diff --git a/nix/module-eval.nix b/nix/module-eval.nix
index 71931666..91130618 100644
--- a/nix/module-eval.nix
+++ b/nix/module-eval.nix
@@ -453,8 +453,85 @@ let
     in
     builtins.foldl' (acc: v: builtins.seq v acc) true vals;
 
+  # The swarm's shared services hosted HERE without the all-local mode — a
+  # services box with hives elsewhere, the shape ./host-modules/
+  # swarm-required-services.nix documents the switch for.
+  swarmServicesHere = hive { deploy.allSwarmServices = true; };
+
+  # Same, with one of those services placed on another host. Every derivation
+  # in that module is `mkDefault` so this stays expressible.
+  swarmServicesBaoElsewhere = hive {
+    deploy.allSwarmServices = true;
+    deploy.bao.enable = false;
+  };
+
+  # The enables that switch owns. ⚠️ `otel` is the per-hive collector's own
+  # option and is NOT under `deploy` — spelled at the wrong path it would be
+  # undeclared rather than false, and a roster that quietly loses a member is
+  # what the count guard in the cases below exists to catch. Its membership
+  # here is deliberate and was the fix for a gap, not an oversight: the hive
+  # tier lands wherever the swarm services do.
+  swarmServiceEnables =
+    machine:
+    let
+      h = machine.services.hyperhive;
+    in
+    {
+      matrix = h.deploy.matrix.enable;
+      otel = h.otel.enable;
+      authelia = h.deploy.authelia.enable;
+      nats = h.deploy.nats.enable;
+      swarm-otel = h.deploy.swarm-otel.enable;
+      victoriametrics = h.deploy.victoriametrics.enable;
+      grafana = h.deploy.grafana.enable;
+      victorialogs = h.deploy.victorialogs.enable;
+      bao = h.deploy.bao.enable;
+    };
+
   # Each case: a name stating the property, and `ok`.
   cases = [
+    {
+      # `lib.all` over an empty set holds vacuously, so the roster is counted
+      # before it is read: a helper that lost a member would otherwise turn
+      # this case green by measuring nothing.
+      name = "hosting the swarm's shared services turns on every service that switch owns";
+      ok =
+        let
+          es = swarmServiceEnables swarmServicesHere;
+        in
+        lib.length (lib.attrNames es) == 9 && lib.all lib.id (lib.attrValues es);
+    }
+    {
+      name = "a hive that does not host the swarm's shared services runs none of them";
+      ok =
+        let
+          es = swarmServiceEnables bare;
+        in
+        lib.length (lib.attrNames es) == 9 && !lib.any lib.id (lib.attrValues es);
+    }
+    {
+      # The switch fills in for an operator who has not spoken and yields to
+      # one who has — that is what keeps a shared service placeable on a host
+      # of its own. A plain assignment or `mkForce` would satisfy both cases
+      # above and break this one. `nats` is the control: without it the case
+      # also passes on a fixture where nothing came on at all.
+      name = "placing one shared service elsewhere survives the switch that would enable it";
+      ok =
+        let
+          es = swarmServiceEnables swarmServicesBaoElsewhere;
+        in
+        !es.bao && es.nats;
+    }
+    {
+      # The controller sits on the OTHER tier: `singleHostSwarm` places it
+      # (./host-modules/local-defaults.nix) and swarm-ui follows the
+      # controller. Pinned so that moving a service between tiers is a
+      # decision someone makes rather than a merge nobody reads.
+      name = "hosting the swarm's shared services does not make a hive the swarm's control plane";
+      ok =
+        !swarmServicesHere.services.hyperhive.deploy.swarm-controller.enable
+        && !swarmServicesHere.services.hyperhive.deploy.swarm-ui.enable;
+    }
     {
       name = "a hive that has not opted into all-local runs no swarm controller";
       ok = !bare.services.hyperhive.deploy.swarm-controller.enable;