diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index a8afd429cf..62b212d722 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -2710,6 +2710,13 @@ MBEDTLS_CHECK_RETURN_CRITICAL
 int mbedtls_ssl_tls13_parse_record_size_limit_ext(mbedtls_ssl_context *ssl,
                                                   const unsigned char *buf,
                                                   const unsigned char *end);
+
+MBEDTLS_CHECK_RETURN_CRITICAL
+int mbedtls_ssl_tls13_write_record_size_limit_ext(mbedtls_ssl_context *ssl,
+                                                  uint16_t record_size_limit,
+                                                  unsigned char *buf,
+                                                  const unsigned char *end,
+                                                  size_t *out_len);
 #endif /* MBEDTLS_SSL_RECORD_SIZE_LIMIT */
 
 #if defined(MBEDTLS_SSL_ALPN)