Reasons to use viv, and reasons not to

Reasons to use viv

Besides matching Composer's output faster, viv does six things Composer does not.

See Merging composer.lock without conflicts, Running tools without installing them, A PHP per project, Automatic normalisation and Two plugins, one library for each in full, and Cache and offline use for the shared store.

Reasons not to use viv

• It stays 0.x. No 1.0 is planned. Minor releases can change behaviour and flags; the release notes and JOURNAL.md call those out. The output contract (vendor/ and composer.lock identical to Composer's) is the one thing that does not move.¹
• Windows is not supported. Linux and macOS only.
• Maintenance is on demand. The compatible mode is complete and no new plugin adapters or Composer commands are planned. A bug in it that a real project hits gets fixed; open an issue with the project's composer.json and lock. Releases continue as research chapters land, and the compat sweep and benchmark gates run on every change so the drop-in behaviour does not regress.
• Some Composer plugins stop the install. symfony/flex and any plugin without a native adapter make viv exit with an error naming the plugin. --no-plugins installs as Composer would without them, but the plugin's work is not done. Of the 10 pinned test projects, 1 is in this position: symfony/demo, for symfony/flex, which viv refuses by design.
• The shim needs a real Composer for everything else. It maps install, dump-autoload, normalize, create-project, update, require and remove to viv; search and the rest go to the Composer on your PATH, and with no real Composer installed they fail.
• vendor/ files are read-only by default. viv hardlinks them from a shared store, so an edit inside vendor/ fails instead of changing every project on the machine. If you patch vendor files by hand, install with --link-mode copy, or --link-mode clone for writable files sharing the store's disk space where the filesystem supports it.
• A minimum-stability gap. A version filtered out by minimum-stability is still reported as not found rather than as filtered; Composer names the cause.

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