Hi     I am trying to run a simulation ensemble where the system MPI coordinates many tasks, each of which is a call to a plain MPICH-built app (ExaEpi/AMReX). The app runs on its own, but when called from inside the bigger MPI job, it causes an error in MPI_Init(). This approach works on other systems like Perlmutter. I think I can reproduce this with something as simple as: /opt/cray/pals/1.8/bin/mpiexec \ env $UNSETS \ /lus/flare/projects/EpiCalib/sfw/mpich-git/bin/mpiexec -n 2 \ ~/../main.x where main.x is a toy MPI program, mpich-git is my MPICH build on Aurora, and UNSETS is a variety of attempts to unset environment variables, none of this has worked so far. Is there any known reason why I am unable to run MPICH inside this context, when it does run on its own? Also, any other tips for debugging MPI startup might help. I am using the following but not getting much detail: export PMI_DEBUG=1 export PMIX_DEBUG=1 export HYDRA_DEBUG=1 I get: [mpiexec@x4217c5s7b0n0] Launch arguments: /lus/flare/projects/EpiCalib/sfw/mpich-git/bin/hydra_pmi_proxy --control-port x4217c5s7b0n0:37193 --debug --rmk user --launcher ssh --demux poll --pgid 0 --retries 10 --usize -2 --pmi-port 0 --gpus-per-proc -2 --gpu-subdevs-per-proc -2 --proxy-id 0 [proxy:0@x4217c5s7b0n0] Sending upstream hdr.cmd = CMD_PID_LIST [proxy:0@x4217c5s7b0n0] Sending upstream hdr.cmd = CMD_STDERR Abort(16): Fatal error in internal_Init: Internal MPI error! [proxy:0@x4217c5s7b0n0] Sending upstream hdr.cmd = CMD_STDERR Abort(16): Fatal error in internal_Init: Internal MPI error! [proxy:0@x4217c5s7b0n0] Sending upstream hdr.cmd = CMD_EXIT_STATUS x4217c5s7b0n0.hsn.cm.aurora.alcf.anl.gov: rank 0 exited with code 16     Thanks     Justin -- Justin M Wozniak