diff --git a/runtime/handler.go b/runtime/handler.go
index 8fe6a77efd5..63dc812b781 100644
--- a/runtime/handler.go
+++ b/runtime/handler.go
@@ -6,6 +6,7 @@ import (
 	"io"
 	"net/http"
 	"net/textproto"
+	"strconv"
 	"strings"
 
 	"google.golang.org/genproto/googleapis/api/httpbody"
@@ -176,6 +177,10 @@ func ForwardResponseMessage(ctx context.Context, mux *ServeMux, marshaler Marsha
 		return
 	}
 
+	if !doForwardTrailers {
+		w.Header().Set("Content-Length", strconv.Itoa(len(buf)))
+	}
+
 	if _, err = w.Write(buf); err != nil {
 		grpclog.Infof("Failed to write response: %v", err)
 	}