diff --git a/CMakeLists.txt b/CMakeLists.txt
index d90e1341..c8828ad7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -164,7 +164,14 @@ SET(CMAKE_POLICY_VERSION_MINIMUM 3.5)
 
 # libzmq must be made available BEFORE sls_detector_package for the override above to take effect.
 FetchContent_MakeAvailable(libzmq)
-FetchContent_MakeAvailable(zstd sls_detector_package catch2 hdf5 spdlog httplib)
+IF (JFJOCH_VIEWER_ONLY)
+    # A viewer-only build still needs zstd/hdf5/spdlog/httplib (JFJochReader uses httplib).
+    # Only sls_detector_package (detector) and catch2 (tests) are not built here -- and sls
+    # in particular does not configure under MSVC -- so skip just those two.
+    FetchContent_MakeAvailable(zstd hdf5 spdlog httplib)
+ELSE()
+    FetchContent_MakeAvailable(zstd sls_detector_package catch2 hdf5 spdlog httplib)
+ENDIF()
 
 IF (JFJOCH_VIEWER_ONLY)
     # Minimal subtree: jfjoch_viewer and only the libraries it transitively links.