diff --git a/Modules/Compiler/CMakeCommonCompilerMacros.cmake b/Modules/Compiler/CMakeCommonCompilerMacros.cmake
index 9eead201bd..75a9da228f 100644
--- a/Modules/Compiler/CMakeCommonCompilerMacros.cmake
+++ b/Modules/Compiler/CMakeCommonCompilerMacros.cmake
@@ -211,6 +211,9 @@ function(cmake_create_cxx_import_std std variable)
     26)
   list(FIND _cmake_supported_import_std_features "${std}" _cmake_supported_import_std_idx)
   if (_cmake_supported_import_std_idx EQUAL "-1")
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Unsupported C++ standard: C++${std}\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
   # If the target exists, skip. A toolchain file may have provided it.
@@ -219,14 +222,23 @@ function(cmake_create_cxx_import_std std variable)
   endif ()
   # The generator must support imported C++ modules.
   if (NOT CMAKE_GENERATOR MATCHES "Ninja")
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Unsupported generator: ${CMAKE_GENERATOR}\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
   # Check if the compiler understands how to `import std;`.
   include("${CMAKE_ROOT}/Modules/Compiler/${CMAKE_CXX_COMPILER_ID}-CXX-CXXImportStd.cmake" OPTIONAL RESULT_VARIABLE _cmake_import_std_res)
   if (NOT _cmake_import_std_res)
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Toolchain does not support discovering `import std` support\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
   if (NOT COMMAND _cmake_cxx_import_std)
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Toolchain does not provide `import std` discovery command\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
@@ -237,6 +249,9 @@ function(cmake_create_cxx_import_std std variable)
     "CxxImportStd"
     _cmake_supported_import_std_experimental)
   if (NOT _cmake_supported_import_std_experimental)
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Experimental `import std` support not enabled when detecting toolchain\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
diff --git a/Modules/Compiler/Clang-CXX-CXXImportStd.cmake b/Modules/Compiler/Clang-CXX-CXXImportStd.cmake
index 3934db92ff..55dbfb85bf 100644
--- a/Modules/Compiler/Clang-CXX-CXXImportStd.cmake
+++ b/Modules/Compiler/Clang-CXX-CXXImportStd.cmake
@@ -14,11 +14,17 @@ function (_cmake_cxx_import_std std variable)
     OUTPUT_STRIP_TRAILING_WHITESPACE
     ERROR_STRIP_TRAILING_WHITESPACE)
   if (_clang_libcxx_modules_json_file_res)
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Could not find `libc++.modules.json` resource\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
   # Without this file, we do not have modules installed.
   if (NOT EXISTS "${_clang_libcxx_modules_json_file}")
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`libc++.modules.json` resource does not exist\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
@@ -26,6 +32,9 @@ function (_cmake_cxx_import_std std variable)
     # The original PR had a key spelling mismatch internally. Do not support it
     # and instead require a release known to have the fix.
     # https://github.com/llvm/llvm-project/pull/83036
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"LLVM 18.1.2 is required for `libc++.modules.json` format fix\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
@@ -34,12 +43,18 @@ function (_cmake_cxx_import_std std variable)
   string(JSON _clang_modules_json_revision GET "${_clang_libcxx_modules_json}" "revision")
   # Require version 1.
   if (NOT _clang_modules_json_version EQUAL "1")
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`libc++.modules.json` version ${_clang_modules_json_version}.${_clang_modules_json_revision} is not recognized\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
   string(JSON _clang_modules_json_nmodules LENGTH "${_clang_libcxx_modules_json}" "modules")
   # Don't declare the target without any modules.
   if (NOT _clang_modules_json_nmodules)
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`libc++.modules.json` does not list any available modules\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
diff --git a/Modules/Compiler/MSVC-CXX-CXXImportStd.cmake b/Modules/Compiler/MSVC-CXX-CXXImportStd.cmake
index f253f9ec66..08199f7e6c 100644
--- a/Modules/Compiler/MSVC-CXX-CXXImportStd.cmake
+++ b/Modules/Compiler/MSVC-CXX-CXXImportStd.cmake
@@ -12,6 +12,9 @@ function (_cmake_cxx_import_std std variable)
     NO_CACHE)
   # Without this file, we do not have modules installed.
   if (NOT EXISTS "${_msvc_modules_json_file}")
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"Could not find `modules.json` resource\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
@@ -20,18 +23,27 @@ function (_cmake_cxx_import_std std variable)
   string(JSON _msvc_json_revision GET "${_msvc_modules_json}" "revision")
   # Require version 1.
   if (NOT _msvc_json_version EQUAL "1")
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`modules.json` version ${_msvc_json_version}.${_msvc_json_revision} is not recognized\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
   string(JSON _msvc_json_library GET "${_msvc_modules_json}" "library")
   # Bail if we don't understand the library.
   if (NOT _msvc_json_library STREQUAL "microsoft/STL")
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`modules.json` library `${_msvc_json_library}` is not recognized\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
   string(JSON _msvc_json_nmodules LENGTH "${_msvc_modules_json}" "module-sources")
   # Don't declare the target without any modules.
   if (NOT _msvc_json_nmodules)
+    set("${variable}"
+      "set(CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE \"`modules.json` does not list any available modules\")\n"
+      PARENT_SCOPE)
     return ()
   endif ()
 
diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 75f3b6d7ed..c1c04358a4 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -8583,15 +8583,22 @@ bool cmGeneratorTarget::ApplyCXXStdTargets()
       continue;
     }
 
