reality: Add support_x25519mlkem768 option

common/tls/reality_client.go

@@ -45,10 +45,11 @@ import (
 var _ ConfigCompat = (*RealityClientConfig)(nil)
 
 type RealityClientConfig struct {
-	ctx       context.Context
-	uClient   *UTLSClientConfig
-	publicKey []byte
-	shortID   [8]byte
+	ctx                   context.Context
+	uClient               *UTLSClientConfig
+	publicKey             []byte
+	shortID               [8]byte
+	supportX25519MLKEM768 bool
 }
 
 func NewRealityClient(ctx context.Context, logger logger.ContextLogger, serverAddress string, options option.OutboundTLSOptions) (Config, error) {
@@ -84,7 +85,7 @@ func newRealityClient(ctx context.Context, logger logger.ContextLogger, serverAd
 		return nil, E.New("invalid short_id")
 	}
 
-	var config Config = &RealityClientConfig{ctx, uClient.(*UTLSClientConfig), publicKey, shortID}
+	var config Config = &RealityClientConfig{ctx, uClient.(*UTLSClientConfig), publicKey, shortID, options.Reality.SupportX25519MLKEM768}
 	if options.KernelRx || options.KernelTx {
 		if !C.IsLinux {
 			return nil, E.New("kTLS is only supported on Linux")
@@ -145,22 +146,24 @@ func (e *RealityClientConfig) ClientHandshake(ctx context.Context, conn net.Conn
 	if err != nil {
 		return nil, err
 	}
-	for _, extension := range uConn.Extensions {
-		if ce, ok := extension.(*utls.SupportedCurvesExtension); ok {
-			ce.Curves = common.Filter(ce.Curves, func(curveID utls.CurveID) bool {
-				return curveID != utls.X25519MLKEM768
-			})
+	if !e.supportX25519MLKEM768 {
+		for _, extension := range uConn.Extensions {
+			if ce, ok := extension.(*utls.SupportedCurvesExtension); ok {
+				ce.Curves = common.Filter(ce.Curves, func(curveID utls.CurveID) bool {
+					return curveID != utls.X25519MLKEM768
+				})
+			}
+			if ks, ok := extension.(*utls.KeyShareExtension); ok {
+				ks.KeyShares = common.Filter(ks.KeyShares, func(share utls.KeyShare) bool {
+					return share.Group != utls.X25519MLKEM768
+				})
+			}
 		}
-		if ks, ok := extension.(*utls.KeyShareExtension); ok {
-			ks.KeyShares = common.Filter(ks.KeyShares, func(share utls.KeyShare) bool {
-				return share.Group != utls.X25519MLKEM768
-			})
+		err = uConn.BuildHandshakeState()
+		if err != nil {
+			return nil, err
 		}
 	}
-	err = uConn.BuildHandshakeState()
-	if err != nil {
-		return nil, err
-	}
 
 	if len(uConfig.NextProtos) > 0 {
 		for _, extension := range uConn.Extensions {
@@ -271,6 +274,7 @@ func (e *RealityClientConfig) Clone() Config {
 		e.uClient.Clone().(*UTLSClientConfig),
 		e.publicKey,
 		e.shortID,
+		e.supportX25519MLKEM768,
 	}
 }
 

docs/configuration/shared/tls.md

@@ -152,7 +152,8 @@ icon: material/new-box
   "reality": {
     "enabled": false,
     "public_key": "jNXHt1yRo0vDuchQlIP6Z0ZvjT3KtzVI-T4E7RoLJS0",
-    "short_id": "0123456789abcdef"
+    "short_id": "0123456789abcdef",
+    "support_x25519mlkem768": false
   }
 }
 ```
@@ -825,6 +826,16 @@ Public key, generated by `sing-box generate reality-keypair`.
 
 A hexadecimal string with zero to eight digits.
 
+#### support_x25519mlkem768
+
+==Client only==
+
+Keep the `X25519MLKEM768` post-quantum hybrid key share in the ClientHello instead of stripping it.
+
+Required by REALITY servers running `xtls/reality` built after 2026-09-08 (Xray-core >= v26.9.8), which reject
+ClientHellos that don't offer it before any plain `X25519` share. Older REALITY servers may fail the handshake
+if this key share is present, so it is disabled by default.
+
 #### max_time_difference
 
 ==Server only==

option/tls.go

@@ -250,7 +250,8 @@ type OutboundUTLSOptions struct {
 }
 
 type OutboundRealityOptions struct {
-	Enabled   bool   `json:"enabled,omitempty"`
-	PublicKey string `json:"public_key,omitempty"`
-	ShortID   string `json:"short_id,omitempty"`
+	Enabled               bool   `json:"enabled,omitempty"`
+	PublicKey             string `json:"public_key,omitempty"`
+	ShortID               string `json:"short_id,omitempty"`
+	SupportX25519MLKEM768 bool   `json:"support_x25519mlkem768,omitempty"`
 }