diff --git a/tests/src/test_utils.hpp b/tests/src/test_utils.hpp
index c6371e6e9..baa802f71 100644
--- a/tests/src/test_utils.hpp
+++ b/tests/src/test_utils.hpp
@@ -32,15 +32,12 @@ inline std::vector<std::uint8_t> read_binary_file(const std::string& filename)
 
 // sentinel for istreambuf_iterator; compares != true until EOF is reached
 // lets tests read a file directly via the new iterator+sentinel overloads
-// instead of buffering the whole file into a vector first
+// instead of buffering the whole file into a vector first.
+// Only the iterator-first direction (it != sentinel) is ever evaluated by
+// the library's parse loop, so no reversed-order overload is needed.
 struct istreambuf_sentinel
 {
-    friend bool operator!=(const std::istreambuf_iterator<char>& it, const istreambuf_sentinel&) noexcept
-    {
-        return it != std::istreambuf_iterator<char>();
-    }
-
-    friend bool operator!=(const istreambuf_sentinel&, const std::istreambuf_iterator<char>& it) noexcept
+    friend bool operator!=(const std::istreambuf_iterator<char>& it, const istreambuf_sentinel& /*unused*/) noexcept
     {
         return it != std::istreambuf_iterator<char>();
     }
diff --git a/tests/src/unit-bjdata.cpp b/tests/src/unit-bjdata.cpp
index 3c272236f..5af4afac1 100644
--- a/tests/src/unit-bjdata.cpp
+++ b/tests/src/unit-bjdata.cpp
@@ -3703,7 +3703,7 @@ TEST_CASE("Parse BJData directly from a file using iterator and sentinel")
 {
     std::string const filename = TEST_DATA_DIRECTORY "/json_testsuite/sample.json.bjdata";
     std::ifstream file(filename, std::ios::binary);
-    std::istreambuf_iterator<char> first(file);
+    const std::istreambuf_iterator<char> first(file);
     const json parsed = json::from_bjdata(first, utils::istreambuf_sentinel{});
     CHECK((parsed.is_object() || parsed.is_array()));
 }
diff --git a/tests/src/unit-bson.cpp b/tests/src/unit-bson.cpp
index 84b974b0f..28837d75e 100644
--- a/tests/src/unit-bson.cpp
+++ b/tests/src/unit-bson.cpp
@@ -1216,7 +1216,7 @@ TEST_CASE("Parse BSON directly from a file using iterator and sentinel")
     const json expected = json::parse(f_json);
 
     std::ifstream file(filename + ".bson", std::ios::binary);
-    std::istreambuf_iterator<char> first(file);
+    const std::istreambuf_iterator<char> first(file);
     const json parsed = json::from_bson(first, utils::istreambuf_sentinel{});
     CHECK(parsed == expected);
 }
diff --git a/tests/src/unit-cbor.cpp b/tests/src/unit-cbor.cpp
index c0cd5cde3..af7364299 100644
--- a/tests/src/unit-cbor.cpp
+++ b/tests/src/unit-cbor.cpp
@@ -1925,7 +1925,7 @@ TEST_CASE("Parse CBOR directly from a file using iterator and sentinel")
 {
     std::string const filename = TEST_DATA_DIRECTORY "/json_testsuite/sample.json.cbor";
     std::ifstream file(filename, std::ios::binary);
-    std::istreambuf_iterator<char> first(file);
+    const std::istreambuf_iterator<char> first(file);
     const json parsed = json::from_cbor(first, utils::istreambuf_sentinel{});
     CHECK((parsed.is_object() || parsed.is_array()));
 }
diff --git a/tests/src/unit-msgpack.cpp b/tests/src/unit-msgpack.cpp
index d35fe4198..4358cd464 100644
--- a/tests/src/unit-msgpack.cpp
+++ b/tests/src/unit-msgpack.cpp
@@ -1645,7 +1645,7 @@ TEST_CASE("Parse MessagePack directly from a file using iterator and sentinel")
 {
     std::string const filename = TEST_DATA_DIRECTORY "/json_testsuite/sample.json.msgpack";
     std::ifstream file(filename, std::ios::binary);
-    std::istreambuf_iterator<char> first(file);
+    const std::istreambuf_iterator<char> first(file);
     const json parsed = json::from_msgpack(first, utils::istreambuf_sentinel{});
     CHECK((parsed.is_object() || parsed.is_array()));
 }
diff --git a/tests/src/unit-ubjson.cpp b/tests/src/unit-ubjson.cpp
index 9dae264ed..8242a0cc8 100644
--- a/tests/src/unit-ubjson.cpp
+++ b/tests/src/unit-ubjson.cpp
@@ -2397,7 +2397,7 @@ TEST_CASE("Parse UBJSON directly from a file using iterator and sentinel")
 {
     std::string const filename = TEST_DATA_DIRECTORY "/json_testsuite/sample.json.ubjson";
     std::ifstream file(filename, std::ios::binary);
-    std::istreambuf_iterator<char> first(file);
+    const std::istreambuf_iterator<char> first(file);
     const json parsed = json::from_ubjson(first, utils::istreambuf_sentinel{});
     CHECK((parsed.is_object() || parsed.is_array()));
 }