diff --git a/docs/swarm/services.md b/docs/swarm/services.md
index 0ad85e84..b3e88c19 100644
--- a/docs/swarm/services.md
+++ b/docs/swarm/services.md
@@ -69,11 +69,11 @@ provider, differentiated by roles and claims rather than by mechanism —
 there is one IdP and one auth path.
 
 - **`deploy.authelia`** — run the container here.
-- **`swarm.authelia.url`** — where clients go to authenticate. Present
-  on **every** hive and the same value on all of them:
-  `https://<swarm.authelia.domain>`, whether or not this host runs the
-  container. Set it explicitly when joining a swarm whose IdP is under
-  another name.
+- **`swarm.authelia.url`** — where clients go to authenticate.
+  Present on **every** hive, defaulting to this host's own instance only
+  when this module is the thing running it; otherwise `null`, and a hive
+  joining someone else's swarm sets it explicitly. Null means "no SSO
+  configured", and consumers say so rather than guessing an address.
 
 swarm-controller writes the users database, not by hand: hive-c0re
 creates and destroys agents continuously, so the subject set is dynamic.
diff --git a/nix/host-modules/hive-forge/default.nix b/nix/host-modules/hive-forge/default.nix
index 4a7f29de..1b22d878 100644
--- a/nix/host-modules/hive-forge/default.nix
+++ b/nix/host-modules/hive-forge/default.nix
@@ -573,6 +573,20 @@ in
           host: see docs/swarm/ for which secret goes where.
         '';
       }
+      {
+        # Without a provider URL there is nothing to discover against,
+        # and the rendered unit would ask `null/.well-known/…`.
+        assertion = autheliaUrl != null;
+        message = ''
+          The forge's SSO login source requires
+          services.hyperhive.swarm.authelia.url — the base URL of the
+          swarm's SSO provider.
+
+          It defaults to this host's own instance only when this host
+          runs authelia. A hive that federates with a swarm sets it
+          explicitly to wherever that provider lives.
+        '';
+      }
       {
         assertion = cfg.rootUrl == null || lib.hasSuffix "/" cfg.rootUrl;
         message = ''
diff --git a/nix/host-modules/hive-matrix.nix b/nix/host-modules/hive-matrix.nix
index 0f4fc3cb..1304d9b2 100644
--- a/nix/host-modules/hive-matrix.nix
+++ b/nix/host-modules/hive-matrix.nix
@@ -968,6 +968,20 @@ in
           host: see docs/swarm/ for which secret goes where.
         '';
       }
+      {
+        # Without a provider URL there is nothing to discover against, and
+        # the rendered config would name `null` as its issuer.
+        assertion = autheliaUrl != null;
+        message = ''
+          This homeserver's SSO login flow requires
+          services.hyperhive.swarm.authelia.url — the base URL of the
+          swarm's SSO provider.
+
+          It defaults to this host's own instance only when this host
+          runs authelia. A hive that federates with a swarm sets it
+          explicitly to wherever that provider lives.
+        '';
+      }
       {
         # The callback URL must name the homeserver itself, and with no
         # gateway vhost there is no public name for it to be built from.
diff --git a/nix/host-modules/swarm-authelia.nix b/nix/host-modules/swarm-authelia.nix
index dcdafdc3..c1c0d035 100644
--- a/nix/host-modules/swarm-authelia.nix
+++ b/nix/host-modules/swarm-authelia.nix
@@ -457,25 +457,23 @@ in
     };
 
     url = lib.mkOption {
-      type = lib.types.str;
-      default = "https://${cfg.domain}";
-      defaultText = lib.literalExpression ''"https://''${domain}"'';
+      type = lib.types.nullOr lib.types.str;
+      default = if deployCfg.authelia.enable then "https://${cfg.domain}" else null;
+      defaultText = lib.literalExpression ''if enable then "https://''${domain}" else null'';
       example = "https://auth.example.com";
       description = ''
         Base URL clients are sent to for authentication — the half of
         this module that exists on **every** hive, not just the one
         running the container.
 
-        Names {option}`domain`, and does **not** ask whether this host
-        runs the container: a swarm has one SSO provider, so every hive
-        addresses the same name and resolution decides where it is —
-        dnsmasq locally on the host serving the vhost, the real network
-        anywhere else. There is no loopback-vs-remote branch to get
-        wrong, the same way {option}`services.hyperhive.swarm.otel.domain`
-        has none.
-
-        Not having SSO is not a supported deployment: every swarm has an
-        IdP, so this is never `null`.
+        Defaults to this host's own instance **only when this module is
+        the thing running it**; in that case the URL is not a guess, it
+        is where this module just put the container. Otherwise `null`,
+        and a hive that federates with a swarm sets it explicitly to
+        wherever the swarm's authelia lives. Null means "no SSO
+        configured" and consumers say so rather than inventing an
+        address — an endpoint baked in as a fallback is one that
+        resolves cleanly and points at the wrong machine.
       '';
     };
 
