diff --git a/examples/asio-cl.cc b/examples/asio-cl.cc
index ffb1d9a0..b8908265 100644
--- a/examples/asio-cl.cc
+++ b/examples/asio-cl.cc
@@ -73,7 +73,7 @@ int main(int argc, char *argv[]) {
         return;
       }
 
-      req->on_response([&sess, req](response &res) {
+      req->on_response([&sess, req](const response &res) {
         std::cerr << "response header was received" << std::endl;
         print_header(res);
 
@@ -87,12 +87,12 @@ int main(int argc, char *argv[]) {
         std::cerr << "request done with error_code=" << error_code << std::endl;
       });
 
-      req->on_push([](request &push_req) {
+      req->on_push([](const request &push_req) {
         std::cerr << "push request was received" << std::endl;
 
         print_header(push_req);
 
-        push_req.on_response([](response &res) {
+        push_req.on_response([](const response &res) {
           std::cerr << "push response header was received" << std::endl;
 
           res.on_data([](const uint8_t *data, std::size_t len) {
diff --git a/src/asio_client_request.cc b/src/asio_client_request.cc
index b1194940..4aa83866 100644
--- a/src/asio_client_request.cc
+++ b/src/asio_client_request.cc
@@ -36,13 +36,15 @@ request::request() : impl_(make_unique<request_impl>()) {}
 
 request::~request() {}
 
-void request::cancel() { impl_->cancel(); }
+void request::cancel() const { impl_->cancel(); }
 
-void request::on_response(response_cb cb) { impl_->on_response(std::move(cb)); }
+void request::on_response(response_cb cb) const {
+  impl_->on_response(std::move(cb));
+}
 
-void request::on_push(request_cb cb) { impl_->on_push(std::move(cb)); }
+void request::on_push(request_cb cb) const { impl_->on_push(std::move(cb)); }
 
-void request::on_close(close_cb cb) { impl_->on_close(std::move(cb)); }
+void request::on_close(close_cb cb) const { impl_->on_close(std::move(cb)); }
 
 const std::string &request::method() const { return impl_->method(); }
 
diff --git a/src/asio_client_response.cc b/src/asio_client_response.cc
index 6c66ef16..e38355a0 100644
--- a/src/asio_client_response.cc
+++ b/src/asio_client_response.cc
@@ -36,7 +36,7 @@ response::response() : impl_(make_unique<response_impl>()) {}
 
 response::~response() {}
 
-void response::on_data(data_cb cb) { impl_->on_data(std::move(cb)); }
+void response::on_data(data_cb cb) const { impl_->on_data(std::move(cb)); }
 
 int response::status_code() const { return impl_->status_code(); }
 
diff --git a/src/asio_client_session.cc b/src/asio_client_session.cc
index a060cf44..046a48a6 100644
--- a/src/asio_client_session.cc
+++ b/src/asio_client_session.cc
@@ -57,22 +57,24 @@ void session::shutdown() { impl_->shutdown(); }
 
 boost::asio::io_service &session::io_service() { return impl_->io_service(); }
 
-request *session::submit(boost::system::error_code &ec,
-                         const std::string &method, const std::string &uri,
-                         header_map h) {
+const request *session::submit(boost::system::error_code &ec,
+                               const std::string &method,
+                               const std::string &uri, header_map h) {
   return impl_->submit(ec, method, uri, read_cb(), std::move(h));
 }
 
-request *session::submit(boost::system::error_code &ec,
-                         const std::string &method, const std::string &uri,
-                         std::string data, header_map h) {
+const request *session::submit(boost::system::error_code &ec,
+                               const std::string &method,
+                               const std::string &uri, std::string data,
+                               header_map h) {
   return impl_->submit(ec, method, uri, string_reader(std::move(data)),
                        std::move(h));
 }
 
