`ExecStart=|` does not work in user instances

[Stebalien]

systemd version the issue has been seen with

258

Used distribution

Arch Linux (CachyOS, technically)

Linux kernel version used

6.16.8-2-cachyos

CPU architectures issue was seen on

x86_64

Component

systemd

Expected behaviour you didn't see

I expected the following unit to work when in my systemd user manager:

[Unit]
Description=Test

[Service]
Type=oneshot
ExecStart=|echo test

Unexpected behaviour you saw

Starting the service failed and I see the following in my journal:

2025-09-19 14:30:22.837601 ERROR (sh)                 test.service: Failed to determine supplementary groups: Operation not permitted
2025-09-19 14:30:22.838182 ERROR (sh)                 test.service: Failed at step GROUP spawning /bin/sh: Operation not permitted

Steps to reproduce the problem

Create $XDG_CONFIG_HOME/systemd/user/test.service with the contents:

[Unit]
Description=Test

[Service]
Type=oneshot
ExecStart=|echo test

Run systemctl --user daemon-reload then systemctl --user start test.service.

Additional program output to the terminal or log subsystem illustrating the issue