diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4b9588d..bfec365 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -62,6 +62,23 @@ target_compile_definitions(shum
   PUBLIC
   ${SHUM_DEFINES})
 
+# BUILD_OPENMP goes as far as find_package(OpenMP) in ShumOptions.cmake, but
+# the imported targets have to be linked for the option to have any effect.
+# Without this, no -fopenmp reaches a compile line, _OPENMP is never defined
+# and the OpenMP regions are preprocessed away, so a CMake build produces a
+# serial library where the Makefile build (SHUM_OPENMP ?= true) produces a
+# threaded one.
+#
+# PRIVATE is sufficient: _OPENMP is used inside c_shum_byteswap.c, never in an
+# installed header, so consumers of shumlib do not need OpenMP flags of their
+# own.
+if(BUILD_OPENMP)
+  target_link_libraries(shum
+    PRIVATE
+    OpenMP::OpenMP_C
+    OpenMP::OpenMP_Fortran)
+endif()
+
 # Install shumlib and the C header files for the sub-libraries that
 # have them
 install(TARGETS shum
@@ -95,6 +112,16 @@ if(BUILD_TESTS)
     PUBLIC ${CMAKE_Fortran_MODULE_DIRECTORY})
   target_link_libraries(shumlib-tests PUBLIC fruit shum)
 
+  # The thread_utils tests exercise the OpenMP paths directly, and shumlib's
+  # OpenMP linkage is PRIVATE, so the test executable needs the flags in its
+  # own right.
+  if(BUILD_OPENMP)
+    target_link_libraries(shumlib-tests
+      PRIVATE
+      OpenMP::OpenMP_C
+      OpenMP::OpenMP_Fortran)
+  endif()
+
   add_subdirectory(fruit)
   setup_shum_fruit()
 
diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index e4f90b3..4d1f42f 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -1,8 +1,9 @@
 # Contributors
 
-| GitHub user     | Real Name        | Affiliation | Date       |
-| --------------- | ---------------- | ----------- | ---------- |
-| james-bruten-mo | James Bruten     | Met Office  | 2025-12-09 |
-| t00sa           | Sam Clarke-Green | Met Office  | 2026-02-10 |
-| jennyhickson    | Jenny Hickson    | Met Office  | 2026-03-02 |
-| yaswant         | Yaswant Pradhan  | Met Office  | 2026-07-17 |
+| GitHub user     | Real Name        | Affiliation          | Date       |
+| --------------- | ---------------- | -----------          | ---------- |
+| james-bruten-mo | James Bruten     | Met Office           | 2025-12-09 |
+| t00sa           | Sam Clarke-Green | Met Office           | 2026-02-10 |
+| jennyhickson    | Jenny Hickson    | Met Office           | 2026-03-02 |
+| yaswant         | Yaswant Pradhan  | Met Office           | 2026-07-17 |
+| ickc            | Kolen Cheung     | University of Exeter | 2026-09-16 |
diff --git a/shum_thread_utils/test/c_fruit_test_shum_thread_utils.c b/shum_thread_utils/test/c_fruit_test_shum_thread_utils.c
index 2e19faa..76e45cb 100644
--- a/shum_thread_utils/test/c_fruit_test_shum_thread_utils.c
+++ b/shum_thread_utils/test/c_fruit_test_shum_thread_utils.c
@@ -58,6 +58,7 @@ typedef struct DISPATCH_PACK {
   int64_t iend;
   int64_t incr;
   int64_t threads;
+  int64_t team;
   size_t exp_iter;
   size_t *act_iter;
   size_t tot_act_iter;
@@ -1135,6 +1136,7 @@ void c_test_startOMPparallel(bool *test_ret, int64_t *threads)
 
   pack.test_ret = 0;
   pack.threads = *threads;
+  pack.team = 1;
 
   f_shum_startOMPparallel((void **)&pack_ptr,
                           &OMPparallel_dispatch);
@@ -1162,6 +1164,7 @@ void c_test_startOMPparallelfor(bool *test_ret, int64_t *in_istart,
   pack.iend = *in_iend;
   pack.incr = *in_incr;
   pack.threads = *threads;
+  pack.team = 1;
 
   pack.exp_iter = 0;
   pack.tot_act_iter = 0;
@@ -1185,8 +1188,10 @@ void c_test_startOMPparallelfor(bool *test_ret, int64_t *in_istart,
                              in_iend,
                              in_incr);
 