-request *session::submit(boost::system::error_code &ec,
-                         const std::string &method, const std::string &uri,
-                         read_cb cb, header_map h) {
+const request *session::submit(boost::system::error_code &ec,
+                               const std::string &method,
+                               const std::string &uri, read_cb cb,
+                               header_map h) {
   return impl_->submit(ec, method, uri, std::move(cb), std::move(h));
 }
 
diff --git a/src/asio_client_session_impl.cc b/src/asio_client_session_impl.cc
index 72153a19..58985380 100644
--- a/src/asio_client_session_impl.cc
+++ b/src/asio_client_session_impl.cc
@@ -358,9 +358,10 @@ std::unique_ptr<stream> session_impl::create_stream() {
   return strm;
 }
 
-request *session_impl::submit(boost::system::error_code &ec,
-                              const std::string &method, const std::string &uri,
-                              read_cb cb, header_map h) {
+const request *session_impl::submit(boost::system::error_code &ec,
+                                    const std::string &method,
+                                    const std::string &uri, read_cb cb,
+                                    header_map h) {
   ec.clear();
 
   auto nva = std::vector<nghttp2_nv>();
diff --git a/src/asio_client_session_impl.h b/src/asio_client_session_impl.h
index b3469b9c..913998cd 100644
--- a/src/asio_client_session_impl.h
+++ b/src/asio_client_session_impl.h
@@ -66,8 +66,9 @@ public:
   stream *create_push_stream(int32_t stream_id);
   stream *find_stream(int32_t stream_id);
 
-  request *submit(boost::system::error_code &ec, const std::string &method,
-                  const std::string &uri, read_cb cb, header_map h);
+  const request *submit(boost::system::error_code &ec,
+                        const std::string &method, const std::string &uri,
+                        read_cb cb, header_map h);
 
   virtual void start_connect(tcp::resolver::iterator endpoint_it) = 0;
   virtual tcp::socket &socket() = 0;
diff --git a/src/includes/nghttp2/asio_http2.h b/src/includes/nghttp2/asio_http2.h
index 6df3f600..8c2a8033 100644
--- a/src/includes/nghttp2/asio_http2.h
+++ b/src/includes/nghttp2/asio_http2.h
@@ -287,7 +287,7 @@ public:
   response();
   ~response();
 
-  void on_data(data_cb cb);
+  void on_data(data_cb cb) const;
 
   int status_code() const;
 
@@ -303,8 +303,8 @@ private:
 
 class request;
 
-using response_cb = std::function<void(response &)>;
-using request_cb = std::function<void(request &)>;
+using response_cb = std::function<void(const response &)>;
+using request_cb = std::function<void(const request &)>;
 
 class request_impl;
 
@@ -313,11 +313,11 @@ public:
   request();
   ~request();
 
-  void on_response(response_cb cb);
-  void on_push(request_cb cb);
-  void on_close(close_cb cb);
+  void on_response(response_cb cb) const;
+  void on_push(request_cb cb) const;
+  void on_close(close_cb cb) const;
 
-  void cancel();
+  void cancel() const;
 
   const std::string &method() const;
   const std::string &scheme() const;
@@ -351,12 +351,15 @@ public:
 
   boost::asio::io_service &io_service();
 
-  request *submit(boost::system::error_code &ec, const std::string &method,
-                  const std::string &uri, header_map h = {});
-  request *submit(boost::system::error_code &ec, const std::string &method,
-                  const std::string &uri, std::string data, header_map h = {});
-  request *submit(boost::system::error_code &ec, const std::string &method,
-                  const std::string &uri, read_cb cb, header_map h = {});
+  const request *submit(boost::system::error_code &ec,
+                        const std::string &method, const std::string &uri,
+                        header_map h = {});
+  const request *submit(boost::system::error_code &ec,
+                        const std::string &method, const std::string &uri,
+                        std::string data, header_map h = {});
+  const request *submit(boost::system::error_code &ec,
+                        const std::string &method, const std::string &uri,
+                        read_cb cb, header_map h = {});
 
 private:
   std::unique_ptr<session_impl> impl_;