diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0c3079bdbf8069e9e4f333c01f07acf5eaf5955a..ee4149fabd4c4e62d4c3922d648e9b768abaad4d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,10 +4,18 @@ All notable changes to this project are documented below.
 The format is based on [keep a changelog](http://keepachangelog.com) and this project uses [semantic versioning](http://semver.org).
 
 ## [Unreleased]
+### Added
+- Handle thrown JS runtime custom exceptions containing a message and a grpc code to be returned in the server response.
+
 ### Changed
 - Size limit for status messages increased from 128 to 2048 characters.
 - Improve unfiltered group listings responses.
 - Improve error when attempting to create a group with the system user.
+- Add userId field for permission validation in JS/Lua runtimes group update functions.
+
+### Fixed
+- Fix creator id being read from the wrong argument in JS runtime group update function.
+- Fix max count being incorrectly validated in group update JS runtime function.
 
 ## [3.4.0] - 2021-07-08
 ### Added
diff --git a/server/runtime_javascript.go b/server/runtime_javascript.go
index ab38346a9b83b72ef7af8bc4d6a79f73c23719ec..966724fdfb7a85e434b02797c0040f4f42d17b62 100644
--- a/server/runtime_javascript.go
+++ b/server/runtime_javascript.go
@@ -72,26 +72,10 @@ func (r *RuntimeJS) GetCallback(e RuntimeExecutionMode, key string) string {
 	return ""
 }
 
-type JsErrorType int
-
-func (e JsErrorType) String() string {
-	switch e {
-	case JsErrorException:
-		return "exception"
-	default:
-		return ""
-	}
-}
-
-const (
-	JsErrorException JsErrorType = iota
-	JsErrorRuntime
-)
-
 type jsError struct {
-	StackTrace string
-	Type       string
-	Message    string `json:",omitempty"`
+	StackTrace string `json:"stackTrace,omitempty"`
+	Type       string `json:"type,omitempty"`
+	custom     bool
 	error      error
 }
 
@@ -99,19 +83,22 @@ func (e *jsError) Error() string {
 	return e.error.Error()
 }
 
-func newJsExceptionError(t JsErrorType, error, st string) *jsError {
-	return &jsError{
-		StackTrace: st,
-		Type:       t.String(),
-		error:      errors.New(error),
+func newJsUncaughtExceptionError(error error, st string, custom bool) *jsError {
+	jsErr := &jsError{
+		Type:   "uncaughtExceptionError",
+		error:  error,
+		custom: custom,
 	}
+	if !custom {
+		jsErr.StackTrace = st
+	}
+	return jsErr
 }
 
-func newJsError(t JsErrorType, err error) *jsError {
+func newJsRuntimeError(err error) *jsError {
 	return &jsError{
-		Message: err.Error(),
-		Type:    t.String(),
-		error:   err,
+		Type:  "runtimeError",
+		error: err,
 	}
 }
 
@@ -240,7 +227,11 @@ func (rp *RuntimeProviderJS) BeforeRt(ctx context.Context, id string, logger *za
 	rp.Put(r)
 
 	if fnErr != nil {
-		logger.Error("Runtime Before function caused an error.", zap.String("id", id), zap.Error(fnErr))
+		if jsErr, ok := fnErr.(*jsError); ok {
+			if !jsErr.custom {
+				logger.Error("Runtime Before function caused an error.", zap.String("id", id), zap.Error(fnErr))
+			}
+		}
 		return nil, fnErr
 	}
 
@@ -301,7 +292,11 @@ func (rp *RuntimeProviderJS) AfterRt(ctx context.Context, id string, logger *zap
 	rp.Put(r)
 
 	if fnErr != nil {
-		logger.Error("Runtime After function caused an error.", zap.String("id", id), zap.Error(fnErr))
+		if jsErr, ok := fnErr.(*jsError); ok {
+			if !jsErr.custom {
+				logger.Error("Runtime After function caused an error.", zap.String("id", id), zap.Error(fnErr))
+			}
+		}
 		return fnErr
 	}
 
@@ -358,7 +353,11 @@ func (rp *RuntimeProviderJS) BeforeReq(ctx context.Context, id string, logger *z
 	rp.Put(r)
 
 	if fnErr != nil {
-		logger.Error("Runtime Before function caused an error.", zap.String("id", id), zap.Error(err))
+		if jsErr, ok := fnErr.(*jsError); ok {
+			if !jsErr.custom {
+				logger.Error("Runtime Before function caused an error.", zap.String("id", id), zap.Error(err))
+			}
+		}
 		return nil, fnErr, code
 	}
 
