diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 9029078..7c3a3d9 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1669,6 +1669,12 @@
      * Miscellaneous
      */
     int MBEDTLS_PRIVATE(state);                  /*!< SSL handshake: current state     */
+
+    /** Mask of `MBEDTLS_SSL_CONTEXT_FLAG_XXX`.
+     * This field is not saved by mbedtls_ssl_session_save().
+     */
+    uint32_t MBEDTLS_PRIVATE(flags);
+
 #if defined(MBEDTLS_SSL_RENEGOTIATION)
     int MBEDTLS_PRIVATE(renego_status);          /*!< Initial, in progress, pending?   */
     int MBEDTLS_PRIVATE(renego_records_seen);    /*!< Records since renego request, or with DTLS,