There aren't any documented settings to disable this, but there is a feature flag it currently sits behind. You can flip it in your ~/.codex/config.toml[1]:

  [features]
  daemon_auto_start = false

There are also some files that it writes to in ~/.codex at startup: pid files, lock files, things like that. You can delete them and replace them with symlinks to /dev/null, then mark those unwritable with `chattr +i` or similar. Presumably then the background daemon will crash on startup, if the feature flag ever goes away.

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1: https://github.com/openai/codex/blob/0c329ad2c9d5a2592128a1d...