diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index e57a80f..8dcf350 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -144,3 +144,26 @@ jobs:
 
       - name: Run memory tests
         run: make test-memory TEST_DATA_BRANCH=${{env.TEST_DATA_BRANCH_NAME}}
+
+  cxx-modules-test:
+    runs-on: macos-latest
+    name: Build with C++20 modules
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          repository: ${{ github.event.pull_request.head.repo.full_name || 'Eppo-exp/cpp-sdk' }}
+          ref: ${{ env.SDK_BRANCH_NAME }}
+
+      - name: Set up build environment
+        run: |
+          brew install re2
+          brew install cmake
+
+      - name: Build with -fmodules flag
+        run: |
+          mkdir -p build-modules
+          cd build-modules
+          cmake .. -DCMAKE_CXX_FLAGS="-fmodules" -DEPPOCLIENT_BUILD_TESTS=OFF -DEPPOCLIENT_ERR_ON_WARNINGS=OFF
+          cmake --build .
diff --git a/third_party/nlohmann/json.hpp b/third_party/nlohmann/json.hpp
index 8b72ea6..8961e50 100644
--- a/third_party/nlohmann/json.hpp
+++ b/third_party/nlohmann/json.hpp
@@ -6159,6 +6159,7 @@ NLOHMANN_JSON_NAMESPACE_END
 #ifndef JSON_NO_IO
     #include <cstdio>   // FILE *
     #include <istream>  // istream
+    #include <streambuf> // streambuf
 #endif                  // JSON_NO_IO
 
 // #include <nlohmann/detail/iterators/iterator_traits.hpp>
@@ -7378,6 +7379,7 @@ NLOHMANN_JSON_NAMESPACE_END
 #include <array> // array
 #include <clocale> // localeconv
 #include <cstddef> // size_t
+#include <cerrno> // errno
 #include <cstdio> // snprintf
 #include <cstdlib> // strtof, strtod, strtold, strtoll, strtoull
 #include <initializer_list> // initializer_list