@@ -453,7 +452,11 @@ func (rp *RuntimeProviderJS) AfterReq(ctx context.Context, id string, logger *za
 	rp.Put(r)
 
 	if fnErr != nil {
-		logger.Error("JavaScript runtime After function caused an error.", zap.String("id", id), zap.Error(fnErr))
+		if jsErr, ok := fnErr.(*jsError); ok {
+			if !jsErr.custom {
+				logger.Error("JavaScript runtime After function caused an error.", zap.String("id", id), zap.Error(fnErr))
+			}
+		}
 		return fnErr
 	}
 
@@ -487,11 +490,32 @@ func (r *RuntimeJS) invokeFunction(execMode RuntimeExecutionMode, id string, fn
 	retVal, err := fn(goja.Null(), args...)
 	if err != nil {
 		if exErr, ok := err.(*goja.Exception); ok {
-			r.logger.Error("JavaScript runtime function raised an uncaught exception", zap.String("mode", execMode.String()), zap.String("id", id), zap.Error(err))
-			return nil, newJsExceptionError(JsErrorException, exErr.Error(), exErr.String()), codes.Internal
+			errMsg := exErr.Error()
+			errCode := codes.Internal
+			custom := false
+			if errMap, ok := exErr.Value().Export().(map[string]interface{}); ok {
+				// Custom exception with message and code
+				if msg, ok := errMap["message"]; ok {
+					if msgStr, ok := msg.(string); ok {
+						errMsg = msgStr
+						custom = true
+					}
+				}
+				if code, ok := errMap["code"]; ok {
+					if codeInt, ok := code.(int64); ok {
+						errCode = codes.Code(codeInt)
+						custom = true
+					}
+				}
+			}
+
+			if !custom {
+				r.logger.Error("JavaScript runtime function raised an uncaught exception", zap.String("mode", execMode.String()), zap.String("id", id), zap.Error(err))
+			}
+			return nil, newJsUncaughtExceptionError(errors.New(errMsg), exErr.String(), custom), errCode
 		}
 		r.logger.Error("JavaScript runtime function caused an error", zap.String("mode", execMode.String()), zap.String("id", id), zap.Error(err))
-		return nil, newJsError(JsErrorRuntime, err), codes.Internal
+		return nil, newJsRuntimeError(err), codes.Internal
 	}
 	if retVal == nil || retVal == goja.Undefined() || retVal == goja.Null() {
 		return nil, nil, codes.OK
@@ -1613,6 +1637,9 @@ func (rp *RuntimeProviderJS) MatchmakerMatched(ctx context.Context, entries []*M
 
 	retValue, err, _ := r.InvokeFunction(RuntimeExecutionModeMatchmaker, "matchmakerMatched", fn, jsLogger, nil, "", "", nil, 0, "", "", "", "", r.vm.ToValue(entriesSlice))
 	rp.Put(r)
+	if err != nil {
+		return "", false, fmt.Errorf("Error running runtime Matchmaker Matched hook: %v", err.Error())
+	}
 
 	if retValue == nil {
 		// No return value or hook decided not to return an authoritative match ID.
diff --git a/server/runtime_javascript_context.go b/server/runtime_javascript_context.go
index 0003eae50503e3fdc5e06bba252d1aba2f007487..2fd95032ccbb8ce2ab87d282b092d81ea07fcb32 100644
--- a/server/runtime_javascript_context.go
+++ b/server/runtime_javascript_context.go
@@ -43,7 +43,7 @@ func NewRuntimeJsContext(r *goja.Runtime, node string, env goja.Value, mode Runt
 	ctxObj := r.NewObject()
 	ctxObj.Set(__RUNTIME_JAVASCRIPT_CTX_NODE, node)
 	ctxObj.Set(__RUNTIME_JAVASCRIPT_CTX_ENV, env)
-	ctxObj.Set(__RUNTIME_JAVASCRIPT_CTX_MODE, mode)
+	ctxObj.Set(__RUNTIME_JAVASCRIPT_CTX_MODE, mode.String())
 	if queryParams != nil {
 		ctxObj.Set(__RUNTIME_JAVASCRIPT_CTX_QUERY_PARAMS, queryParams)
 	}
diff --git a/server/runtime_javascript_nakama.go b/server/runtime_javascript_nakama.go
index cac26a5b59c097a6d71400a20a3d0b91ce94124c..ef323d13a9eb4b758c79ceb9693aa64d94d92d91 100644
--- a/server/runtime_javascript_nakama.go
+++ b/server/runtime_javascript_nakama.go
@@ -5172,8 +5172,8 @@ func (n *runtimeJavascriptNakamaModule) groupCreate(r *goja.Runtime) func(goja.F
 		}
 
