diff --git a/backend/mpc-system/pkg/tss/keygen.go b/backend/mpc-system/pkg/tss/keygen.go
index cfdc3519..2f6643b7 100644
--- a/backend/mpc-system/pkg/tss/keygen.go
+++ b/backend/mpc-system/pkg/tss/keygen.go
@@ -117,8 +117,11 @@ func NewKeygenSession(
 	sortedPartyIDs := tss.SortPartyIDs(tssPartyIDs)
 
 	// Create peer context and parameters
+	// IMPORTANT: TSS-lib threshold convention: threshold=t means (t+1) signers required
+	// User says "2-of-3" meaning 2 signers needed, so we pass (Threshold-1) to TSS-lib
 	peerCtx := tss.NewPeerContext(sortedPartyIDs)
-	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), config.Threshold)
+	tssThreshold := config.Threshold - 1
+	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), tssThreshold)
 
 	return &KeygenSession{
 		config:      config,
diff --git a/backend/mpc-system/pkg/tss/signing.go b/backend/mpc-system/pkg/tss/signing.go
index f9d01e40..ae319af7 100644
--- a/backend/mpc-system/pkg/tss/signing.go
+++ b/backend/mpc-system/pkg/tss/signing.go
@@ -132,14 +132,15 @@ func NewSigningSession(
 		keygenIndexToSortedIndex, selfParty.PartyID)
 
 	// Create peer context and parameters
-	// CRITICAL: threshold must match keygen exactly!
-	// Use len(sortedPartyIDs) as partyCount - this is the number of CURRENT signers
+	// IMPORTANT: TSS-lib threshold convention: threshold=t means (t+1) signers required
+	// This MUST match keygen exactly! Both use (Threshold-1)
 	// The BuildLocalSaveDataSubset call in Start() will filter the save data to match
 	peerCtx := tss.NewPeerContext(sortedPartyIDs)
-	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), config.Threshold)
+	tssThreshold := config.Threshold - 1
+	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), tssThreshold)
 
-	fmt.Printf("[TSS-SIGN] NewParameters: partyCount=%d, threshold=%d party_id=%s\n",
-		len(sortedPartyIDs), config.Threshold, selfParty.PartyID)
+	fmt.Printf("[TSS-SIGN] NewParameters: partyCount=%d, tssThreshold=%d (from config.Threshold=%d) party_id=%s\n",
+		len(sortedPartyIDs), tssThreshold, config.Threshold, selfParty.PartyID)
 
 	// Convert message hash to big.Int
 	msgHash := new(big.Int).SetBytes(messageHash)
diff --git a/backend/mpc-system/services/service-party-app/tss-party/main.go b/backend/mpc-system/services/service-party-app/tss-party/main.go
index 5a2e3a57..5f8fb8bd 100644
--- a/backend/mpc-system/services/service-party-app/tss-party/main.go
+++ b/backend/mpc-system/services/service-party-app/tss-party/main.go
@@ -241,13 +241,14 @@ func executeKeygen(
 	sortedPartyIDs := tss.SortPartyIDs(tssPartyIDs)
 
 	// Create peer context and parameters
-	// NOTE: We use thresholdT directly (not thresholdT-1) to maintain backward compatibility
-	// with existing wallets. In our system, thresholdT=2 means "need 2 signers" but TSS-lib
-	// interprets threshold=2 as "need 3 signers". This is intentional to match existing keygen data.
+	// IMPORTANT: TSS-lib threshold convention: threshold=t means (t+1) signers required
+	// User says "2-of-3" meaning 2 signers needed, so we pass (thresholdT-1) to TSS-lib
+	// For 2-of-3: thresholdT=2, tss-lib threshold=1, signers_needed=1+1=2 ✓
 	peerCtx := tss.NewPeerContext(sortedPartyIDs)
-	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), thresholdT)
-	fmt.Fprintf(os.Stderr, "[TSS-KEYGEN] NewParameters: partyCount=%d, threshold=%d\n",
-		len(sortedPartyIDs), thresholdT)
+	tssThreshold := thresholdT - 1
+	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), tssThreshold)
+	fmt.Fprintf(os.Stderr, "[TSS-KEYGEN] NewParameters: partyCount=%d, tssThreshold=%d (from thresholdT=%d, means %d signers needed)\n",
+		len(sortedPartyIDs), tssThreshold, thresholdT, thresholdT)
 