@@ -952,6 +950,18 @@ in
       # would reject it, but three layers away and at boot — naming both
       # sources here is the cheaper failure.
       ++ [
+        {
+          # The bridge introspects by name, so a null URL becomes a nix
+          # coercion error several files from its cause. Only reachable by
+          # enabling authelia and clearing `url` by hand — an assertion
+          # rather than a fallback, because a guessed URL that evaluates
+          # cleanly is worse than a refused build.
+          assertion = cfg.url != null;
+          message =
+            "services.hyperhive.swarm.authelia.url must not be null when authelia "
+            + "is enabled: swarm-authelia-bridge introspects at "
+            + "`\${url}/api/oidc/introspection` from inside its container.";
+        }
         {
           assertion = lib.length (lib.unique (map (c: c.id) cfg.oidc.clients)) == lib.length cfg.oidc.clients;
           message =
diff --git a/nix/host-modules/swarm-controller.nix b/nix/host-modules/swarm-controller.nix
index fb11ad80..383e768e 100644
--- a/nix/host-modules/swarm-controller.nix
+++ b/nix/host-modules/swarm-controller.nix
@@ -382,7 +382,7 @@ in
 
       tokenEndpoint = lib.mkOption {
         type = lib.types.str;
-        default = "${autheliaCfg.url}/api/oidc/token";
+        default = lib.optionalString (autheliaCfg.url != null) "${autheliaCfg.url}/api/oidc/token";
         defaultText = lib.literalExpression ''"''${swarm.authelia.url}/api/oidc/token"'';
         description = ''
           The OIDC token endpoint the controller mints its own access
@@ -773,13 +773,12 @@ in
       {
         assertion = cfg.queue.tokenEndpoint != "";
         message = ''
-          services.hyperhive.swarm.controller.queue.tokenEndpoint is unset.
+          services.hyperhive.swarm.controller.queue.tokenEndpoint is unset,
+          which means services.hyperhive.swarm.authelia.url is null.
 
-          It defaults from services.hyperhive.swarm.authelia.url, so this
-          only happens when it has been cleared by hand. The controller
-          mints its own access token before it may connect to the queue,
-          so it needs to know where the swarm's identity provider lives —
-          set this endpoint directly.
+          The controller mints its own access token before it may connect
+          to the queue, so it needs to know where the swarm's identity
+          provider lives — set that URL, or set this endpoint directly.
         '';
       }
       {
diff --git a/nix/host-modules/swarm-grafana.nix b/nix/host-modules/swarm-grafana.nix
index 006e3abf..5e6ac674 100644
--- a/nix/host-modules/swarm-grafana.nix
+++ b/nix/host-modules/swarm-grafana.nix
@@ -109,6 +109,18 @@ let
   # operator sees, not a coercion error from here.
   domainBase = if swarmDomain == null then "invalid" else swarmDomain;
 
+  # Is SSO configured for this SWARM. Swarm-wide by construction — `swarm.*` is
+  # identical on every host — and the option's own description is what makes
+  # this the right question to ask: a null URL means "no SSO configured".
+  #
+  # 🩸 A subject of an assertion below, NOT a gate. Dropping the OIDC block when
+  # this is false looks conservative and is the outage: `disable_login_form` is
+  # unconditional a few hundred lines down, so a Grafana with no OIDC settings
+  # is a Grafana with no login of any kind, arrived at silently. SSO is a
+  # requirement of running this service, so an unconfigured swarm fails to
+  # build and says which option to set.
+  ssoConfigured = hyperhiveCfg.swarm.authelia.url != null;
+
   # A reader of the store is defined by holding a certificate the store
   # accepts, never by standing next to it — the rule
   # ./glue-matrix-bao-token.nix states in full.
@@ -436,11 +448,34 @@ in
     # host that runs authelia, and this whole block is gated on the host that
     # runs Grafana. ./glue-grafana-oidc-client.nix is where it moved to.
 