-  /* Check the thread division was balanced */
-  for(i = 1; i < *threads; i++)
+  /* Check the thread division was balanced across the team we actually got,
+   * which may be smaller than requested (e.g. OMP_THREAD_LIMIT)
+   */
+  for(i = 1; i < pack.team; i++)
   {
     if (pack.act_iter[0] - pack.act_iter[i] > 1)
     {
@@ -1200,7 +1205,7 @@ void c_test_startOMPparallelfor(bool *test_ret, int64_t *in_istart,
 
   /* Check the sum of local iteration counts is the total count */
 
-  for(i = 0; i < *threads; i++)
+  for(i = 0; i < pack.team; i++)
   {
     pack.tot_act_iter += pack.act_iter[i];
   }
@@ -1256,9 +1261,14 @@ static void OMPparallel_dispatch(void **const shareptr)
 {
   dispatch_pack *pack = (dispatch_pack *)*shareptr;
 
-  /* Check we have a parallel region */
+  /* Check we have a parallel region; a team of one is not an active one */
+
+  if (f_shum_threadID() == 0)
+  {
+    pack->team = f_shum_numThreads();
+  }
 
-  pack->test_ret = f_shum_inPar() || (pack->threads == 1);
+  pack->test_ret = f_shum_inPar() || (f_shum_numThreads() == 1);
 }
 
 /******************************************************************************/
@@ -1288,7 +1298,7 @@ static void OMPparallelfor_dispatch(void **const shareptr,
        * This may be on purpose (to cause zero iterations on this thread) if
        * there are more threads than iterations.
        */
-      if ((*iend-*istart) > pack->threads && pack->exp_iter != 0)
+      if ((*iend-*istart) > f_shum_numThreads() && pack->exp_iter != 0)
       {
         pack->test_ret = 0;
       }
@@ -1302,7 +1312,7 @@ static void OMPparallelfor_dispatch(void **const shareptr,
        * This may be on purpose (to cause zero iterations on this thread) if
        * there are more threads than iterations.
        */
-      if ((*iend-*istart) > pack->threads && pack->exp_iter != 0)
+      if ((*iend-*istart) > f_shum_numThreads() && pack->exp_iter != 0)
       {
        pack->test_ret = 0;
       }
diff --git a/shum_thread_utils/test/fruit_test_shum_thread_utils.f90 b/shum_thread_utils/test/fruit_test_shum_thread_utils.f90
index 8b1b966..f4f667b 100644
--- a/shum_thread_utils/test/fruit_test_shum_thread_utils.f90
+++ b/shum_thread_utils/test/fruit_test_shum_thread_utils.f90
@@ -673,6 +673,7 @@ SUBROUTINE test_release_pending_locks
 
 LOGICAL(KIND=C_BOOL) :: test_ret
 INTEGER(KIND=C_INT64_T) :: lock
+INTEGER :: team_size
 
 lock = -1
 
@@ -682,12 +683,19 @@ SUBROUTINE test_release_pending_locks
 CALL assert_true(LOGICAL(lock>0,KIND=C_BOOL),                                  &
                "Did not generate single lock succesfully [invalid lock number]")
 
+team_size = 3
 !$ CALL omp_set_num_threads(3)
-!$OMP PARALLEL DEFAULT(NONE) SHARED(lock) REDUCTION(.AND.:test_ret)
+!$OMP PARALLEL DEFAULT(NONE) SHARED(lock, team_size)                         &
+!$OMP REDUCTION(.AND.:test_ret)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$OMP BARRIER
 
-CALL c_test_release_pending_locks(test_ret,lock)
+IF (team_size >= 3) CALL c_test_release_pending_locks(test_ret,lock)
 
 !$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, 3)
 
