Measured on both shapes: ping returns success:true and healthcheck returns OK at the root whether or not the app is mounted under a prefix, which is why the phone app's connectivity check works before it ever resolves the base path. The paragraph now says which routes were probed instead of asserting 200s in general, and the frontmatter description fits the OKF 120-char budget the linter enforces.
The page owned the URL as a plain host:port; every client now carries the /audiobookshelf prefix. Added the two laws a next operator needs (why the base path is not a choice, why the redirect subfolder must be sent) and the Single sign-on section naming which script owns which half.
The live round trip died on authentik with Redirect URI Error. ABS builds redirect_uri as <origin><authOpenIDSubfolderForRedirectURLs>/auth/openid/callback, and that key is unset on a database created before SSO (the 2.17.4 migration only fills a DB that already had OIDC enabled), so the URL carried the literal string undefined. Arming through the API therefore has to send the value; it is the mounted base path, so it is derived from ABS_URL rather than becoming a second knob an operator has to keep in sync.
authentik-audiobookshelf-sso.sh is the IdP half kept in git: provider, groups, the abs_groups scope mapping and the redirect URIs. That state lives in authentik's database, and a blueprint re-import reverts hand-made rows silently, so it needs a re-runnable door. Tokens are HS256 because no provider on the fleet carries a signing key, which makes the client secret the signature key.
The stub server now records the auth-settings PATCH body, so the seam asserts the subfolder that reaches the wire follows ABS_URL for both the prefixed and root shapes, and a second seam pins that the provider script and the bootstrapper build the same redirect path.
ROUTER_BASE_PATH=/ served a healthy API and a web UI that could never boot. The index.html in the pinned image is built with routerBasePath:/audiobookshelf, so every script request fell through to the SPA fallback and came back as text/html, which Chrome refuses to execute. healthcheck, the API and the phone app all stayed fine, which is exactly how this hid behind an API-only deploy check.
The deployer now reads the mounted base path out of the unit and hands it to the bootstrapper as ABS_URL, so the API door, the login route and the OIDC redirect subfolder cannot drift from the unit. healthcheck answers at the root under either base path, so the health gate is unchanged.
Frank