-    # This arm fires only where Grafana runs — this whole block is under
-    # `deploy.grafana.enable`. SSO itself needs no assertion any more:
-    # `swarm.authelia.url` is non-nullable, so every swarm has one and the
-    # OIDC block below always names a real endpoint.
+    # Both arms are what used to be a silent gate, and both fire only where
+    # Grafana runs — this whole block is under `deploy.grafana.enable`. The
+    # binding itself is asserted rather than a copy of its formula, the way
+    # ./swarm-nats.nix's own `autheliaUrl` arm does it: two spellings of one
+    # boolean is two places for a future edit to land in only one.
     assertions = [
+      {
+        # SSO is not optional for this service, and the reason is a hundred
+        # lines below in `auth.disable_login_form = true`: Grafana ships an
+        # `admin`/`admin` account on a public vhost, so the password box is
+        # off whatever the topology. Emitting no OIDC block when the swarm
+        # names no IdP therefore produces a container with no way in at all —
+        # a state no log names, since nothing failed. Failing the build and
+        # naming the option is the only report that reaches anyone.
+        assertion = ssoConfigured;
+        message = ''
+          services.hyperhive.deploy.grafana.enable requires
+          services.hyperhive.swarm.authelia.url — Grafana's only login is SSO,
+          because its local login form is disabled unconditionally (it ships
+          an admin/admin account and its vhost is on the public gateway).
+
+          It defaults to this host's own instance only when this host runs
+          authelia. A hive that federates with a swarm sets it explicitly to
+          wherever that provider lives. Grafana exchanges its authorization
+          code at `''${url}/api/oidc/token` from inside its container, so a
+          null URL leaves no endpoint to name.
+        '';
+      }
       {
         # The other half of one login: the OIDC block names a `$__file{}` that
         # `swarm-bao-grafana-oidc.service` below writes, and that unit reads
diff --git a/nix/host-modules/swarm-nats.nix b/nix/host-modules/swarm-nats.nix
index c86a3c62..a13974c3 100644
--- a/nix/host-modules/swarm-nats.nix
+++ b/nix/host-modules/swarm-nats.nix
@@ -511,6 +511,18 @@ in
           to have this host mint all four.
         '';
       }
+      {
+        assertion = autheliaUrl != null;
+        message = ''
+          services.hyperhive.deploy.nats.enable requires
+          services.hyperhive.swarm.authelia.url — the queue authenticates
+          clients by validating tokens that authelia issued.
+
+          It defaults to this host's own instance only when this host
+          runs authelia. A hive that federates with a swarm sets it
+          explicitly to wherever that provider lives.
+        '';
+      }
     ];
 
     # One declaration, two readers. The queue knows which client id it
diff --git a/nix/host-modules/swarm-otel.nix b/nix/host-modules/swarm-otel.nix
index 868df8b7..945f79ea 100644
--- a/nix/host-modules/swarm-otel.nix
+++ b/nix/host-modules/swarm-otel.nix
@@ -1027,6 +1027,22 @@ in
           should not be described as both.
         '';
       }
+      {
+        # A hive proves who it is with a token this provider mints, so
+        # there is no version of this collector that runs without one.
+        # Stated as an assertion rather than a fallback because a guessed
+        # issuer URL evaluates cleanly and refuses every hive at runtime.
+        assertion = hyperhiveCfg.swarm.authelia.url != null;
+        message = ''
+          services.hyperhive.deploy.swarm-otel.enable is true but
+          services.hyperhive.swarm.authelia.url is null: every hive
+          authenticates to this collector as itself, and the token comes
+          from the swarm's identity provider.
+
+          Point authelia.url at the swarm's provider, or enable
+          services.hyperhive.swarm.authelia on the host that runs it.
+        '';
+      }
       {
         # A port collision between two listeners on one host is a runtime
         # coin toss with nothing in any log — the failure this whole
diff --git a/nix/host-modules/swarm.nix b/nix/host-modules/swarm.nix
index d0e36dcd..fd1fe9c8 100644
--- a/nix/host-modules/swarm.nix
+++ b/nix/host-modules/swarm.nix
@@ -500,8 +500,8 @@ in
   options.services.hyperhive.swarm.statusPublish = {
     tokenEndpoint = lib.mkOption {
       type = lib.types.nullOr lib.types.str;
-      default = "${swarmCfg.authelia.url}/api/oidc/token";
-      defaultText = lib.literalExpression ''"''${swarm.authelia.url}/api/oidc/token"'';
+      default = if swarmCfg.authelia.url != null then "${swarmCfg.authelia.url}/api/oidc/token" else null;
+      defaultText = lib.literalExpression ''"''${swarm.authelia.url}/api/oidc/token" when that URL is set, else null'';
       example = "https://auth.example.com/api/oidc/token";
       description = ''
         The swarm IdP's OAuth2 token endpoint. This hive mints a
diff --git a/nix/module-eval/grafana.nix b/nix/module-eval/grafana.nix
index ae825800..c1008194 100644
--- a/nix/module-eval/grafana.nix
+++ b/nix/module-eval/grafana.nix
@@ -70,6 +70,17 @@ let
     swarm.authelia.url = "https://auth.example.invalid";
   };
 