 CALL assert_true(test_ret,                                                     &
                 "Did not handle/detect release of a lock with pending requests")
@@ -733,6 +741,7 @@ END SUBROUTINE test_unlock
 
 !$ LOGICAL(KIND=C_BOOL) :: test_ret
 !$ INTEGER(KIND=C_INT64_T) :: lock
+!$ INTEGER :: team_size
 
 !$ lock = -1
 
@@ -744,9 +753,14 @@ END SUBROUTINE test_unlock
 !$             "Did not generate single lock succesfully [invalid lock number]")
 
 !$ CALL omp_set_num_threads(2)
-!$OMP PARALLEL DEFAULT(NONE) SHARED(lock) REDUCTION(.AND.:test_ret)
-!$ CALL c_test_unlock_foreign(test_ret, lock)
+!$OMP PARALLEL DEFAULT(NONE) SHARED(lock, team_size)                         &
+!$OMP REDUCTION(.AND.:test_ret)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$ IF (omp_get_num_threads() >= 2) CALL c_test_unlock_foreign(test_ret, lock)
 !$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, 2)
 !$ CALL assert_true(test_ret,                                                  &
 !$          "Did not correctly handle unlocking a lock owner by another thread")
 
@@ -760,6 +774,7 @@ END SUBROUTINE test_unlock
 
 !$ LOGICAL(KIND=C_BOOL) :: test_ret
 !$ INTEGER(KIND=C_INT64_T) :: lock
+!$ INTEGER :: team_size
 
 !$ CALL set_case_name("test_lock_blocking")
 
@@ -769,9 +784,14 @@ END SUBROUTINE test_unlock
 !$             "Did not generate single lock succesfully [invalid lock number]")
 
 !$ CALL omp_set_num_threads(3)
-!$OMP PARALLEL DEFAULT(NONE) SHARED(lock) REDUCTION(.AND.:test_ret)
-!$ CALL c_test_lock_blocking(test_ret, lock)
+!$OMP PARALLEL DEFAULT(NONE) SHARED(lock, team_size)                         &
+!$OMP REDUCTION(.AND.:test_ret)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$ IF (omp_get_num_threads() >= 3) CALL c_test_lock_blocking(test_ret, lock)
 !$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, 3)
 !$ CALL assert_true(test_ret,"Set locks did not correctly block another thread")
 
 !$ END SUBROUTINE test_lock_blocking
@@ -784,6 +804,7 @@ END SUBROUTINE test_unlock
 
 !$ LOGICAL(KIND=C_BOOL) :: test_ret
 !$ INTEGER(KIND=C_INT64_T) :: lock
+!$ INTEGER :: team_size
 
 !$ CALL set_case_name("test_lockqueue_multi")
 
@@ -793,9 +814,14 @@ END SUBROUTINE test_unlock
 !$             "Did not generate single lock succesfully [invalid lock number]")
 
 !$ CALL omp_set_num_threads(3)
-!$OMP PARALLEL DEFAULT(NONE) SHARED(lock) REDUCTION(.AND.:test_ret)
-!$ CALL c_test_lockqueue_multi(test_ret, lock)
+!$OMP PARALLEL DEFAULT(NONE) SHARED(lock, team_size)                         &
+!$OMP REDUCTION(.AND.:test_ret)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$ IF (omp_get_num_threads() >= 3) CALL c_test_lockqueue_multi(test_ret, lock)
 !$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, 3)
 !$ CALL assert_true(test_ret, "Set locks did not queue correctly")
 
 !$ END SUBROUTINE test_lockqueue_multi
@@ -808,6 +834,7 @@ END SUBROUTINE test_unlock
 
 !$ LOGICAL(KIND=C_BOOL) :: test_ret
 !$ INTEGER(KIND=C_INT64_T) :: lock
+!$ INTEGER :: team_size
 
 !$ CALL set_case_name("test_testlock_nonblocking")
 
@@ -817,14 +844,42 @@ END SUBROUTINE test_unlock
 !$             "Did not generate single lock succesfully [invalid lock number]")
 
