Requirements

A host running PHP-FPM. fpm-tune discovers masters by scanning the process table, parses their effective configuration with php-fpm -tt, scrapes pool status over FastCGI, validates with php-fpm -t, and reloads with SIGUSR2. It runs where php-fpm runs, which in practice means Linux. On macOS it builds and the allocator's logic runs, but the cgroup budget detection and the process scan have no equivalent.
Permission to see and signal the master. Discovery reads other processes' details, and applying signals the master, so it wants to run as root or as the php-fpm user. Running as a user that cannot read the master's cgroup is detected and refused rather than sized against the whole machine; see The budget.
The status page enabled, on the pools you want measured. Worker memory is read from the pids the status page reports, so a pool with no pm.status_path is sized from a profile guess rather than from measurement.
A writable state directory, /var/lib/fpm-tune by default, for the learned baselines and (during a change) the backup and recovery record. It is created on first run; under systemd, StateDirectory=fpm-tune gives it sensible ownership.
Go 1.26 or newer to build from source. Released binaries have no runtime dependency beyond php-fpm itself.

The tool holds no long-running connection to php-fpm and needs no agent inside it. It reads, decides, optionally writes one file and sends one signal, and otherwise stays out of the way.