [
   "BROADCAST_RATE_LIMITED",
   "SENDER_HAND_EXPIRED",
   "NONCE_OCCUPIED",
+  "PAYMENT_IDENTIFIER_CONFLICT",
 ] as const;
 
 export const RpcErrorCodeSchema = z.enum(RPC_ERROR_CODES);
@@ -73,6 +75,8 @@ export const RpcSubmitPaymentWarningSchema = z.object({
 export const RpcSubmitPaymentRequestSchema = z.object({
   txHex: TransactionHexSchema,
   settle: RpcSettleOptionsSchema.optional(),
+  // Caller-controlled idempotency key; see PaymentIdentifierSchema for charset/length constraints.
+  paymentIdentifier: PaymentIdentifierSchema.optional(),
 });
 
 export const RpcSubmitPaymentAcceptedSchema = z.object({
diff --git a/tests/rpc.test.ts b/tests/rpc.test.ts
index 9bb1a84..854c95d 100644
--- a/tests/rpc.test.ts
+++ b/tests/rpc.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 import {
   RpcCheckPaymentResultSchema,
+  RpcSubmitPaymentRequestSchema,
   RpcSubmitPaymentResultSchema,
 } from "../src/index.js";
 
@@ -140,4 +141,60 @@ describe("rpc schemas", () => {
       "https://example.com/payment/pay_01JMVP9QE8XA3BDGM5RN7KWTZ4",
     );
   });
+
+  describe("paymentIdentifier — x402 V2 idempotency parity", () => {
+    const STUB_TX_HEX = "0x" + "ab".repeat(32);
+
+    it("accepts a submit request with an optional paymentIdentifier", () => {
+      const req = RpcSubmitPaymentRequestSchema.parse({
+        txHex: STUB_TX_HEX,
+        paymentIdentifier: "pay_01JMVP9QE8XA3BDGM5",
+      });
+      expect(req.paymentIdentifier).toBe("pay_01JMVP9QE8XA3BDGM5");
+    });
+
+    it("accepts a submit request without paymentIdentifier (backward compat)", () => {
+      const req = RpcSubmitPaymentRequestSchema.parse({ txHex: STUB_TX_HEX });
+      expect(req.paymentIdentifier).toBeUndefined();
+    });
+
+    it("rejects a paymentIdentifier shorter than 16 chars", () => {
+      const result = RpcSubmitPaymentRequestSchema.safeParse({
+        txHex: STUB_TX_HEX,
+        paymentIdentifier: "short",
+      });
+      expect(result.success).toBe(false);
+    });
+
+    it("rejects a paymentIdentifier longer than 128 chars", () => {
+      const result = RpcSubmitPaymentRequestSchema.safeParse({
+        txHex: STUB_TX_HEX,
+        paymentIdentifier: "a".repeat(129),
+      });
+      expect(result.success).toBe(false);
+    });
+
+    it("rejects a paymentIdentifier with disallowed characters", () => {
+      const result = RpcSubmitPaymentRequestSchema.safeParse({
+        txHex: STUB_TX_HEX,
+        paymentIdentifier: "invalid identifier!",
+      });
+      expect(result.success).toBe(false);
+    });
+
+    it("accepts PAYMENT_IDENTIFIER_CONFLICT as a rejected submit error code", () => {
+      const result = RpcSubmitPaymentResultSchema.parse({
+        accepted: false,
+        error: "Same paymentIdentifier submitted with a different transaction",
+        code: "PAYMENT_IDENTIFIER_CONFLICT",
+        retryable: false,
+      });
+
+      expect(result.accepted).toBe(false);
+      if (result.accepted) {
+        throw new Error("Expected a rejected RPC submit result");
+      }
+      expect(result.code).toBe("PAYMENT_IDENTIFIER_CONFLICT");
+    });
+  });
 });