+  # The mirror image: the identity is placed, and the swarm names no IdP. The
+  # other half of "SSO must always be configured", and isolated the same way —
+  # exactly one thing wrong, so the arm reads one refusal.
+  grafanaNoSso = hive {
+    deploy.grafana.enable = true;
+    deploy.grafana.plugins = [ ];
+    deploy.grafana.package = pkgs.emptyDirectory;
+    deploy.bao.clientCertFile = "/etc/pki/bao-client.pem";
+    deploy.bao.clientKeyFile = "/etc/pki/bao-client-key.pem";
+  };
+
   # Did ./host-modules/swarm-grafana.nix refuse this host, and for which of its
   # two reasons. An assertion is a config VALUE until something forces it —
   # `.config` never throws — so a fixture in a state the module refuses is
@@ -153,7 +164,19 @@ let
         && !(grafanaRefusedFor grafanaNoIdentity "swarm.authelia.url");
     }
     {
-      # Without this the arm above proves nothing: a refusal that fires on
+      # "SSO must always be configured", as an eval-time refusal rather than a
+      # gate. A null URL used to drop the OIDC block silently, and
+      # `disable_login_form` is unconditional a hundred lines below it, so that
+      # combination produced a Grafana with no SSO and no password box — an
+      # outage whose cause is a boolean that evaluated to false at build time
+      # and left no trace. Same isolation as the arm above, mirrored.
+      name = "a grafana host in a swarm with no IdP is refused, naming swarm.authelia.url";
+      ok =
+        grafanaRefusedFor grafanaNoSso "services.hyperhive.swarm.authelia.url"
+        && !(grafanaRefusedFor grafanaNoSso "deploy.bao.clientCertFile");
+    }
+    {
+      # Without this the two arms above prove nothing: a refusal that fires on
       # every host is not a check, and both of these are hosts a swarm is
       # expected to have. Read through the same helper, so a message that
       # stopped naming its option would fail the arms above rather than pass
diff --git a/nix/module-eval/nats-authelia.nix b/nix/module-eval/nats-authelia.nix
index cd72aa1b..4f343c04 100644
--- a/nix/module-eval/nats-authelia.nix
+++ b/nix/module-eval/nats-authelia.nix
@@ -59,15 +59,6 @@ let
     swarm.nats.calloutUserSeedFile = "/run/secrets/nats-user.seed";
     swarm.nats.calloutIssuerSeedFile = "/run/secrets/nats-issuer.seed";
   };
-
-  # A hive running NOTHING of the swarm's own services — no IdP here, no
-  # `swarm.authelia.url` set by hand. The whole point of the fixture is what it
-  # does *not* say: it is the shape whose IdP address used to be null, and
-  # before that a co-location-derived guess.
-  autheliaNotColocated = hive { };
-  # The same swarm's IdP host, for the arm that the two agree.
-  autheliaColocated = hive { deploy.authelia.enable = true; };
-
   cases = [
     {
       # Reads the RENDERED settings, not the option: `calloutBlocks {…} // {
@@ -125,34 +116,6 @@ let
       name = "a consumer of authelia's host client-secret dir renders it from the deploy namespace";
       ok = lib.hasInfix "/var/lib/nixos-containers/swarm-authelia/var/lib/authelia-swarm/oidc-clients/" autheliaOldPath.systemd.services.swarm-nats-auth-secrets.script;
     }
-    {
-      # Config NAMES the IdP; it never computes where the IdP is. Both arms
-      # matter together: the address is the swarm's name on a hive that runs
-      # nothing, and it is the SAME string on the hive that serves the vhost —
-      # so a re-introduced co-location branch shows up as the two disagreeing
-      # rather than as a value that merely looks plausible on one of them.
-      name = "the swarm IdP address is its domain on every hive, co-located or not";
-      ok =
-        autheliaNotColocated.services.hyperhive.swarm.authelia.url
-        == "https://${autheliaNotColocated.services.hyperhive.swarm.authelia.domain}"
-        &&
-          autheliaColocated.services.hyperhive.swarm.authelia.url
-          == autheliaNotColocated.services.hyperhive.swarm.authelia.url;
-    }
-    {
-      # …and the name it resolves through is a DOMAIN, not a host this config
-      # picked. `127.0.0.1` is what the old default rendered on the IdP host,
-      # the bridge address is what the queue's defaults still render, and
-      # neither is an address a client may be handed: the domain is allowed to
-      # resolve differently in different places, which is the whole property.
-      name = "the swarm IdP address names no host address";
-      ok =
-        let
-          url = autheliaNotColocated.services.hyperhive.swarm.authelia.url;
-          bridgeIp = autheliaNotColocated.services.hyperhive.network.bridgeIp;
-        in
-        !(lib.hasInfix "127.0.0.1" url) && !(lib.hasInfix "localhost" url) && !(lib.hasInfix bridgeIp url);
-    }
   ];
 in
 runGroup "nats-authelia" cases