diff --git a/.github/workflows/auto-formatting.yml b/.github/workflows/auto-formatting.yml
index 7274f34038..589a626ed4 100644
--- a/.github/workflows/auto-formatting.yml
+++ b/.github/workflows/auto-formatting.yml
@@ -12,7 +12,8 @@ jobs:
       # Checkout the pull request branch, also include all history
       - uses: actions/checkout@v7
         with:
-          ref: ${{ github.head_ref }}
+          # Use the PR ref in the base repository so fork branches are resolved.
+          ref: refs/pull/${{ github.event.pull_request.number }}/head
           fetch-depth: 0
 
       - name: "Set up Python"
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 90fe822aa3..6737ab00e6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -560,10 +560,11 @@ set_target_properties(
 set(CONFIG_LDFLAGS "${CONFIG_LDFLAGS} -L\$BOUT_LIB_PATH -lbout++")
 set(BOUT_INCLUDE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/include")
 set(CONFIG_CFLAGS
-    "${CONFIG_CFLAGS} -I\${BOUT_INCLUDE_PATH} -I${CMAKE_CURRENT_BINARY_DIR}/include ${CMAKE_CXX_FLAGS} -std=c++20"
+    "${CONFIG_CFLAGS} -I\${BOUT_INCLUDE_PATH} -I${CMAKE_CURRENT_BINARY_DIR}/include ${CMAKE_CXX_FLAGS} -std=c++${BOUT_CXX_STANDARD}"
 )
 
-target_compile_features(bout++ PUBLIC cxx_std_20)
+target_compile_features(bout++ PUBLIC cxx_std_${BOUT_CXX_STANDARD})
+bout_target_set_language_standards(bout++)
 set_target_properties(bout++ PROPERTIES CXX_EXTENSIONS OFF)
 
 # Optional compiler features
@@ -783,6 +784,7 @@ option(
 if(BOUT_TESTS)
   set(CMAKE_CTEST_ARGUMENTS "--output-on-failure" "--timeout" "300")
   enable_testing()
+  add_subdirectory(tests/cmake)
   # Targets for just building the tests
   # Tests need to add themselves as dependencies to these targets
   add_custom_target(build-check-unit-tests)
@@ -803,7 +805,10 @@ if(BOUT_TESTS)
   # Targets for running the tests
   if(BOUT_ENABLE_UNIT_TESTS)
     add_custom_target(
-      check-unit-tests COMMAND ctest -R serial_tests --output-on-failure
+      check-unit-tests
+      COMMAND ${CMAKE_CTEST_COMMAND} -R serial_tests --output-on-failure
+      COMMAND ${CMAKE_CTEST_COMMAND} -R cmake-language-standards
+              --output-on-failure
     )
     add_dependencies(check-unit-tests build-check-unit-tests)
   endif()
diff --git a/cmake/BOUT++functions.cmake b/cmake/BOUT++functions.cmake
index be56977996..f8013c21ee 100644
--- a/cmake/BOUT++functions.cmake
+++ b/cmake/BOUT++functions.cmake
@@ -1,5 +1,27 @@
 # Helper functions for building BOUT++ models
 
+set(BOUT_CXX_STANDARD 20)
+
+# Apply the BOUT++ minimum to each dialect without changing directory defaults
+# or downgrading a newer standard selected by the caller.
+function(bout_target_set_language_standards TARGET_NAME)
+  foreach(language IN ITEMS CXX CUDA HIP)
+    get_target_property(standard ${TARGET_NAME} ${language}_STANDARD)
+    if(NOT standard
+       OR standard EQUAL 98
+       OR standard LESS BOUT_CXX_STANDARD
+    )
+      set_property(
+        TARGET ${TARGET_NAME} PROPERTY ${language}_STANDARD
+                                       ${BOUT_CXX_STANDARD}
+      )
+    endif()
+    set_property(
+      TARGET ${TARGET_NAME} PROPERTY ${language}_STANDARD_REQUIRED ON
+    )
+  endforeach()
+endfunction()
+
 # Copy FILENAME from source directory to build directory
 macro(bout_copy_file FILENAME)
   configure_file(
@@ -37,6 +59,7 @@ macro(bout_handle_requires_conflicts TYPENAME TYPEVAR)
 endmacro()
 
