diff --git a/CMakeLists.txt b/CMakeLists.txt
index dea843b23..d54eb58ac 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -293,6 +293,21 @@ option(LINK_STATIC_DEPENDS "Link dependencies for static build"     YES)
 # Enable Azure Monitor / Application Insights end-point support
 option(BUILD_AZMON        "Build for Azure Monitor" YES)
 
+# Common Schema 4.0 (CS4) protocol support (CS3 by default). HAVE_CS4 toggles
+# optional CsProtocol fields whose presence changes the public struct layout, so
+# it is applied as a PUBLIC compile definition on the exported target (see
+# lib/CMakeLists.txt) to keep the SDK and its consumers in sync. HAVE_CS4_FULL
+# adds server/service extensions and implies HAVE_CS4.
+option(MATSDK_HAVE_CS4      "Build with Common Schema 4.0 (CS4) support" OFF)
+option(MATSDK_HAVE_CS4_FULL "Build with CS4 plus server/service extensions" OFF)
+# CS4_FULL requires CS4. Force the options consistent so the cache state and any
+# downstream checks of MATSDK_HAVE_CS4 are reliable (rather than relying on a
+# downstream OR).
+if(MATSDK_HAVE_CS4_FULL AND NOT MATSDK_HAVE_CS4)
+  message(STATUS "MATSDK_HAVE_CS4_FULL implies MATSDK_HAVE_CS4; enabling MATSDK_HAVE_CS4.")
+  set(MATSDK_HAVE_CS4 ON CACHE BOOL "Build with Common Schema 4.0 (CS4) support" FORCE)
+endif()
+
 if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
   option(BUILD_APPLE_HTTP "Build Apple HTTP client" YES)
 endif()
diff --git a/docs/building-custom-SKU.md b/docs/building-custom-SKU.md
index 0668fbe3f..8e5722ffb 100644
--- a/docs/building-custom-SKU.md
+++ b/docs/building-custom-SKU.md
@@ -30,8 +30,8 @@ Build recipe must contain the following preprocessor definitions:
 | HAVE_MAT_STORAGE | on | Enable SQLite persistent offline storage |
 | HAVE_MAT_NETDETECT | on | _Win32 Desktop only_: Use NLM COM object for network cost detection on Windows 8+ |
 | HAVE_MAT_SHORT_NS | off | Use short "MAT::" namespace instead of "Microsoft::Applications::Events::" to reduce the .DLL size |
-| HAVE_CS4 | off | Build with Common Schema 4.0 support. Current default is `off`, i.e. building with Common Schema 3.0 support |
-| HAVE_CS4_FULL | off | Enable additional Common Schema 4.0 protocol features needed by server / services SDK |
+| HAVE_CS4 | off | Preprocessor define enabling Common Schema 4.0 (CS3 by default). Any build system can define it (e.g. via a custom config header or compiler flag); CMake builds should prefer `-DMATSDK_HAVE_CS4=ON`, which defines `HAVE_CS4` and propagates it to consumers via the exported `MSTelemetry::mat` target. It changes the public CsProtocol struct layout and must match across the SDK and its consumers. |
+| HAVE_CS4_FULL | off | Preprocessor define adding the Common Schema 4.0 server / services extensions (implies `HAVE_CS4`). CMake builds should use `-DMATSDK_HAVE_CS4_FULL=ON`. |
 | COMPACT_SDK | off | Built-in build recipe for smallest possible SDK. Turns most features off. Includes_mat/config-compact.h_ |
 
 ## Building custom SDK SKU: MSBuild example
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 533144c06..ad771e067 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -338,6 +338,19 @@ target_include_directories(mat
     ${CMAKE_CURRENT_SOURCE_DIR}/utils
 )
 
+# Common Schema 4.0 support. HAVE_CS4 toggles optional CsProtocol fields whose
+# presence changes the public struct layout, so it must be defined consistently
+# across the SDK and its consumers. Expose it as a PUBLIC (propagating) compile
+# definition on the exported target rather than a raw build flag, so consumers
+# linking MSTelemetry::mat inherit it automatically. MATSDK_HAVE_CS4_FULL forces
+# MATSDK_HAVE_CS4 on in the root CMakeLists, so checking MATSDK_HAVE_CS4 suffices.
+if(MATSDK_HAVE_CS4)
+  target_compile_definitions(mat PUBLIC HAVE_CS4)
+endif()
+if(MATSDK_HAVE_CS4_FULL)
+  target_compile_definitions(mat PUBLIC HAVE_CS4_FULL)
+endif()
+
 if(APPLE AND BUILD_OBJC_WRAPPER)
   if(MATSDK_OBJC_PRIVACYGUARD_AVAILABLE)
     target_compile_definitions(mat PRIVATE MATSDK_OBJC_PRIVACYGUARD_AVAILABLE=1)