diff --git a/include/ircd/server/request.h b/include/ircd/server/request.h
index 94c1f1021..8502d37cb 100644
--- a/include/ircd/server/request.h
+++ b/include/ircd/server/request.h
@@ -42,6 +42,13 @@ struct ircd::server::out
 {
 	const_buffer head;
 	const_buffer content;
+
+	/// The progress closure is an optional callback invoked every time more
+	/// content is written to the socket. The first argument is a view of the
+	/// data most recently written. The second argument is a view of all data
+	/// written so far. This is only invoked for content. At the first
+	/// invocation, the head has been fully written.
+	std::function<void (const_buffer, const_buffer) noexcept> progress;
 };
 
 /// Request data and options related to the receive side of the request.
diff --git a/ircd/server.cc b/ircd/server.cc
index b47b5009a..8806b6f23 100644
--- a/ircd/server.cc
+++ b/ircd/server.cc
@@ -1655,6 +1655,11 @@ ircd::server::tag::wrote_buffer(const const_buffer &buffer)
 	{
 		assert(data(buffer) == data(req.out.content));
 		assert(written <= write_total());
+
+		// Invoke the user's optional progress callback; this function
+		// should be marked noexcept and has no reason to throw yet.
+		if(req.out.progress)
+			req.out.progress(buffer, const_buffer{data(req.out.content), written});
 	}
 	else
 	{