diff --git a/nix/host-modules/swarm-otel.nix b/nix/host-modules/swarm-otel.nix
index baf64d13..99c2c37b 100644
--- a/nix/host-modules/swarm-otel.nix
+++ b/nix/host-modules/swarm-otel.nix
@@ -72,6 +72,14 @@ let
   # would be an eval error rather than a silently missing guard.
   reservedOwners = import ../reserved-names.nix;
 
+  # The second, narrower blacklist: words no hive name may CONTAIN, because
+  # they are fixed segments of identifiers built FROM a hive name
+  # (`hive-<name>`, `hive-<name>-agent`). Substring rather than equality is
+  # the whole point — a hive called `foo-agent` collides in the composed
+  # string while colliding with nothing in the list above. That file explains
+  # why it is not merged into this one.
+  reservedFragments = import ../reserved-hive-fragments.nix;
+
   # A published target is declared as ONE url, because that url is also the
   # audience its token is minted for — but prometheus wants the same fact in
   # three fields. Split it here rather than asking a service to state it
@@ -685,6 +693,44 @@ in
           Rename the hive. Reserved: ${lib.concatMapStringsSep ", " (n: "'${n}'") reservedOwners}.
         '';
       }
+      {
+        # The substring half, and a genuinely different failure from the one
+        # above: these words are fixed SEGMENTS of identifiers composed from a
+        # hive name, so the collision is between two composed strings while
+        # the names themselves differ. `hive-foo-agent` is hive `foo-agent`'s
+        # own client id and hive `foo`'s AGENT client id at the same time; the
+        # queue's responder resolves it as the agents, so `foo-agent` silently
+        # receives an agent grant instead of its own — and a NATS denial
+        # reaches a client as a timeout, so nothing names the cause.
+        #
+        # Lives here rather than in `swarm-authelia.nix` (which composes those
+        # ids) because this module already owns the hive-roster check and a
+        # second site for "is this hive name legal" is how the two drift.
+        assertion =
+          !lib.any (h: lib.any (frag: lib.hasInfix frag h) reservedFragments) (
+            lib.attrNames hyperhiveCfg.swarm.hives
+          );
+        message = ''
+          services.hyperhive.swarm.hives contains ${
+            lib.concatMapStringsSep ", " (h: "'${h}'") (
+              lib.filter (h: lib.any (frag: lib.hasInfix frag h) reservedFragments) (
+                lib.attrNames hyperhiveCfg.swarm.hives
+              )
+            )
+          }, which CONTAIN a word the swarm composes identifiers from.
+
+          Hive-scoped identifiers are built by joining a hive name with these
+          words — `hive-<name>` for a hive, `hive-<name>-agent` for its agent
+          containers — so a name containing one produces an identifier that is
+          also somebody else's. The failure is a wrong grant, not an error:
+          the client authenticates and receives permissions meant for another
+          principal.
+
+          Rename the hive. Forbidden as substrings: ${
+            lib.concatMapStringsSep ", " (n: "'${n}'") reservedFragments
+          }.
+        '';
+      }
       {
         # A published target that is not an `https://host/path` url. Without
         # this the split returns null and the failure surfaces as
diff --git a/nix/reserved-hive-fragments.nix b/nix/reserved-hive-fragments.nix
new file mode 100644
index 00000000..ebf95a8e
--- /dev/null
+++ b/nix/reserved-hive-fragments.nix
@@ -0,0 +1,45 @@
+# Words no HIVE name may CONTAIN — a substring blacklist, and the second half
+# of `./reserved-names.nix`.
+#
+# Two files rather than one list, because the two differ in every dimension
+# that matters and folding them would silently widen one of them:
+#
+#   | | `reserved-names.nix` | this file |
+#   |---|---|---|
+#   | subject | agent AND hive names | hive names only |
+#   | match | equality | substring |
+#   | why | the name IS a value some component emits | the word is a fixed SEGMENT of a derived identifier |
+#
+# ⚠️ That last row is the whole argument. `hive-<name>` and `hive-<name>-agent`
+# are built by composing a hive name with these words, so a hive called
+# `foo-agent` produces an id another hive's AGENTS also produce — the composed
+# strings collide even though the names do not. Equality cannot see that; only
+# containment can.
+#
+# ⇒ And it is why the sender words (`operator`, `system`, `forge`, …) are NOT
+# here: those are compared against a whole name, never embedded in one, so
+# applying containment to them would refuse `forgeworks` for no failure that
+# exists. `reserved-names.nix`'s own rule — an entry must name a real failure,
+# not a word that looked risky — governs this file too.
+#
+# Read by `host-modules/swarm-otel.nix`, which owns the hive-roster assertion.
+# Deliberately NOT exported to the Rust side: `HIVE_RESERVED_NAMES` answers
+# "may an AGENT be called this", which stays an equality question.
+[
+  # `hive-<name>` — every hive's own OIDC client id (`swarm-authelia.nix`'s
+  # `hiveClientPrefix`), and the string the queue's auth-callout responder
+  # strips to decide which hive a connection is.
+  "hive"
+  # `hive-<name>-agent` — the client its agent containers present
+  # (`agentClientSuffix`). A hive named `foo-agent` mints exactly the id hive
+  # `foo`'s agents do, and the responder resolves it as the agents: that hive
+  # silently receives an agent grant instead of its own, and a NATS denial
+  # arrives as a timeout, so nothing names the cause.
+  "agent"
+  # Already in `reserved-names.nix` as an exact match, listed again here for
+  # the wider rule: the swarm collector names components `<kind>/<owner>` with
+  # the hive name as owner, and a name merely CONTAINING it is not a collision
+  # today — but `swarm` is the one word this swarm composes identifiers from
+  # everywhere, so the operator's rule covers it and the two files agree.
+  "swarm"
+]
diff --git a/nix/reserved-names.nix b/nix/reserved-names.nix
index f7fbb6b2..60ecefdf 100644
--- a/nix/reserved-names.nix
+++ b/nix/reserved-names.nix
@@ -25,6 +25,9 @@
 # `from`/`to`, or a component name the collector builds pipelines from — not a
 # word that merely looked risky. A name in here that nothing emits is a refusal
 # with no failure behind it.
+#
+# ⚠️ Matched by EQUALITY. Words forbidden *inside* a hive name live in
+# `./reserved-hive-fragments.nix` — read its header before merging the two.
 [
   # ---- message-layer senders -------------------------------------------
   # The human at the dashboard: a broker recipient (the T4LK box sends