diff --git a/src/main/java/com/stripe/mpp/methods/tempo/TempoChargeIntent.java b/src/main/java/com/stripe/mpp/methods/tempo/TempoChargeIntent.java
index f5ab16c..3504572 100644
--- a/src/main/java/com/stripe/mpp/methods/tempo/TempoChargeIntent.java
+++ b/src/main/java/com/stripe/mpp/methods/tempo/TempoChargeIntent.java
@@ -29,11 +29,12 @@
  * </ul>
  *
  * <p>A qualifying Transfer of the requested token, recipient and amount is not
- * enough. Unless the merchant set an explicit memo, the matched logs must include
- * a {@code TransferWithMemo} whose memo is bound to this challenge (MPP attribution
- * tag, server fingerprint of the challenge realm, and nonce
- * {@code keccak256(challengeId)[0..6]}). That is what stops a third party from
- * presenting someone else's settled transaction as their own payment.
+ * enough. The matched logs must include a {@code TransferWithMemo} whose memo
+ * is bound to this challenge (MPP attribution tag, server fingerprint of the
+ * challenge realm, and nonce {@code keccak256(challengeId)[0..6]}). A merchant
+ * memo, when present, is an additional exact-match constraint — it does not
+ * skip that binding. That is what stops a third party from presenting someone
+ * else's settled transaction as their own payment.
  *
  * <p>Create the intent once and reuse it so its replay store is shared across requests:
  *
@@ -159,9 +160,7 @@ private Receipt awaitReceipt(
                         "transaction logs contain no Transfer matching the request currency, recipient, and amount"
                     );
                 }
-                if (memoFrom(request) == null) {
-                    assertChallengeBoundMemo(matched, credential);
-                }
+                assertChallengeBoundMemo(matched, credential);
                 return Receipt.success(txHash, "tempo");
             }
             if (i < maxRetries - 1) {
diff --git a/src/main/java/com/stripe/mpp/methods/tempo/TempoMethod.java b/src/main/java/com/stripe/mpp/methods/tempo/TempoMethod.java
index ee0a8f9..3eb71ce 100644
--- a/src/main/java/com/stripe/mpp/methods/tempo/TempoMethod.java
+++ b/src/main/java/com/stripe/mpp/methods/tempo/TempoMethod.java
@@ -104,12 +104,13 @@ public Builder store(Store store) {
         }
 
         /**
-         * Sets an explicit TIP-20 memo that payments must match.
+         * Sets an explicit TIP-20 memo that payments must match in addition to
+         * the challenge-bound MPP attribution memo.
          *
-         * <p>When omitted, clients write an MPP attribution memo bound to the
-         * challenge and the server requires that binding. An explicit memo is
-         * matched exactly and is not challenge-bound — the caller must make it
-         * unique per challenge if hash reuse across challenges should be rejected.
+         * <p>Verification always requires an on-chain memo bound to the
+         * challenge id and realm. A method-wide static value cannot satisfy
+         * that binding across distinct challenges, so omit this unless the
+         * advertised memo is itself unique per challenge.
          */
         public Builder memo(String memo) {
             this.memo = Objects.requireNonNull(memo, "memo");
diff --git a/src/test/java/com/stripe/mpp/methods/tempo/TempoChargeIntentTest.java b/src/test/java/com/stripe/mpp/methods/tempo/TempoChargeIntentTest.java
index c61adf9..0056d70 100644
--- a/src/test/java/com/stripe/mpp/methods/tempo/TempoChargeIntentTest.java
+++ b/src/test/java/com/stripe/mpp/methods/tempo/TempoChargeIntentTest.java
@@ -49,13 +49,17 @@ static Credential txCredential(String rawTx) {
     }
 
     static Credential hashCredential(String txHash) {
-        return hashCredential(txHash, null);
+        return hashCredential(txHash, (String) null);
     }
 
     static Credential hashCredential(String txHash, String source) {
         return new Credential(ECHO, Map.of("type", "hash", "hash", txHash), source);
     }
 
+    static Credential hashCredential(String txHash, ChallengeEcho echo) {
+        return new Credential(echo, Map.of("type", "hash", "hash", txHash), null);
+    }
+
     static String didPkh(int chainId, String address) {
         return "did:pkh:eip155:" + chainId + ":" + address;
     }
@@ -370,12 +374,11 @@ void pushAcceptsChallengeBoundMemoAlongsideAPlainTransfer() {
 
     @Test
     void explicitMemoMustMatchExactly() {
-        String merchantMemo = "0x" + "ab".repeat(32);
         Map<String, Object> request = new HashMap<>(REQUEST);
-        request.put("memo", merchantMemo);
+        request.put("memo", BOUND_MEMO);
 
