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 670e92f0..35c26a47 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
@@ -209,6 +209,20 @@ class TssRepository @Inject constructor(
     // Session event callback (set by ViewModel)
     private var sessionEventCallback: ((SessionEventData) -> Unit)? = null
 
+    // Pending sign info cache (to handle session_started race condition)
+    // Stores sign session info before session_started event arrives
+    // Pattern matches server-party-co-managed's PendingSessionCache
+    private data class PendingSignInfo(
+        val sessionId: String,
+        val shareId: Long,
+        val password: String,
+        val messageHash: String
+    )
+    private var pendingSignInfo: PendingSignInfo? = null
+
+    // Track if signing has already been triggered (prevent double execution)
+    private var signingTriggered: Boolean = false
+
     /**
      * Register this party with the router
      * Also subscribes to session events (matching Electron behavior)
@@ -272,7 +286,48 @@ class TssRepository @Inject constructor(
                     when (event.eventType) {
                         "session_started" -> {
                             android.util.Log.d("TssRepository", "  → Processing session_started event")
-                            // Notify callback
+
+                            // CRITICAL: Auto-trigger signing if we have cached pendingSignInfo
+                            // This handles race condition where session_started arrives before
+                            // MainViewModel's result.fold completes (typically 4ms race window)
+                            // Pattern matches server-party-co-managed's auto-participation
+                            val signInfo = pendingSignInfo
+                            if (signInfo != null && signInfo.sessionId == event.sessionId && !signingTriggered) {
+                                android.util.Log.d("TssRepository", "[RACE-FIX] session_started arrived! Auto-triggering signing from TssRepository")
+                                android.util.Log.d("TssRepository", "[RACE-FIX] sessionId=${signInfo.sessionId}, shareId=${signInfo.shareId}")
+
+                                signingTriggered = true  // Mark as triggered BEFORE launching coroutine
+
+                                // Launch signing in background (don't block event handler)
+                                repositoryScope.launch {
+                                    android.util.Log.d("TssRepository", "[RACE-FIX] Calling startSigning from TssRepository...")
+                                    val startResult = startSigning(signInfo.sessionId, signInfo.shareId, signInfo.password)
+
+                                    if (startResult.isSuccess) {
+                                        android.util.Log.d("TssRepository", "[RACE-FIX] startSigning succeeded, calling waitForSignature...")
+                                        val signResult = waitForSignature()
+                                        android.util.Log.d("TssRepository", "[RACE-FIX] waitForSignature completed: isSuccess=${signResult.isSuccess}")
+
+                                        // Note: Signature will be available via repository._signature flow
+                                        // MainViewModel will pick it up automatically
+                                    } else {
+                                        android.util.Log.e("TssRepository", "[RACE-FIX] startSigning FAILED: ${startResult.exceptionOrNull()?.message}")
+                                    }
+
+                                    // Clean up after signing (whether success or failure)
+                                    pendingSignInfo = null
+                                }
+                            } else {
+                                if (signInfo == null) {
+                                    android.util.Log.d("TssRepository", "[RACE-FIX] No pendingSignInfo cached, skipping auto-trigger")
+                                } else if (signInfo.sessionId != event.sessionId) {
+                                    android.util.Log.d("TssRepository", "[RACE-FIX] sessionId mismatch (cached=${signInfo.sessionId}, event=${event.sessionId})")
+                                } else if (signingTriggered) {
+                                    android.util.Log.d("TssRepository", "[RACE-FIX] Signing already triggered, skipping duplicate")
+                                }
+                            }
+
+                            // Notify callback (MainViewModel may also try to trigger, but防重入检查 will prevent duplicate)
                             sessionEventCallback?.invoke(event)
                         }
                         "party_joined", "participant_joined" -> {
@@ -339,6 +394,12 @@ class TssRepository @Inject constructor(
         sessionEventCallback = callback
     }
 
+    /**
+     * Check if signing has already been triggered (防重入检查)
+     * Used by MainViewModel to prevent double execution
+     */
+    fun isSigningTriggered(): Boolean = signingTriggered
+
     /**
      * Set keygen timeout callback (called by ViewModel)
      * This callback is invoked when the 5-minute polling timeout is reached
@@ -3933,6 +3994,19 @@ data class ParticipantStatusInfo(
                         startMessageRouting(sessionId, myPartyIndex, signingPartyId)
                         ensureSessionEventSubscriptionActive(signingPartyId)
 
+                        // CRITICAL: Cache sign info BEFORE session_started arrives
+                        // This prevents race condition where session_started arrives before
+                        // MainViewModel's result.fold callback completes
+                        // Pattern matches server-party-co-managed's PendingSessionCache
+                        pendingSignInfo = PendingSignInfo(
+                            sessionId = sessionId,
+                            shareId = shareId,
+                            password = password,
+                            messageHash = messageHash
+                        )
+                        signingTriggered = false  // Reset flag for new signing session
+                        android.util.Log.d("TssRepository", "[CO-SIGN-OPTIONS] Cached pendingSignInfo for sessionId=$sessionId to handle session_started event")
+
                         if (joinData.sessionStatus == "in_progress") {
                             android.util.Log.d("TssRepository", "[CO-SIGN-OPTIONS] Session already in_progress, will trigger sign immediately")
                             sessionAlreadyInProgress = true
diff --git a/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt b/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
index 50d43b27..cdd7fbf3 100644
--- a/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
+++ b/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
@@ -1470,6 +1470,10 @@ class MainViewModel @Inject constructor(
     /**
      * Start sign as initiator (called when session_started event is received)
      * Matches Electron's handleCoSignStart for initiator
+     *
+     * CRITICAL: This method includes防重入检查 to prevent double execution
+     * Race condition fix: TssRepository may have already triggered signing via
+     * its session_started handler. This callback serves as a fallback.
      */
     private fun startSignAsInitiator(selectedParties: List<String>) {
         val info = pendingSignInitiatorInfo
@@ -1478,6 +1482,13 @@ class MainViewModel @Inject constructor(
             return
         }
 
+        // CRITICAL: Prevent double execution if TssRepository already started signing
+        // TssRepository sets signingTriggered=true when it auto-triggers from session_started
+        if (repository.isSigningTriggered()) {
+            android.util.Log.d("MainViewModel", "[RACE-FIX] Signing already triggered by TssRepository, skipping duplicate from MainViewModel")
+            return
+        }
+
         android.util.Log.d("MainViewModel", "Starting sign as initiator: sessionId=${info.sessionId}, selectedParties=$selectedParties")
         startSigningProcess(info.sessionId, info.shareId, info.password)
     }