 	// Create channels
 	outCh := make(chan tss.Message, thresholdN*10)
@@ -600,13 +601,15 @@ func executeSign(
 	sortedPartyIDs := tss.SortPartyIDs(tssPartyIDs)
 
 	// Create peer context and parameters
-	// CRITICAL: threshold must match keygen exactly!
-	// Keygen uses: NewParameters(..., len(sortedPartyIDs), thresholdT)
-	// Sign must use the same threshold value for Lagrange coefficients to work correctly.
+	// IMPORTANT: TSS-lib threshold convention: threshold=t means (t+1) signers required
+	// User says "2-of-3" meaning 2 signers needed, so we pass (thresholdT-1) to TSS-lib
+	// This MUST match keygen exactly!
+	// For 2-of-3: thresholdT=2, tss-lib threshold=1, signers_needed=1+1=2 ✓
 	peerCtx := tss.NewPeerContext(sortedPartyIDs)
-	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), thresholdT)
-	fmt.Fprintf(os.Stderr, "[TSS-SIGN] NewParameters: partyCount=%d, threshold=%d (must match keygen)\n",
-		len(sortedPartyIDs), thresholdT)
+	tssThreshold := thresholdT - 1
+	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), tssThreshold)
+	fmt.Fprintf(os.Stderr, "[TSS-SIGN] NewParameters: partyCount=%d, tssThreshold=%d (from thresholdT=%d, means %d signers needed)\n",
+		len(sortedPartyIDs), tssThreshold, thresholdT, thresholdT)
 
 	// CRITICAL: Build a subset of the keygen save data for the current signing parties
 	// This is required when signing with a subset of the original keygen participants.
diff --git a/backend/mpc-system/services/tss-wasm/main.go b/backend/mpc-system/services/tss-wasm/main.go
index 76919b3e..aeec102f 100644
--- a/backend/mpc-system/services/tss-wasm/main.go
+++ b/backend/mpc-system/services/tss-wasm/main.go
@@ -160,8 +160,11 @@ func startKeygen(this js.Value, args []js.Value) interface{} {
 	sortedPartyIDs := tss.SortPartyIDs(tssPartyIDs)
 
 	// Create peer context and parameters
+	// IMPORTANT: TSS-lib threshold convention: threshold=t means (t+1) signers required
+	// User says "2-of-3" meaning 2 signers needed, so we pass (thresholdT-1) to TSS-lib
 	peerCtx := tss.NewPeerContext(sortedPartyIDs)
-	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), thresholdT)
+	tssThreshold := thresholdT - 1
+	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), tssThreshold)
 
 	// Build party index map
 	session.PartyIndexMap = make(map[int]*tss.PartyID)
@@ -281,9 +284,11 @@ func startSigning(this js.Value, args []js.Value) interface{} {
 	sortedPartyIDs := tss.SortPartyIDs(tssPartyIDs)
 
 	// Create peer context and parameters
-	// CRITICAL: threshold must match keygen exactly!
+	// IMPORTANT: TSS-lib threshold convention: threshold=t means (t+1) signers required
+	// This MUST match keygen exactly! Both use (thresholdT-1)
 	peerCtx := tss.NewPeerContext(sortedPartyIDs)
-	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), thresholdT)
+	tssThreshold := thresholdT - 1
+	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), tssThreshold)
 
 	// Build party index map
 	session.PartyIndexMap = make(map[int]*tss.PartyID)