`kill_signal = 'SIGUSR2'` rejected but docs explicitly state that it is supported

[bcheidemann]

Please only report specific issues with flyctl behavior. Anything like a support request for your application should go to https://community.fly.io. More people watch that space and can help you faster!

I'm not sure if this is an issue with flyctl or the fly.io infrastructure, so perhaps I'm posting in the wrong place. In any case, previous reports on https://community.fly.io were closed without response or resolution. In any case, I've also posted this issue here.

Describe the bug
Briefly, describe what broke and provide the following details:

• Operating system: Linux
• fly version: flyctl v0.4.1 linux/amd64 Commit: 7e342cd5ef931c5d037a33924de85f66839958ef BuildDate: 2026-01-05T16:42:17Z

** Paste your fly.toml

app = '...'
primary_region = 'iad'
# EXPLANATION: Use lame duck mode to gracefully shutdown the NATS server.
# SEE: https://docs.nats.io/running-a-nats-service/nats_admin/lame_duck_mode
# SEE: https://docs.nats.io/running-a-nats-service/nats_admin/signals#command-line
kill_signal = 'SIGUSR2'
# EXPLANATION: Wait for NATS server to gracefully shutdown (lame_duck_duration + 30s)
# SEE: https://docs.nats.io/running-a-nats-service/configuration#runtime-configuration
kill_timeout = 150

[build]
    dockerfile = 'Dockerfile'

[[vm]]
    memory = '256mb'
    cpus = 1
    memory_mb = 256

[deploy]
    strategy = 'rolling'

** Command output: **

$ flyctl deploy
...
Watch your deployment at https://fly.io/apps/obsidian-nats-primary/monitoring

-------
Updating existing machines in 'obsidian-nats-primary' with rolling strategy
Failed to update machines: failed to update machine d8d1774a1e57e8: failed to update VM d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGSEGV SIGUSR1 SIGALRM SIGFPE SIGHUP SIGQUIT SIGTERM SIGTRAP SIGABRT SIGILL SIGINT SIGKILL SIGPIPE] Retrying...7e8: invalid stop_config.signal, must be one of [SIGSEGV SIGUSR1 SIGALRM SIG…
Failed to update machines: failed to update machine d8d1774a1e57e8: failed to update VM d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGUSR1 SIGALRM SIGFPE SIGHUP SIGQUIT SIGTERM SIGTRAP SIGABRT SIGILL SIGINT SIGKILL SIGPIPE SIGSEGV] Retrying...7e8: invalid stop_config.signal, must be one of [SIGUSR1 SIGALRM SIGFPE SIGH…
Failed to update machines: failed to update machine d8d1774a1e57e8: failed to update VM d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGUSR1 SIGALRM SIGFPE SIGHUP SIGQUIT SIGTERM SIGTRAP SIGABRT SIGILL SIGINT SIGKILL SIGPIPE SIGSEGV] Retrying...7e8: invalid stop_config.signal, must be one of [SIGUSR1 SIGALRM SIGFPE SIGH…
Failed to update machines: failed to update machine d8d1774a1e57e8: failed to update VM d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGUSR1 SIGALRM SIGFPE SIGHUP SIGQUIT SIGTERM SIGTRAP SIGABRT SIGILL SIGINT SIGKILL SIGPIPE SIGSEGV] Retrying...7e8: invalid stop_config.signal, must be one of [SIGUSR1 SIGALRM SIGFPE SIGH…
Failed to update machines: failed to update machine d8d1774a1e57e8: failed to update VM d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGPIPE SIGSEGV SIGUSR1 SIGALRM SIGFPE SIGHUP SIGQUIT SIGTERM SIGTRAP SIGABRT SIGILL SIGINT SIGKILL] Retrying...7e8: invalid stop_config.signal, must be one of [SIGPIPE SIGSEGV SIGUSR1 SIG…
Failed to update machines: failed to update machine d8d1774a1e57e8: failed to update VM d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGABRT SIGILL SIGINT SIGKILL SIGPIPE SIGSEGV SIGUSR1 SIGALRM SIGFPE SIGHUP SIGQUIT SIGTERM SIGTRAP] Retrying...7e8: invalid stop_config.signal, must be one of [SIGABRT SIGILL SIGINT SIGKI…
Failed to update machines: failed to update machine d8d1774a1e57e8: failed to update VM d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGILL SIGINT SIGKILL SIGPIPE SIGSEGV SIGUSR1 SIGALRM SIGFPE SIGHUP SIGQUIT SIGTERM SIGTRAP SIGABRT] d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGABRT SIGILL SIGINT SIGKI…

-------
 ✔ Cleared lease for d8d1774a1e57e8
-------
Error: failed to update machine d8d1774a1e57e8: failed to update VM d8d1774a1e57e8: invalid stop_config.signal, must be one of [SIGILL SIGINT SIGKILL SIGPIPE SIGSEGV SIGUSR1 SIGALRM SIGFPE SIGHUP SIGQUIT SIGTERM SIGTRAP SIGABRT] (Request ID: 01KECNZ62V55KRCQ80ECDCEVW3-lhr) (Trace ID: c5a4a3bd29874a8b18c96e6d994a1c31)

Fly docs:

... The kill_signal option lets you override that with a different signal so that you can trigger a softer, less disruptive shutdown: SIGTERM, SIGQUIT, SIGUSR1, SIGUSR2, SIGKILL, or SIGSTOP.