diff --git a/docs/README.md b/docs/README.md
index cfdbb463..c5644bc0 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -806,10 +806,13 @@ Create client Websocket connection.
 ### mg\_ws\_upgrade()
 
 ```c
-void mg_ws_upgrade(struct mg_connection *, struct mg_http_message *);
+void mg_ws_upgrade(struct mg_connection *, struct mg_http_message *,
+                   const char *fmt, ...);
 ```
 
-Upgrade given HTTP connection to Websocket.
+Upgrade given HTTP connection to Websocket. The `fmt` is a printf-like
+format string for the extra HTTP headers returned to the client in a
+Websocket handshake. Set `fmt` to `NULL` if no extra headers needs to be passed.
 
 
 ### mg\_ws\_send()
diff --git a/examples/timers/main.c b/examples/timers/main.c
index ae0104ce..f00bf200 100644
--- a/examples/timers/main.c
+++ b/examples/timers/main.c
@@ -18,7 +18,7 @@ static const char *s_listen_on = "http://localhost:8000";
 static void fn(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
   if (ev == MG_EV_HTTP_MSG) {
     struct mg_http_message *hm = (struct mg_http_message *) ev_data;
-    mg_ws_upgrade(c, hm);
+    mg_ws_upgrade(c, hm, NULL);
   } else if (ev == MG_EV_WS_MSG) {
     // Got websocket frame. Received data is wm->data. Echo it back!
     struct mg_ws_message *wm = (struct mg_ws_message *) ev_data;
@@ -42,9 +42,9 @@ static void timer_fn(void *arg) {
 }
 
 int main(void) {
-  struct mg_mgr mgr;                            // Event manager
-  struct mg_timer t1;                           // Timer
-  mg_mgr_init(&mgr);                            // Initialise event manager
+  struct mg_mgr mgr;   // Event manager
+  struct mg_timer t1;  // Timer
+  mg_mgr_init(&mgr);   // Initialise event manager
   mg_timer_init(&t1, 300, MG_TIMER_REPEAT, timer_fn, &mgr);  // Init timer
   mg_http_listen(&mgr, s_listen_on, fn, NULL);  // Create HTTP listener
   for (;;) mg_mgr_poll(&mgr, 1000);             // Infinite event loop
diff --git a/examples/websocket-server/main.c b/examples/websocket-server/main.c
index d9df7cc0..80c279d1 100644
--- a/examples/websocket-server/main.c
+++ b/examples/websocket-server/main.c
@@ -21,7 +21,7 @@ static void fn(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
     if (mg_http_match_uri(hm, "/websocket")) {
       // Upgrade to websocket. From now on, a connection is a full-duplex
       // Websocket connection, which will receive MG_EV_WS_MSG events.
-      mg_ws_upgrade(c, hm);
+      mg_ws_upgrade(c, hm, NULL);
     } else if (mg_http_match_uri(hm, "/rest")) {
       // Serve REST response
       mg_http_reply(c, 200, "", "{\"result\": %d}\n", 123);
diff --git a/mongoose.c b/mongoose.c
index 5fae3525..ec4cdf48 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -3967,25 +3967,27 @@ struct ws_msg {
 };
 
 static void ws_handshake(struct mg_connection *c, const char *key,
-                         size_t key_len) {
+                         size_t key_len, const char *fmt, va_list ap) {
   const char *magic = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
   unsigned char sha[20], b64_sha[30];
-  // mem[256], *buf = mem;
-  // int len = 0;
+  char mem[128], *buf = mem;
+
   mg_sha1_ctx sha_ctx;
   mg_sha1_init(&sha_ctx);
   mg_sha1_update(&sha_ctx, (unsigned char *) key, key_len);
   mg_sha1_update(&sha_ctx, (unsigned char *) magic, 36);
   mg_sha1_final(sha, &sha_ctx);
   mg_base64_encode(sha, sizeof(sha), (char *) b64_sha);
+  buf[0] = '\0';
+  if (fmt != NULL) mg_vasprintf(&buf, sizeof(mem), fmt, ap);
   mg_printf(c,
             "HTTP/1.1 101 Switching Protocols\r\n"
             "Upgrade: websocket\r\n"
             "Connection: Upgrade\r\n"
-            "Sec-WebSocket-Accept: %s\r\n\r\n",
-            b64_sha);
-  // mg_send(c, buf, len);
-  // if (buf != mem) free(buf);
+            "Sec-WebSocket-Accept: %s\r\n"
+            "%s\r\n",
+            b64_sha, buf);
+  if (buf != mem) free(buf);
 }
 
 static size_t ws_process(uint8_t *buf, size_t len, struct ws_msg *msg) {
@@ -4145,9 +4147,15 @@ struct mg_connection *mg_ws_connect(struct mg_mgr *mgr, const char *url,
   return c;
 }
 