 function(bout_target_enable_hip TARGET_NAME)
+  bout_target_set_language_standards(${TARGET_NAME})
   if(BOUT_HAS_HIP)
     get_target_property(model_sources ${TARGET_NAME} SOURCES)
     list(FILTER model_sources INCLUDE REGEX ".*\\.(cxx|cpp|cc)$")
@@ -44,11 +67,7 @@ function(bout_target_enable_hip TARGET_NAME)
       set_source_files_properties(${model_sources} PROPERTIES LANGUAGE HIP)
     endif()
     set_target_properties(
-      ${TARGET_NAME}
-      PROPERTIES HIP_STANDARD 20
-                 HIP_STANDARD_REQUIRED ON
-                 HIP_EXTENSIONS OFF
-                 LINKER_LANGUAGE HIP
+      ${TARGET_NAME} PROPERTIES HIP_EXTENSIONS OFF LINKER_LANGUAGE HIP
     )
   endif()
 endfunction()
diff --git a/cmake/SetupBOUTThirdParty.cmake b/cmake/SetupBOUTThirdParty.cmake
index d6d3f18b1e..799b40c9e7 100644
--- a/cmake/SetupBOUTThirdParty.cmake
+++ b/cmake/SetupBOUTThirdParty.cmake
@@ -22,8 +22,6 @@ if(BOUT_HAS_CUDA)
   set(BOUT_SOURCES_CXX ${BOUT_SOURCES})
   list(FILTER BOUT_SOURCES_CXX INCLUDE REGEX ".*\.cxx")
 
-  # NOTE: CUDA inherits the CXX standard setting from the top-level
-  # compile features, set for the bout++ target.
   set_source_files_properties(${BOUT_SOURCES_CXX} PROPERTIES LANGUAGE CUDA)
 
   # timer.cxx contains no device code, and compiling its C++20 formatting and
@@ -43,10 +41,7 @@ if(BOUT_HAS_HIP)
   list(FILTER BOUT_HIP_SOURCES INCLUDE REGEX ".*\\.cxx$")
   set_source_files_properties(${BOUT_HIP_SOURCES} PROPERTIES LANGUAGE HIP)
   set_target_properties(
-    bout++
-    PROPERTIES HIP_STANDARD 20
-               HIP_STANDARD_REQUIRED ON
-               LINKER_LANGUAGE HIP
+    bout++ PROPERTIES HIP_EXTENSIONS OFF LINKER_LANGUAGE HIP
   )
   find_package(hip REQUIRED CONFIG)
   target_link_libraries(bout++ PUBLIC hip::host)
diff --git a/cmake/SetupCompilers.cmake b/cmake/SetupCompilers.cmake
index 8b8256a59c..4d76643ac0 100644
--- a/cmake/SetupCompilers.cmake
+++ b/cmake/SetupCompilers.cmake
@@ -56,9 +56,6 @@ if(BOUT_ENABLE_HIP)
     )
   endif()
   enable_language(HIP)
-  set(CMAKE_HIP_STANDARD 20)
-  set(CMAKE_HIP_STANDARD_REQUIRED ON)
-  set(CMAKE_HIP_EXTENSIONS OFF)
 endif()
 set(BOUT_HAS_HIP ${BOUT_ENABLE_HIP})
 set(CUDA_ARCH
diff --git a/tests/README.md b/tests/README.md
index 1defe24de5..211dfda42b 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -8,6 +8,17 @@ The BOUT++ test suite is split into three parts:
 
