diff --git a/chttp.c b/chttp.c
index f9ef146..36048ef 100644
--- a/chttp.c
+++ b/chttp.c
@@ -4327,6 +4327,54 @@ void http_response_builder_body(HTTP_ResponseBuilder builder, HTTP_String str)
     byte_queue_write(&conn->output, str.ptr, str.len);
 }
 
+void http_response_builder_body_cap(HTTP_ResponseBuilder builder, int cap)
+{
+    HTTP_ServerConn *conn = builder_to_conn(builder);
+    if (conn == NULL)
+        return;
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_HEADER) {
+        append_special_headers(conn);
+        conn->state = HTTP_SERVER_CONN_WAIT_BODY;
+    }
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_BODY)
+        return;
+
+    byte_queue_write_setmincap(&conn->output, cap);
+}
+
+char *http_response_builder_body_buf(HTTP_ResponseBuilder builder, int *cap)
+{
+    HTTP_ServerConn *conn = builder_to_conn(builder);
+    if (conn == NULL)
+        return NULL;
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_HEADER) {
+        append_special_headers(conn);
+        conn->state = HTTP_SERVER_CONN_WAIT_BODY;
+    }
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_BODY)
+        return NULL;
+
+    ByteView tmp = byte_queue_write_buf(&conn->output);
+    *cap = tmp.len;
+    return tmp.ptr;
+}
+
+void http_response_builder_body_ack(HTTP_ResponseBuilder builder, int num)
+{
+    HTTP_ServerConn *conn = builder_to_conn(builder);
+    if (conn == NULL)
+        return;
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_BODY)
+        return;
+
+    byte_queue_write_ack(&conn->output, num);
+}
+
 void http_response_builder_send(HTTP_ResponseBuilder builder)
 {
     HTTP_ServerConn *conn = builder_to_conn(builder);
diff --git a/chttp.h b/chttp.h
index 9419184..64bed6f 100644
--- a/chttp.h
+++ b/chttp.h
@@ -1142,6 +1142,11 @@ void http_response_builder_header(HTTP_ResponseBuilder builder, HTTP_String str)
 // Append some bytes to the response's body
 void http_response_builder_body(HTTP_ResponseBuilder builder, HTTP_String str);
 
+// TODO: comment
+void  http_response_builder_body_cap(HTTP_ResponseBuilder builder, int cap);
+char *http_response_builder_body_buf(HTTP_ResponseBuilder builder, int *cap);
+void  http_response_builder_body_ack(HTTP_ResponseBuilder builder, int num);
+
 // Mark the response as complete. This will invalidate
 // the response builder handle.
 void http_response_builder_send(HTTP_ResponseBuilder builder);
diff --git a/src/server.c b/src/server.c
index 605d75b..4fde5f0 100644
--- a/src/server.c
+++ b/src/server.c
@@ -491,6 +491,54 @@ void http_response_builder_body(HTTP_ResponseBuilder builder, HTTP_String str)
     byte_queue_write(&conn->output, str.ptr, str.len);
 }
 
+void http_response_builder_body_cap(HTTP_ResponseBuilder builder, int cap)
+{
+    HTTP_ServerConn *conn = builder_to_conn(builder);
+    if (conn == NULL)
+        return;
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_HEADER) {
+        append_special_headers(conn);
+        conn->state = HTTP_SERVER_CONN_WAIT_BODY;
+    }
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_BODY)
+        return;
+
+    byte_queue_write_setmincap(&conn->output, cap);
+}
+
+char *http_response_builder_body_buf(HTTP_ResponseBuilder builder, int *cap)
+{
+    HTTP_ServerConn *conn = builder_to_conn(builder);
+    if (conn == NULL)
+        return NULL;
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_HEADER) {
+        append_special_headers(conn);
+        conn->state = HTTP_SERVER_CONN_WAIT_BODY;
+    }
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_BODY)
+        return NULL;
+
+    ByteView tmp = byte_queue_write_buf(&conn->output);
+    *cap = tmp.len;
+    return tmp.ptr;
+}
+
+void http_response_builder_body_ack(HTTP_ResponseBuilder builder, int num)
+{
+    HTTP_ServerConn *conn = builder_to_conn(builder);
+    if (conn == NULL)
+        return;
+
+    if (conn->state != HTTP_SERVER_CONN_WAIT_BODY)
+        return;
+
+    byte_queue_write_ack(&conn->output, num);
+}
+
 void http_response_builder_send(HTTP_ResponseBuilder builder)
 {
     HTTP_ServerConn *conn = builder_to_conn(builder);
diff --git a/src/server.h b/src/server.h
index c99bfa8..fa79a0c 100644
--- a/src/server.h
+++ b/src/server.h
@@ -220,6 +220,11 @@ void http_response_builder_header(HTTP_ResponseBuilder builder, HTTP_String str)
 // Append some bytes to the response's body
 void http_response_builder_body(HTTP_ResponseBuilder builder, HTTP_String str);
 
+// TODO: comment
+void  http_response_builder_body_cap(HTTP_ResponseBuilder builder, int cap);
+char *http_response_builder_body_buf(HTTP_ResponseBuilder builder, int *cap);
+void  http_response_builder_body_ack(HTTP_ResponseBuilder builder, int num);
+
 // Mark the response as complete. This will invalidate
 // the response builder handle.
 void http_response_builder_send(HTTP_ResponseBuilder builder);