 		creatorIDString := uuid.Nil.String()
-		if f.Argument(3) != goja.Undefined() && f.Argument(3) != goja.Null() {
-			creatorIDString = getJsString(r, f.Argument(3))
+		if f.Argument(2) != goja.Undefined() && f.Argument(2) != goja.Null() {
+			creatorIDString = getJsString(r, f.Argument(2))
 		}
 		creatorID, err := uuid.FromString(creatorIDString)
 		if err != nil {
@@ -5259,15 +5259,24 @@ func (n *runtimeJavascriptNakamaModule) groupUpdate(r *goja.Runtime) func(goja.F
 			panic(r.NewTypeError("expects group ID to be a valid identifier"))
 		}
 
-		var name *wrapperspb.StringValue
+		userId := uuid.Nil
 		if f.Argument(1) != goja.Undefined() && f.Argument(1) != goja.Null() {
-			nameStr := getJsString(r, f.Argument(1))
+			userIdStr := getJsString(r, f.Argument(1))
+			userId, err = uuid.FromString(userIdStr)
+			if err != nil {
+				panic(r.NewTypeError("expects user ID to be a valid identifier"))
+			}
+		}
+
+		var name *wrapperspb.StringValue
+		if f.Argument(2) != goja.Undefined() && f.Argument(2) != goja.Null() {
+			nameStr := getJsString(r, f.Argument(2))
 			name = &wrapperspb.StringValue{Value: nameStr}
 		}
 
 		creatorID := uuid.Nil
-		if f.Argument(2) != goja.Undefined() && f.Argument(2) != goja.Null() {
-			creatorIDStr := getJsString(r, f.Argument(2))
+		if f.Argument(3) != goja.Undefined() && f.Argument(3) != goja.Null() {
+			creatorIDStr := getJsString(r, f.Argument(3))
 			creatorID, err = uuid.FromString(creatorIDStr)
 			if err != nil {
 				panic(r.NewTypeError("expects creator ID to be a valid identifier"))
@@ -5275,30 +5284,30 @@ func (n *runtimeJavascriptNakamaModule) groupUpdate(r *goja.Runtime) func(goja.F
 		}
 
 		var lang *wrapperspb.StringValue
-		if f.Argument(3) != goja.Undefined() && f.Argument(3) != goja.Null() {
-			langStr := getJsString(r, f.Argument(3))
+		if f.Argument(4) != goja.Undefined() && f.Argument(4) != goja.Null() {
+			langStr := getJsString(r, f.Argument(4))
 			lang = &wrapperspb.StringValue{Value: langStr}
 		}
 
 		var desc *wrapperspb.StringValue
-		if f.Argument(4) != goja.Undefined() && f.Argument(4) != goja.Null() {
-			descStr := getJsString(r, f.Argument(4))
+		if f.Argument(5) != goja.Undefined() && f.Argument(5) != goja.Null() {
+			descStr := getJsString(r, f.Argument(5))
 			desc = &wrapperspb.StringValue{Value: descStr}
 		}
 
 		var avatarURL *wrapperspb.StringValue
-		if f.Argument(5) != goja.Undefined() && f.Argument(5) != goja.Null() {
-			avatarStr := getJsString(r, f.Argument(5))
+		if f.Argument(6) != goja.Undefined() && f.Argument(6) != goja.Null() {
+			avatarStr := getJsString(r, f.Argument(6))
 			avatarURL = &wrapperspb.StringValue{Value: avatarStr}
 		}
 
 		var open *wrapperspb.BoolValue
-		if f.Argument(6) != goja.Undefined() && f.Argument(6) != goja.Null() {
-			open = &wrapperspb.BoolValue{Value: getJsBool(r, f.Argument(6))}
+		if f.Argument(7) != goja.Undefined() && f.Argument(7) != goja.Null() {
+			open = &wrapperspb.BoolValue{Value: getJsBool(r, f.Argument(7))}
 		}
 
 		var metadata *wrapperspb.StringValue
-		metadataIn := f.Argument(7)
+		metadataIn := f.Argument(8)
 		if metadataIn != goja.Undefined() && metadataIn != goja.Null() {
 			metadataMap, ok := metadataIn.Export().(map[string]interface{})
 			if !ok {
@@ -5312,16 +5321,16 @@ func (n *runtimeJavascriptNakamaModule) groupUpdate(r *goja.Runtime) func(goja.F
 		}
 
 		maxCount := 0
-		if f.Argument(8) != goja.Undefined() && f.Argument(8) != goja.Null() {
-			maxCountIn := int(getJsInt(r, f.Argument(8)))
-			if maxCount > 0 && maxCount <= 100 {
+		if f.Argument(9) != goja.Undefined() && f.Argument(9) != goja.Null() {
+			maxCountIn := int(getJsInt(r, f.Argument(9)))
+			if maxCountIn > 0 && maxCountIn <= 100 {
 				maxCount = maxCountIn
 			} else {
 				panic(r.NewTypeError("max count must be 1-100"))
 			}
 		}
 
