diff --git a/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/data/repository/TssRepository.kt b/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/data/repository/TssRepository.kt
index 46fe98a0..8115b911 100644
--- a/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/data/repository/TssRepository.kt
+++ b/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/data/repository/TssRepository.kt
@@ -1216,6 +1216,13 @@ class TssRepository @Inject constructor(
             android.util.Log.d("TssRepository", "Starting TSS sign with ${allParticipants.size} participants, thresholdT=$thresholdT")
 
             // Start TSS sign
+            // Remove 0x prefix from messageHash - TSS library expects pure hex
+            val cleanMessageHash = if (messageHash.startsWith("0x") || messageHash.startsWith("0X")) {
+                messageHash.substring(2)
+            } else {
+                messageHash
+            }
+            android.util.Log.d("TssRepository", "Starting TSS sign with cleanMessageHash=${cleanMessageHash.take(20)}...")
             val startResult = tssNativeBridge.startSign(
                 sessionId = sessionId,
                 partyId = partyId,
@@ -1223,7 +1230,7 @@ class TssRepository @Inject constructor(
                 thresholdT = thresholdT,
                 thresholdN = shareEntity.thresholdN,  // Use original N from keygen
                 participants = allParticipants,
-                messageHash = messageHash,
+                messageHash = cleanMessageHash,
                 shareData = shareEntity.encryptedShare,
                 password = password
             )
@@ -2158,7 +2165,14 @@ class TssRepository @Inject constructor(
                 }
 
                 // Start TSS sign
-                android.util.Log.d("TssRepository", "[CO-SIGN] Calling tssNativeBridge.startSign...")
+                // Remove 0x prefix from messageHash - TSS library expects pure hex
+                val rawMessageHash = session.messageHash ?: ""
+                val cleanMessageHash = if (rawMessageHash.startsWith("0x") || rawMessageHash.startsWith("0X")) {
+                    rawMessageHash.substring(2)
+                } else {
+                    rawMessageHash
+                }
+                android.util.Log.d("TssRepository", "[CO-SIGN] Calling tssNativeBridge.startSign with cleanMessageHash=${cleanMessageHash.take(20)}...")
                 val startResult = tssNativeBridge.startSign(
                     sessionId = sessionId,
                     partyId = partyId,
@@ -2166,7 +2180,7 @@ class TssRepository @Inject constructor(
                     thresholdT = session.thresholdT,
                     thresholdN = shareEntity.thresholdN,
                     participants = session.participants,
-                    messageHash = session.messageHash ?: "",
+                    messageHash = cleanMessageHash,
                     shareData = shareEntity.encryptedShare,
                     password = password
                 )