-    auto const targetName = cmStrCat(
-      "__CMAKE::CXX", standardResolver.GetLevelString("CXX", *explicitLevel));
+    auto const stdLevel =
+      standardResolver.GetLevelString("CXX", *explicitLevel);
+    auto const targetName = cmStrCat("__CMAKE::CXX", stdLevel);
     if (!this->Makefile->FindTargetToUse(targetName)) {
+      auto basicReason = this->Makefile->GetDefinition(cmStrCat(
+        "CMAKE_CXX", stdLevel, "_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE"));
+      std::string reason;
+      if (!basicReason.IsEmpty()) {
+        reason = cmStrCat("  Reason:\n  ", basicReason);
+      }
       this->Makefile->IssueMessage(
         MessageType::FATAL_ERROR,
-        cmStrCat(
-          R"(The "CXX_MODULE_STD" property on the target ")", this->GetName(),
-          "\" requires that the \"", targetName,
-          "\" target exist, but it was not provided by the toolchain."));
+        cmStrCat(R"(The "CXX_MODULE_STD" property on the target ")",
+                 this->GetName(), "\" requires that the \"", targetName,
+                 "\" target exist, but it was not provided by the toolchain.",
+                 reason));
       break;
     }
 
diff --git a/Tests/RunCMake/CXXModules/NoCXX23TargetRequired-stderr.txt b/Tests/RunCMake/CXXModules/NoCXX23TargetRequired-stderr.txt
index 56468d0db7..e3d31c5a6e 100644
--- a/Tests/RunCMake/CXXModules/NoCXX23TargetRequired-stderr.txt
+++ b/Tests/RunCMake/CXXModules/NoCXX23TargetRequired-stderr.txt
@@ -1,4 +1,9 @@
 CMake Error in CMakeLists.txt:
   The "CXX_MODULE_STD" property on the target "nocxx23target" requires that
   the "__CMAKE::CXX23" target exist, but it was not provided by the
-  toolchain.
+  toolchain.  Reason:
+
+    (Toolchain does not support discovering `import std` support|Experimental `import std` support not enabled when detecting toolchain|Unsupported generator: [^\n]*)
+
+
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NoCXX26TargetNotRequired.cmake b/Tests/RunCMake/CXXModules/NoCXX26TargetNotRequired.cmake
new file mode 100644
index 0000000000..b91fa9b719
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX26TargetNotRequired.cmake
@@ -0,0 +1,11 @@
+enable_language(CXX)
+set(CMAKE_CXX_SCANDEP_SOURCE "echo")
+
+set(CMAKE_CXX_MODULE_STD 0)
+
+add_library(nocxx26target)
+target_sources(nocxx26target
+  PRIVATE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(nocxx26target PRIVATE cxx_std_26)
diff --git a/Tests/RunCMake/CXXModules/NoCXX26TargetRequired-result.txt b/Tests/RunCMake/CXXModules/NoCXX26TargetRequired-result.txt
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX26TargetRequired-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/NoCXX26TargetRequired-stderr.txt b/Tests/RunCMake/CXXModules/NoCXX26TargetRequired-stderr.txt
new file mode 100644
index 0000000000..8addcb2452
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX26TargetRequired-stderr.txt
@@ -0,0 +1,9 @@
+CMake Error in CMakeLists.txt:
+  The "CXX_MODULE_STD" property on the target "nocxx26target" requires that
+  the "__CMAKE::CXX26" target exist, but it was not provided by the
+  toolchain.  Reason:
+
+    (Toolchain does not support discovering `import std` support|Experimental `import std` support not enabled when detecting toolchain|Unsupported generator: [^\n]*)
+
+
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NoCXX26TargetRequired.cmake b/Tests/RunCMake/CXXModules/NoCXX26TargetRequired.cmake
new file mode 100644
index 0000000000..79dcb792e2
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX26TargetRequired.cmake
@@ -0,0 +1,11 @@
+enable_language(CXX)
+set(CMAKE_CXX_SCANDEP_SOURCE "echo")
+
+set(CMAKE_CXX_MODULE_STD 1)
+
+add_library(nocxx26target)
+target_sources(nocxx26target
+  PRIVATE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(nocxx26target PRIVATE cxx_std_26)
diff --git a/Tests/RunCMake/CXXModules/NoCXX26TargetUnset.cmake b/Tests/RunCMake/CXXModules/NoCXX26TargetUnset.cmake
new file mode 100644
index 0000000000..85108ff24a
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX26TargetUnset.cmake
@@ -0,0 +1,13 @@
+enable_language(CXX)
+set(CMAKE_CXX_SCANDEP_SOURCE "echo")
+
+# TODO(cxxmodules): Add instances of this test which test the policy
+# of the property's unset behavior.
+# set(CMAKE_CXX_MODULE_STD …)
+
+add_library(nocxx26target)
+target_sources(nocxx26target
+  PRIVATE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(nocxx26target PRIVATE cxx_std_26)
diff --git a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
index 8c76afcc40..8b7bc8628b 100644
--- a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
+++ b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
@@ -97,6 +97,13 @@ if ("cxx_std_23" IN_LIST CMAKE_CXX_COMPILE_FEATURES AND
   run_cmake(NoCXX23TargetRequired)
 endif ()
 
+if ("cxx_std_26" IN_LIST CMAKE_CXX_COMPILE_FEATURES AND
+    NOT have_cxx26_import_std)
+  run_cmake(NoCXX26TargetUnset)
+  run_cmake(NoCXX26TargetNotRequired)
+  run_cmake(NoCXX26TargetRequired)
+endif ()
+
 run_cmake(InstallBMI)
 run_cmake(InstallBMIGenericArgs)
 run_cmake(InstallBMIIgnore)