diff --git a/include/nlohmann/detail/output/serializer.hpp b/include/nlohmann/detail/output/serializer.hpp
index 7d6b3d2ce..948ae420a 100644
--- a/include/nlohmann/detail/output/serializer.hpp
+++ b/include/nlohmann/detail/output/serializer.hpp
@@ -730,12 +730,17 @@ class serializer
         write_buffer_pos += length;
     }
 
+  JSON_PRIVATE_UNLESS_TESTED:
     /*!
     @brief flush the write buffer to the output adapter
 
     Writing zero characters is a well-defined no-op for every output adapter, so
     the buffered length is passed through unconditionally (no empty-guard branch
     to leave uncovered).
+
+    @note dump_escaped() and dump_integer()/dump_float() write into the internal
+    write buffer; callers that invoke them directly (rather than through the
+    public dump()) must call flush() before inspecting the output.
     */
     void flush()
     {
@@ -743,6 +748,7 @@ class serializer
         write_buffer_pos = 0;
     }
 
+  private:
     /*!
     @brief count digits
 
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index a08320d33..ffbac824c 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -21313,12 +21313,17 @@ class serializer
         write_buffer_pos += length;
     }
 
+  JSON_PRIVATE_UNLESS_TESTED:
     /*!
     @brief flush the write buffer to the output adapter
 
     Writing zero characters is a well-defined no-op for every output adapter, so
     the buffered length is passed through unconditionally (no empty-guard branch
     to leave uncovered).
+
+    @note dump_escaped() and dump_integer()/dump_float() write into the internal
+    write buffer; callers that invoke them directly (rather than through the
+    public dump()) must call flush() before inspecting the output.
     */
     void flush()
     {
@@ -21326,6 +21331,7 @@ class serializer
         write_buffer_pos = 0;
     }
 
+  private:
     /*!
     @brief count digits
 
diff --git a/tests/src/unit-convenience.cpp b/tests/src/unit-convenience.cpp
index 037a4e589..839f1c86e 100644
--- a/tests/src/unit-convenience.cpp
+++ b/tests/src/unit-convenience.cpp
@@ -100,6 +100,7 @@ void check_escaped(const char* original, const char* escaped, const bool ensure_
     std::stringstream ss;
     json::serializer s(nlohmann::detail::output_adapter<char>(ss), ' ');
     s.dump_escaped(original, ensure_ascii);
+    s.flush(); // dump_escaped writes into the serializer's internal buffer
     CHECK(ss.str() == escaped);
 }
 } // namespace