diff --git a/pkgs/by-name/nl/nlohmann_json/make-tests-build-clang-19.diff b/pkgs/by-name/nl/nlohmann_json/make-tests-build-clang-19.diff
new file mode 100644
index 000000000000..ceb869251469
--- /dev/null
+++ b/pkgs/by-name/nl/nlohmann_json/make-tests-build-clang-19.diff
@@ -0,0 +1,98 @@
+diff --git a/tests/src/unit-bson.cpp b/tests/src/unit-bson.cpp
+index 13216f2..fdfc350 100644
+--- a/tests/src/unit-bson.cpp
++++ b/tests/src/unit-bson.cpp
+@@ -621,7 +621,7 @@ TEST_CASE("BSON input/output_adapters")
+     {
+         SECTION("std::ostringstream")
+         {
+-            std::basic_ostringstream<std::uint8_t> ss;
++            std::basic_ostringstream<char> ss;
+             json::to_bson(json_representation, ss);
+             json j3 = json::from_bson(ss.str());
+             CHECK(json_representation == j3);
+diff --git a/tests/src/unit-cbor.cpp b/tests/src/unit-cbor.cpp
+index be94d2f..2b396b7 100644
+--- a/tests/src/unit-cbor.cpp
++++ b/tests/src/unit-cbor.cpp
+@@ -1881,7 +1881,7 @@ TEST_CASE("single CBOR roundtrip")
+         {
+             SECTION("std::ostringstream")
+             {
+-                std::basic_ostringstream<std::uint8_t> ss;
++                std::basic_ostringstream<char> ss;
+                 json::to_cbor(j1, ss);
+                 json j3 = json::from_cbor(ss.str());
+                 CHECK(j1 == j3);
+diff --git a/tests/src/unit-deserialization.cpp b/tests/src/unit-deserialization.cpp
+index 3bc161f..e4918b0 100644
+--- a/tests/src/unit-deserialization.cpp
++++ b/tests/src/unit-deserialization.cpp
+@@ -1131,13 +1131,11 @@ TEST_CASE("deserialization")
+     }
+ }
+ 
++
+ TEST_CASE_TEMPLATE("deserialization of different character types (ASCII)", T,
+-                   char, unsigned char, signed char,
++                   char,
+                    wchar_t,
+-                   char16_t, char32_t,
+-                   std::uint8_t, std::int8_t,
+-                   std::int16_t, std::uint16_t,
+-                   std::int32_t, std::uint32_t)
++                   char16_t, char32_t)
+ {
+     std::vector<T> const v = {'t', 'r', 'u', 'e'};
+     CHECK(json::parse(v) == json(true));
+@@ -1163,7 +1161,7 @@ TEST_CASE_TEMPLATE("deserialization of different character types (UTF-8)", T,
+ }
+ 
+ TEST_CASE_TEMPLATE("deserialization of different character types (UTF-16)", T,
+-                   char16_t, std::uint16_t)
++                   char16_t)
+ {
+     // a star emoji
+     std::vector<T> const v = {static_cast<T>('"'), static_cast<T>(0x2b50), static_cast<T>(0xfe0f), static_cast<T>('"')};
+@@ -1176,7 +1174,7 @@ TEST_CASE_TEMPLATE("deserialization of different character types (UTF-16)", T,
+ }
+ 
+ TEST_CASE_TEMPLATE("deserialization of different character types (UTF-32)", T,
+-                   char32_t, std::uint32_t)
++                   char32_t)
+ {
+     // a star emoji
+     std::vector<T> const v = {static_cast<T>('"'), static_cast<T>(0x2b50), static_cast<T>(0xfe0f), static_cast<T>('"')};
+diff --git a/tests/src/unit-msgpack.cpp b/tests/src/unit-msgpack.cpp
+index 61162af..cfbb1fa 100644
+--- a/tests/src/unit-msgpack.cpp
++++ b/tests/src/unit-msgpack.cpp
+@@ -1604,7 +1604,7 @@ TEST_CASE("single MessagePack roundtrip")
+         {
+             SECTION("std::ostringstream")
+             {
+-                std::basic_ostringstream<std::uint8_t> ss;
++                std::basic_ostringstream<char> ss;
+                 json::to_msgpack(j1, ss);
+                 json j3 = json::from_msgpack(ss.str());
+                 CHECK(j1 == j3);
+diff --git a/tests/src/unit-regression2.cpp b/tests/src/unit-regression2.cpp
+index fab9aae..98947c5 100644
+--- a/tests/src/unit-regression2.cpp
++++ b/tests/src/unit-regression2.cpp
+@@ -674,6 +674,7 @@ TEST_CASE("regression tests 2")
+         CHECK(j.dump() == "{}");
+     }
+ 
++#if 0
+ #ifdef JSON_HAS_CPP_20
+ #if __has_include(<span>)
+     SECTION("issue #2546 - parsing containers of std::byte")
+@@ -684,6 +685,7 @@ TEST_CASE("regression tests 2")
+         CHECK(j.dump() == "\"Hello, world!\"");
+     }
+ #endif
++#endif
+ #endif
+ 
+     SECTION("issue #2574 - Deserialization to std::array, std::pair, and std::tuple with non-default constructable types fails")
diff --git a/pkgs/by-name/nl/nlohmann_json/package.nix b/pkgs/by-name/nl/nlohmann_json/package.nix
index d690cc45b2f9..8acc3d2d27ec 100644
--- a/pkgs/by-name/nl/nlohmann_json/package.nix
+++ b/pkgs/by-name/nl/nlohmann_json/package.nix
@@ -21,6 +21,12 @@ in stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-7F0Jon+1oWL7uqet5i1IgHX0fUw/+z0QwEcA3zs5xHg=";
   };
 
+  patches = lib.optionals stdenv.cc.isClang [
+    # tests fail to compile on clang-19
+    # https://github.com/nlohmann/json/issues/4490
+    ./make-tests-build-clang-19.diff
+  ];
+
   nativeBuildInputs = [ cmake ];
 
   cmakeFlags = [