-		if err = UpdateGroup(context.Background(), n.logger, n.db, groupID, uuid.Nil, creatorID, name, lang, desc, avatarURL, metadata, open, maxCount); err != nil {
+		if err = UpdateGroup(context.Background(), n.logger, n.db, groupID, userId, creatorID, name, lang, desc, avatarURL, metadata, open, maxCount); err != nil {
 			panic(r.NewGoError(fmt.Errorf("error while trying to update group: %v", err.Error())))
 		}
 
diff --git a/server/runtime_lua_nakama.go b/server/runtime_lua_nakama.go
index 92a7d462f17935aa53b94183fd6cfe39500de806..ef80df3e1026596eb40b10e9f6204a654fc9ddd9 100644
--- a/server/runtime_lua_nakama.go
+++ b/server/runtime_lua_nakama.go
@@ -6764,13 +6764,19 @@ func (n *RuntimeLuaNakamaModule) groupUpdate(l *lua.LState) int {
 		return 0
 	}
 
-	nameStr := l.OptString(2, "")
+	userID, err := uuid.FromString(l.OptString(2, uuid.Nil.String()))
+	if err != nil {
+		l.ArgError(1, "expects user ID to be a valid identifier")
+		return 0
+	}
+
+	nameStr := l.OptString(3, "")
 	var name *wrapperspb.StringValue
 	if nameStr != "" {
 		name = &wrapperspb.StringValue{Value: nameStr}
 	}
 
-	creatorIDStr := l.OptString(3, "")
+	creatorIDStr := l.OptString(4, "")
 	creatorID := uuid.Nil
 	if creatorIDStr != "" {
 		var err error
@@ -6781,31 +6787,31 @@ func (n *RuntimeLuaNakamaModule) groupUpdate(l *lua.LState) int {
 		}
 	}
 
-	langStr := l.OptString(4, "")
+	langStr := l.OptString(5, "")
 	var lang *wrapperspb.StringValue
 	if langStr != "" {
 		lang = &wrapperspb.StringValue{Value: langStr}
 	}
 
-	descStr := l.OptString(5, "")
+	descStr := l.OptString(6, "")
 	var desc *wrapperspb.StringValue
 	if descStr != "" {
 		desc = &wrapperspb.StringValue{Value: descStr}
 	}
 
-	avatarURLStr := l.OptString(6, "")
+	avatarURLStr := l.OptString(7, "")
 	var avatarURL *wrapperspb.StringValue
 	if avatarURLStr != "" {
 		avatarURL = &wrapperspb.StringValue{Value: avatarURLStr}
 	}
 
-	openV := l.Get(7)
+	openV := l.Get(8)
 	var open *wrapperspb.BoolValue
 	if openV != lua.LNil {
 		open = &wrapperspb.BoolValue{Value: l.OptBool(7, false)}
 	}
 
-	metadataTable := l.OptTable(8, nil)
+	metadataTable := l.OptTable(9, nil)
 	var metadata *wrapperspb.StringValue
 	if metadataTable != nil {
 		metadataMap := RuntimeLuaConvertLuaTable(metadataTable)
@@ -6817,13 +6823,13 @@ func (n *RuntimeLuaNakamaModule) groupUpdate(l *lua.LState) int {
 		metadata = &wrapperspb.StringValue{Value: string(metadataBytes)}
 	}
 
-	maxCountInt := l.OptInt(9, 0)
+	maxCountInt := l.OptInt(10, 0)
 	maxCount := 0
 	if maxCountInt > 0 && maxCountInt <= 100 {
 		maxCount = maxCountInt
 	}
 
-	if err = UpdateGroup(l.Context(), n.logger, n.db, groupID, uuid.Nil, creatorID, name, lang, desc, avatarURL, metadata, open, maxCount); err != nil {
+	if err = UpdateGroup(l.Context(), n.logger, n.db, groupID, userID, creatorID, name, lang, desc, avatarURL, metadata, open, maxCount); err != nil {
 		l.RaiseError("error while trying to update group: %v", err.Error())
 		return 0
 	}