 !$ CALL omp_set_num_threads(3)
-!$OMP PARALLEL DEFAULT(NONE) SHARED(lock) REDUCTION(.AND.:test_ret)
-!$ CALL c_test_testlock_nonblocking(test_ret, lock)
+!$OMP PARALLEL DEFAULT(NONE) SHARED(lock, team_size)                         &
+!$OMP REDUCTION(.AND.:test_ret)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$ IF (omp_get_num_threads() >= 3)                                             &
+!$   CALL c_test_testlock_nonblocking(test_ret, lock)
 !$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, 3)
 !$ CALL assert_true(test_ret,"Set locks did not correctly block another thread")
 
 !$ END SUBROUTINE test_testlock_nonblocking
 
 
+!------------------------------------------------------------------------------!
+
+! omp_set_num_threads only requests a team size; the runtime may give fewer
+! threads (e.g. OMP_THREAD_LIMIT, nesting). Tests that depend on the exact
+! team skip their checks in that case and fail here instead: the multi-
+! threaded C lock tests would otherwise spin forever waiting for a thread ID
+! that doesn't exist.
+
+!$ SUBROUTINE assert_team_size(team_size, required)
+
+!$ IMPLICIT NONE
+
+!$ INTEGER, INTENT(IN) :: team_size
+!$ INTEGER, INTENT(IN) :: required
+!$ CHARACTER(LEN=80) :: message
+
+!$ WRITE(message, "(A,I0,A,I0)") "Test needs an OpenMP team of ", required,    &
+!$                               " threads, got ", team_size
+!$ CALL assert_true(LOGICAL(team_size >= required, KIND=C_BOOL), TRIM(message))
+
+!$ END SUBROUTINE assert_team_size
+
 !------------------------------------------------------------------------------!
 
 SUBROUTINE test_lock_invalid_lock
@@ -990,6 +1045,7 @@ SUBROUTINE test_inpar
 
 LOGICAL(KIND=C_BOOL) :: test_ret
 !$ LOGICAL :: working
+!$ INTEGER :: team_size
 INTEGER(KIND=C_INT64_T) :: par
 
 par = -1
@@ -1008,15 +1064,22 @@ SUBROUTINE test_inpar
 !$OMP END PARALLEL
 !$ CALL assert_true(test_ret, "Did not succesfully call c_shum_inPar()")
 
-!$OMP PARALLEL DEFAULT(NONE) PRIVATE(par, working) REDUCTION(.AND.:test_ret)
-!$ par = -1
-!$ CALL c_test_inpar(test_ret,par)
-!$ working = (par == 1)
-!$ IF (working .NEQV. omp_in_parallel()) THEN
-!$   working = .FALSE.
+!$OMP PARALLEL DEFAULT(NONE) SHARED(team_size) PRIVATE(par, working)         &
+!$OMP REDUCTION(.AND.:test_ret)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$ IF (omp_get_num_threads() >= 2) THEN
+!$   par = -1
+!$   CALL c_test_inpar(test_ret,par)
+!$   working = (par == 1)
+!$   IF (working .NEQV. omp_in_parallel()) THEN
+!$     working = .FALSE.
+!$   END IF
+!$   test_ret = working
 !$ END IF
-!$ test_ret = working
 !$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, 2)
 !$ CALL assert_true(test_ret, "c_shum_inPar() did not detect a parallel region")
 
 END SUBROUTINE test_inpar
@@ -1031,6 +1094,7 @@ SUBROUTINE test_threadid
 INTEGER(KIND=C_INT64_T) :: tid
 
 !$ INTEGER :: i
+!$ INTEGER :: team_size
 
 tid = -1
 
@@ -1042,14 +1106,22 @@ SUBROUTINE test_threadid
 CALL assert_true(test_ret, "Thread ID not zero outside parallel region")
 