 <!-- TODO: Pylib, etc. -->
 
+## CMake tests
+
+The `cmake-language-standards` tests check target-local C++, CUDA and HIP
+standard settings and compile a C++23 probe when supported (C++20 otherwise).
+The GPU properties are inspected
+without enabling GPU languages, so these tests do not require CUDA or HIP
+toolchains and do not replace GPU compilation tests.
+
+Run them from a configured build directory with
+`ctest -R cmake-language-standards --output-on-failure`.
+
 ## Unit tests
 
 These aim to be a comprehensive set of tests that run *very* fast and
diff --git a/tests/cmake/CMakeLists.txt b/tests/cmake/CMakeLists.txt
new file mode 100644
index 0000000000..52a711e2a9
--- /dev/null
+++ b/tests/cmake/CMakeLists.txt
@@ -0,0 +1,14 @@
+add_test(
+  NAME cmake-language-standards
+  COMMAND
+    ${CMAKE_COMMAND} -S ${CMAKE_CURRENT_SOURCE_DIR}/language-standards -B
+    ${CMAKE_CURRENT_BINARY_DIR}/language-standards
+    -DBOUT_SOURCE_DIR=${PROJECT_SOURCE_DIR}
+)
+add_test(NAME cmake-language-standards-build
+         COMMAND ${CMAKE_COMMAND} --build
+                 ${CMAKE_CURRENT_BINARY_DIR}/language-standards
+)
+set_tests_properties(
+  cmake-language-standards-build PROPERTIES DEPENDS cmake-language-standards
+)
diff --git a/tests/cmake/language-standards/CMakeLists.txt b/tests/cmake/language-standards/CMakeLists.txt
new file mode 100644
index 0000000000..6494ee8d85
--- /dev/null
+++ b/tests/cmake/language-standards/CMakeLists.txt
@@ -0,0 +1,109 @@
+cmake_minimum_required(VERSION 3.17)
+project(BoutLanguageStandards LANGUAGES CXX)
+include("${BOUT_SOURCE_DIR}/cmake/BOUT++functions.cmake")
+
+function(expect_property target property expected)
+  get_target_property(actual ${target} ${property})
+  if(NOT actual STREQUAL expected)
+    message(
+      FATAL_ERROR "${target}.${property}: expected ${expected}, got ${actual}"
+    )
+  endif()
+endfunction()
+
+# GPU properties can be checked without a GPU compiler. This is a configuration
+# regression test, not a CUDA/HIP compilation or runtime test.
+add_library(defaults INTERFACE)
+bout_target_set_language_standards(defaults)
+foreach(language IN ITEMS CXX CUDA HIP)
+  expect_property(defaults ${language}_STANDARD 20)
+  expect_property(defaults ${language}_STANDARD_REQUIRED ON)
+  if(DEFINED CMAKE_${language}_STANDARD)
+    message(FATAL_ERROR "Helper changed ${language} directory default")
+  endif()
+  set(CMAKE_${language}_STANDARD 23)
+endforeach()
+
+add_library(newer INTERFACE)
+# Model an explicitly chosen target standard independently of CMake defaults.
+foreach(language IN ITEMS CXX CUDA HIP)
+  set_property(TARGET newer PROPERTY ${language}_STANDARD 23)
+endforeach()
+bout_target_set_language_standards(newer)
+foreach(language IN ITEMS CXX CUDA HIP)
+  expect_property(newer ${language}_STANDARD 23)
+  if(NOT CMAKE_${language}_STANDARD EQUAL 23)
+    message(FATAL_ERROR "Helper changed caller's ${language} default")
+  endif()
+endforeach()
+
+foreach(old_standard IN ITEMS 98 17)
+  add_library(older_${old_standard} INTERFACE)
+  foreach(language IN ITEMS CXX CUDA HIP)
+    set_property(
+      TARGET older_${old_standard} PROPERTY ${language}_STANDARD
