Admin surface visibility model — public, capability-gated, superuser-only

Context

Read capabilities (system.*.read, <domain>.read) let an operator hold a read-only admin role without the system.admin wildcard. The admin menu then gates per tile on the capability each surface actually enforces, so a read-only auditor sees the sections their caps permit.

That left one question unanswered: what does a tile with no declared capability mean? The original gate resolved it as

canSeeTile(tile) { return this.can(tile.cap ?? SYSTEM_ADMIN); }

— i.e. no capability implied system.admin. That is safe, but it also hid genuinely public content: the entire help section (docs, keyboard shortcuts, about, release notes, support) declares no capability, so a read-capability holder could not see About at all. On desktop the link survived only because the layout hard-coded a second path to it; the mobile drawer had no such hard-code, so the surface silently differed by viewport.

The obvious fix — treat a missing capability as “visible to everyone” — is wrong, and dangerously so. Only 28 of 100 declared tiles carry a capability. Flipping the default would have exposed the other 72, including Federation, System, and Storage, to any read-capability holder. That was caught only because the change was applied and then driven in a browser.

Decision

Admin tile visibility resolves against three explicit tiers:

public: true — visible to anyone who can reach the admin surface. Reserved for content that is not sensitive in any deployment: help, documentation, keyboard shortcuts, about, release notes, support.
cap: '<code>' — visible if and only if the caller holds that capability. system.admin wildcards every capability check, so a superuser always sees these.
Neither declared — superuser-only. This is the deliberate, conservative default for every tile that has not yet been migrated to a read capability.

canSeeTile(tile) {
  if (tile.public) return true;
  return this.can(tile.cap ?? SYSTEM_ADMIN);
}

Two further rules follow from this:

Navigation is not authorization. The tile flag decides what is listed; the backend independently enforces the capability on every request. A visible tile whose endpoint refuses the caller is a bug, but a hidden tile is never the security boundary.
Opening a surface to a read capability is an explicit, per-tile migration — declare the capability on the tile and enforce it in the handler. There is no bulk default that opens surfaces implicitly.

Consequences

The safe default holds for the ~72 tiles that predate read capabilities: they stay superuser-only until someone deliberately migrates them.
Public help content is reachable by read-only operators on every viewport, and the desktop hard-coded link could be deleted, so desktop and mobile now derive navigation from one source.
The three-tier rule is pinned by a contract test over the matrix ({public} → visible; {} → superuser-only; {cap} → capability-gated; superuser → all).
Writing this down is itself the point: two separate implementation briefs proposed the “cap-less means public” inversion, and only an in-browser check caught it. The model needs to be stated somewhere durable rather than re-derived per sprint.

References

Read capabilities backend + the read-only operator role.
Per-tile capability gating of the admin menu.
The help-visibility defect and its correction, which established the public tier.