         Receipt result = intent(new StubRpc(null,
-            receiptWithMemoLog(TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, merchantMemo), 0))
+            receiptWithMemoLog(TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, BOUND_MEMO), 0))
             .verify(hashCredential("0xpushedtx"), request);
         assertThat(result.status()).isEqualTo("success");
     }
@@ -383,31 +386,31 @@ void explicitMemoMustMatchExactly() {
     @Test
     void explicitMemoMismatchIsRejected() {
         String merchantMemo = "0x" + "ab".repeat(32);
-        String otherMemo = "0x" + "cd".repeat(32);
         Map<String, Object> request = new HashMap<>(REQUEST);
         request.put("memo", merchantMemo);
 
         assertThatThrownBy(() -> intent(new StubRpc(null,
-            receiptWithMemoLog(TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, otherMemo), 0))
+            receiptWithMemoLog(TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, BOUND_MEMO), 0))
             .verify(hashCredential("0xpushedtx"), request))
             .isInstanceOf(VerificationFailedException.class)
             .hasMessageContaining("Transfer");
     }
 
     @Test
-    void explicitMemoDoesNotRequireChallengeBinding() {
+    void explicitMemoDoesNotBypassChallengeBinding() {
         String merchantMemo = "0x" + "ab".repeat(32);
         Map<String, Object> request = new HashMap<>(REQUEST);
         request.put("memo", merchantMemo);
 
-        Receipt result = intent(new StubRpc(null,
+        assertThatThrownBy(() -> intent(new StubRpc(null,
             receiptWithMemoLog(TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, merchantMemo), 0))
-            .verify(hashCredential("0xpushedtx"), request);
-        assertThat(result.status()).isEqualTo("success");
+            .verify(hashCredential("0xpushedtx"), request))
+            .isInstanceOf(VerificationFailedException.class)
+            .hasMessageContaining("memo is not bound to this challenge");
     }
 
     @Test
-    void explicitMemoInMethodDetailsIsHonored() {
+    void explicitMemoInMethodDetailsStillRequiresChallengeBinding() {
         String merchantMemo = "0x" + "ab".repeat(32);
         Map<String, Object> request = Map.of(
             "amount", String.valueOf(AMOUNT_ATOMIC),
@@ -416,10 +419,60 @@ void explicitMemoInMethodDetailsIsHonored() {
             "methodDetails", Map.of("chainId", CHAIN_ID, "memo", merchantMemo)
         );
 
-        Receipt result = intent(new StubRpc(null,
+        assertThatThrownBy(() -> intent(new StubRpc(null,
             receiptWithMemoLog(TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, merchantMemo), 0))
-            .verify(hashCredential("0xpushedtx"), request);
-        assertThat(result.status()).isEqualTo("success");
+            .verify(hashCredential("0xpushedtx"), request))
+            .isInstanceOf(VerificationFailedException.class)
+            .hasMessageContaining("memo is not bound to this challenge");
+    }
+
+    @Test
+    void sharedExplicitMemoCannotSatisfyADifferentChallenge() {
+        ChallengeEcho echoA = new ChallengeEcho(
+            "chal-a", "api.example.com", "tempo", "charge", "e30", "2099-01-01T00:00:00Z", null, null
+        );
+        ChallengeEcho echoB = new ChallengeEcho(
+            "chal-b", "api.example.com", "tempo", "charge", "e30", "2099-01-01T00:00:00Z", null, null
+        );
+        String merchantMemo = "0x" + "ab".repeat(32);
+        Map<String, Object> request = new HashMap<>(REQUEST);
+        request.put("memo", merchantMemo);
+        Map<String, Object> paidForB = receiptWithMemoLog(
+            TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, merchantMemo);
+
+        Store store = new MemoryStore();
+        assertThatThrownBy(() -> intent(new StubRpc(null, paidForB, 0), store)
+            .verify(hashCredential("0xvictimtx", echoA), request))
+            .isInstanceOf(VerificationFailedException.class)
+            .hasMessageContaining("memo is not bound to this challenge");
+
+        assertThatThrownBy(() -> intent(new StubRpc(null, paidForB, 0), store)
+            .verify(hashCredential("0xvictimtx", echoB), request))
+            .isInstanceOf(VerificationFailedException.class)
+            .hasMessageContaining("memo is not bound to this challenge");
+    }
+
+    @Test
+    void stolenBoundPaymentCannotSatisfyADifferentChallenge() {
+        ChallengeEcho echoA = new ChallengeEcho(
+            "chal-a", "api.example.com", "tempo", "charge", "e30", "2099-01-01T00:00:00Z", null, null
+        );
+        ChallengeEcho echoB = new ChallengeEcho(
+            "chal-b", "api.example.com", "tempo", "charge", "e30", "2099-01-01T00:00:00Z", null, null
+        );
+        String memoForB = Attribution.encode(echoB.realm(), echoB.id());
+        Store store = new MemoryStore();
+
+        assertThatThrownBy(() -> intent(new StubRpc(null,
+            receiptWithMemoLog(TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, memoForB), 0), store)
+            .verify(hashCredential("0xvictimtx", echoA), REQUEST))
+            .isInstanceOf(VerificationFailedException.class)
+            .hasMessageContaining("memo is not bound to this challenge");
+
+        Receipt result = intent(new StubRpc(null,
+            receiptWithMemoLog(TOKEN_CONTRACT, SENDER, RECIPIENT, AMOUNT_ATOMIC, memoForB), 0), store)
+            .verify(hashCredential("0xvictimtx", echoB), REQUEST);
+        assertThat(result.reference()).isEqualTo("0xvictimtx");
     }
 
     @Test