-void mg_ws_upgrade(struct mg_connection *c, struct mg_http_message *hm) {
+void mg_ws_upgrade(struct mg_connection *c, struct mg_http_message *hm,
+                   const char *fmt, ...) {
   struct mg_str *wskey = mg_http_get_header(hm, "Sec-WebSocket-Key");
   c->pfn = mg_ws_cb;
-  if (wskey != NULL) ws_handshake(c, wskey->ptr, wskey->len);
+  if (wskey != NULL) {
+    va_list ap;
+    va_start(ap, fmt);
+    ws_handshake(c, wskey->ptr, wskey->len, fmt, ap);
+    va_end(ap);
+  }
   c->is_websocket = 1;
 }
diff --git a/mongoose.h b/mongoose.h
index 8da88991..7fa8f33a 100644
--- a/mongoose.h
+++ b/mongoose.h
@@ -740,7 +740,8 @@ struct mg_ws_message {
 struct mg_connection *mg_ws_connect(struct mg_mgr *, const char *url,
                                     mg_event_handler_t fn, void *fn_data,
                                     const char *fmt, ...);
-void mg_ws_upgrade(struct mg_connection *, struct mg_http_message *);
+void mg_ws_upgrade(struct mg_connection *, struct mg_http_message *,
+                   const char *fmt, ...);
 size_t mg_ws_send(struct mg_connection *, const char *buf, size_t len, int op);
 
 
diff --git a/src/ws.c b/src/ws.c
index f2e2fd2a..4cb883ac 100644
--- a/src/ws.c
+++ b/src/ws.c
@@ -15,25 +15,27 @@ struct ws_msg {
 };
 
 static void ws_handshake(struct mg_connection *c, const char *key,
-                         size_t key_len) {
+                         size_t key_len, const char *fmt, va_list ap) {
   const char *magic = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
   unsigned char sha[20], b64_sha[30];
-  // mem[256], *buf = mem;
-  // int len = 0;
+  char mem[128], *buf = mem;
+
   mg_sha1_ctx sha_ctx;
   mg_sha1_init(&sha_ctx);
   mg_sha1_update(&sha_ctx, (unsigned char *) key, key_len);
   mg_sha1_update(&sha_ctx, (unsigned char *) magic, 36);
   mg_sha1_final(sha, &sha_ctx);
   mg_base64_encode(sha, sizeof(sha), (char *) b64_sha);
+  buf[0] = '\0';
+  if (fmt != NULL) mg_vasprintf(&buf, sizeof(mem), fmt, ap);
   mg_printf(c,
             "HTTP/1.1 101 Switching Protocols\r\n"
             "Upgrade: websocket\r\n"
             "Connection: Upgrade\r\n"
-            "Sec-WebSocket-Accept: %s\r\n\r\n",
-            b64_sha);
-  // mg_send(c, buf, len);
-  // if (buf != mem) free(buf);
+            "Sec-WebSocket-Accept: %s\r\n"
+            "%s\r\n",
+            b64_sha, buf);
+  if (buf != mem) free(buf);
 }
 
 static size_t ws_process(uint8_t *buf, size_t len, struct ws_msg *msg) {
@@ -193,9 +195,15 @@ struct mg_connection *mg_ws_connect(struct mg_mgr *mgr, const char *url,
   return c;
 }
 
-void mg_ws_upgrade(struct mg_connection *c, struct mg_http_message *hm) {
+void mg_ws_upgrade(struct mg_connection *c, struct mg_http_message *hm,
+                   const char *fmt, ...) {
   struct mg_str *wskey = mg_http_get_header(hm, "Sec-WebSocket-Key");
   c->pfn = mg_ws_cb;
-  if (wskey != NULL) ws_handshake(c, wskey->ptr, wskey->len);
+  if (wskey != NULL) {
+    va_list ap;
+    va_start(ap, fmt);
+    ws_handshake(c, wskey->ptr, wskey->len, fmt, ap);
+    va_end(ap);
+  }
   c->is_websocket = 1;
 }
diff --git a/src/ws.h b/src/ws.h
index 37729c96..6413c79c 100644
--- a/src/ws.h
+++ b/src/ws.h
@@ -20,5 +20,6 @@ struct mg_ws_message {
 struct mg_connection *mg_ws_connect(struct mg_mgr *, const char *url,
                                     mg_event_handler_t fn, void *fn_data,
                                     const char *fmt, ...);
-void mg_ws_upgrade(struct mg_connection *, struct mg_http_message *);
+void mg_ws_upgrade(struct mg_connection *, struct mg_http_message *,
+                   const char *fmt, ...);
 size_t mg_ws_send(struct mg_connection *, const char *buf, size_t len, int op);
diff --git a/test/unit_test.c b/test/unit_test.c
index 4f2168a6..6d21fa1c 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -331,7 +331,7 @@ static void eh1(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
     if (mg_http_match_uri(hm, "/foo/*")) {
       mg_http_reply(c, 200, "", "uri: %.*s", hm->uri.len - 5, hm->uri.ptr + 5);
     } else if (mg_http_match_uri(hm, "/ws")) {
-      mg_ws_upgrade(c, hm);
+      mg_ws_upgrade(c, hm, NULL);
     } else if (mg_http_match_uri(hm, "/body")) {
       mg_http_reply(c, 200, "", "%.*s", (int) hm->body.len, hm->body.ptr);
     } else if (mg_http_match_uri(hm, "/bar")) {