+                                            ${old_standard}
+    )
+  endforeach()
+  bout_target_set_language_standards(older_${old_standard})
+  foreach(language IN ITEMS CXX CUDA HIP)
+    expect_property(older_${old_standard} ${language}_STANDARD 20)
+  endforeach()
+endforeach()
+
+add_library(unrelated INTERFACE)
+foreach(language IN ITEMS CXX CUDA HIP)
+  set_property(TARGET unrelated PROPERTY ${language}_STANDARD 17)
+  expect_property(unrelated ${language}_STANDARD 17)
+endforeach()
+
+set(BOUT_HAS_HIP ON)
+bout_target_enable_hip(newer)
+expect_property(newer HIP_STANDARD 23)
+expect_property(newer HIP_EXTENSIONS OFF)
+expect_property(newer LINKER_LANGUAGE HIP)
+bout_target_enable_hip(defaults)
+expect_property(defaults HIP_STANDARD 20)
+
+# Exercise SetupCompilers' HIP branch without requiring a ROCm installation.
+# Only language detection is stubbed; the production configuration runs.
+function(enable_language language)
+  if(NOT language STREQUAL "HIP")
+    message(FATAL_ERROR "Unexpected language detection: ${language}")
+  endif()
+  set(hip_detection_requested
+      ON
+      PARENT_SCOPE
+  )
+endfunction()
+set(BOUT_ENABLE_MPI OFF)
+set(BOUT_ENABLE_HIP ON)
+set(BOUT_ENABLE_CUDA OFF)
+set(BOUT_ENABLE_RAJA ON)
+set(BOUT_ENABLE_UMPIRE ON)
+set(CMAKE_HIP_STANDARD_REQUIRED OFF)
+set(CMAKE_HIP_EXTENSIONS ON)
+include("${BOUT_SOURCE_DIR}/cmake/SetupCompilers.cmake")
+if(NOT hip_detection_requested
+   OR NOT CMAKE_HIP_STANDARD EQUAL 23
+   OR CMAKE_HIP_STANDARD_REQUIRED
+   OR NOT CMAKE_HIP_EXTENSIONS
+)
+  message(FATAL_ERROR "HIP setup changed caller's directory defaults")
+endif()
+
+# Do not raise the project's supported compiler minimum just for this probe.
+set(BOUT_HAS_HIP OFF)
+if("cxx_std_23" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+  set(probe_standard 23)
+else()
+  set(probe_standard 20)
+endif()
+set(CMAKE_CXX_STANDARD ${probe_standard})
+add_executable(cpu_probe probe.cxx)
+bout_target_enable_hip(cpu_probe)
+expect_property(cpu_probe CXX_STANDARD ${probe_standard})
+if(probe_standard EQUAL 23)
+  target_compile_definitions(cpu_probe PRIVATE CHECK_CXX23)
+endif()
+message(STATUS "BOUT language standards regression checks passed")
diff --git a/tests/cmake/language-standards/probe.cxx b/tests/cmake/language-standards/probe.cxx
new file mode 100644
index 0000000000..22d099fb5b
--- /dev/null
+++ b/tests/cmake/language-standards/probe.cxx
@@ -0,0 +1,13 @@
+#ifdef _MSVC_LANG
+#define PROBE_CXX_STANDARD _MSVC_LANG
+#else
+#define PROBE_CXX_STANDARD __cplusplus
+#endif
+
+#if defined(CHECK_CXX23) && PROBE_CXX_STANDARD < 202100L
+#error "The caller's C++23 standard was not preserved"
+#endif
+#if PROBE_CXX_STANDARD < 202002L
+#error "The BOUT++ C++20 minimum was not applied"
+#endif
+int main() { return 0; }
diff --git a/tests/unit/CMakeLists.txt b/tests/unit/CMakeLists.txt
index d931239a70..88548aac89 100644
--- a/tests/unit/CMakeLists.txt
+++ b/tests/unit/CMakeLists.txt
@@ -158,5 +158,4 @@ bout_target_enable_hip(serial_tests)
 
 if(BOUT_HAS_CUDA)
   set_source_files_properties(${serial_tests_source} PROPERTIES LANGUAGE CUDA)
-  set_target_properties(serial_tests PROPERTIES CUDA_STANDARD 20)
 endif()