 !$ CALL omp_set_num_threads(3)
-!$OMP PARALLEL DO SCHEDULE(static, 1) DEFAULT(NONE) PRIVATE(tid, i)            &
+!$OMP PARALLEL DEFAULT(NONE) SHARED(team_size) PRIVATE(tid, i)                &
 !$OMP REDUCTION(.AND.:test_ret)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$OMP DO SCHEDULE(static, 1)
 !$ DO i=0,2
-!$ CALL c_test_threadid(test_ret,tid)
-!$ IF (tid/=i) test_ret = .FALSE.
-!$ IF (tid/=omp_get_thread_num()) test_ret = .FALSE.
+!$ IF (omp_get_num_threads() >= 3) THEN
+!$   CALL c_test_threadid(test_ret,tid)
+!$   IF (tid/=i) test_ret = .FALSE.
+!$   IF (tid/=omp_get_thread_num()) test_ret = .FALSE.
+!$ END IF
 !$ END DO
-!$OMP END PARALLEL DO
+!$OMP END DO
+!$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, 3)
 !$ CALL assert_true(test_ret, "Thread IDs not calculated correctly" //         &
 !$                            " in parallel region")
 
@@ -1089,6 +1161,7 @@ SUBROUTINE test_barrier
 
 LOGICAL(KIND=C_BOOL) :: test_ret, test_ret_arr(max_threads)
 INTEGER(KIND=C_INT64_T) :: shared1(max_threads), tid
+INTEGER :: team_size
 
 CALL set_case_name("test_threadbarrier")
 
@@ -1096,20 +1169,26 @@ SUBROUTINE test_barrier
 test_ret = .FALSE.
 test_ret_arr = .FALSE.
 tid = 1
+team_size = max_threads
 
 !$ CALL omp_set_num_threads(max_threads)
-!$OMP PARALLEL DEFAULT(NONE) SHARED(shared1,test_ret_arr)                      &
+!$OMP PARALLEL DEFAULT(NONE) SHARED(shared1,test_ret_arr,team_size)            &
 !$OMP PRIVATE(tid)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$OMP BARRIER
 
 !$ tid = omp_get_thread_num() + 1
-CALL c_test_barrier(test_ret_arr(tid),shared1)
+IF (team_size >= max_threads) CALL c_test_barrier(test_ret_arr(tid),shared1)
 
 !$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, max_threads)
 
 test_ret = test_ret_arr(1)
 !$ test_ret = ALL(test_ret_arr)
 
-CALL assert_true(test_ret, "Barrier test fails!")
+IF (team_size >= max_threads) CALL assert_true(test_ret, "Barrier test fails!")
 
 END SUBROUTINE test_barrier
 
@@ -1123,6 +1202,7 @@ SUBROUTINE test_numthreads
 INTEGER(KIND=C_INT64_T) :: numthreads
 
 !$ INTEGER :: i
+!$ INTEGER :: team_size
 
 numthreads = -1
 
@@ -1144,14 +1224,22 @@ SUBROUTINE test_numthreads
 
 
 !$ CALL omp_set_num_threads(3)
-!$OMP PARALLEL DO SCHEDULE(static, 1) DEFAULT(NONE) PRIVATE(numthreads, i)     &
+!$OMP PARALLEL DEFAULT(NONE) SHARED(team_size) PRIVATE(numthreads, i)         &
 !$OMP REDUCTION(.AND.:test_ret)
+!$OMP MASTER
+!$ team_size = omp_get_num_threads()
+!$OMP END MASTER
+!$OMP DO SCHEDULE(static, 1)
 !$ DO i=0,2
-!$ CALL c_test_numthreads(test_ret,numthreads)
-!$ IF (numthreads/=3) test_ret = .FALSE.
-!$ IF (numthreads/=omp_get_num_threads()) test_ret = .FALSE.
+!$ IF (omp_get_num_threads() >= 3) THEN
+!$   CALL c_test_numthreads(test_ret,numthreads)
+!$   IF (numthreads/=3) test_ret = .FALSE.
+!$   IF (numthreads/=omp_get_num_threads()) test_ret = .FALSE.
+!$ END IF
 !$ END DO
-!$OMP END PARALLEL DO
+!$OMP END DO
+!$OMP END PARALLEL
+!$ CALL assert_team_size(team_size, 3)
 !$ CALL assert_true(test_ret, "Thread count not consistent across threads" //  &
 !$                            " in parallel region")