diff --git a/src/stan/callbacks/multi_writer.hpp b/src/stan/callbacks/multi_writer.hpp
index 2e2db75fac..e29c2c95e7 100644
--- a/src/stan/callbacks/multi_writer.hpp
+++ b/src/stan/callbacks/multi_writer.hpp
@@ -1,8 +1,9 @@
 #ifndef STAN_CALLBACKS_MULTI_WRITER_HPP
 #define STAN_CALLBACKS_MULTI_WRITER_HPP
 
-#include <stan/callbacks/writer.hpp>
 #include <stan/math/prim/fun/Eigen.hpp>
+#include <stan/callbacks/writer.hpp>
+#include <stan/math/prim/functor/for_each.hpp>
 #include <memory>
 #include <ostream>
 #include <string>
diff --git a/src/stan/callbacks/unique_stream_writer.hpp b/src/stan/callbacks/unique_stream_writer.hpp
index ce82cbde9b..f763b23650 100644
--- a/src/stan/callbacks/unique_stream_writer.hpp
+++ b/src/stan/callbacks/unique_stream_writer.hpp
@@ -113,8 +113,6 @@ class unique_stream_writer final : public writer {
       return;
     *output_ << comment_prefix_ << message << std::endl;
   }
-  using Stream = std::decay_t<Stream>;
-
  private:
   /**
    * Comma formatter for writing Eigen matrices