diff --git a/hive-c0re/src/paths.rs b/hive-c0re/src/paths.rs
index 88eb2366..759f7eae 100644
--- a/hive-c0re/src/paths.rs
+++ b/hive-c0re/src/paths.rs
@@ -255,6 +255,9 @@ pub const FORGE_CORE_TOKEN: &str = "/var/lib/hyperhive/forge-core-token";
 
 /// `matrix-register-token` — shared matrix registration token.
 // nix: bind-mounted into the tuwunel/matrix container (hive-matrix.nix) — must match.
+//      Not operator-option-driven: `registrationTokenFile` is `internal` on the nix
+//      side, and an `assertions` entry there rejects any attempt to move it, so this
+//      literal can never diverge from it.
 #[must_use]
 pub fn matrix_register_token() -> PathBuf {
     state_root().join("matrix-register-token")
diff --git a/nix/host-modules/hive-matrix.nix b/nix/host-modules/hive-matrix.nix
index fe6e39e0..1438b020 100644
--- a/nix/host-modules/hive-matrix.nix
+++ b/nix/host-modules/hive-matrix.nix
@@ -40,6 +40,12 @@ let
   # login into an intermittent one.
   matrixSecretPath = "/var/lib/tuwunel-oidc/${cfg.sso.clientId}.secret";
 
+  # Single source for the registration-token path: the option's `default`,
+  # its `config`-level `mkDefault` self-definition, and the assertion that
+  # rejects a moved path all read this same binding rather than repeating
+  # the string literal. See `registrationTokenFile`'s own comment below.
+  registrationTokenPath = "/var/lib/hyperhive/matrix-register-token";
+
   # ⚠️ tuwunel does NOT read the path above directly, and this indirection
   # is not ceremony. Upstream's own words: "under systemd the path must be
   # visible to the service after sandboxing (ReadWritePaths / ProtectHome),
@@ -452,7 +458,8 @@ in
 
     registrationTokenFile = lib.mkOption {
       type = lib.types.path;
-      default = "/var/lib/hyperhive/matrix-register-token";
+      internal = true;
+      default = registrationTokenPath;
       description = ''
         Host path to a file containing the matrix registration token
         tuwunel reads to authorise new-account creation. The token is
@@ -461,8 +468,28 @@ in
         tuwunel container at the same path. Agents never see this
         token — hive-c0re uses it to provision per-agent accounts
         and the agent only receives the resulting `access_token`.
-        Override only when integrating with externally-managed
-        registration tokens.
+
+        Not operator-settable — `hive-c0re`'s Rust side derives this
+        same path independently (`paths::matrix_register_token()`) with
+        nothing wiring an override across, so a moved path used to
+        desync the two silently. An externally-managed token is
+        delivered by writing into *this* fixed path instead of moving it —
+        see `glue-matrix-bao-token.nix`, which fetches from the swarm
+        secret store and overwrites this file in place.
+
+        Enforced by an `assertions` entry below rather than `readOnly`:
+        the `config` block gives this option its own `mkDefault`
+        definition (lowest priority) so any real override — an operator's,
+        or the pre-rename `swarm.matrix.registrationTokenFile` shim's —
+        still resolves cleanly instead of crashing eval with nixpkgs'
+        generic "read-only, set multiple times" message; the assertion
+        then names the actual problem. (`readOnly` was the first attempt —
+        atlas caught that it only rejects a *second* definition,
+        so with nothing else defining the option a lone override sailed
+        through silently, same desync as before with a lock that wasn't
+        locked. Fixed by defining the value here instead of leaving it on
+        `default` alone, but a proper `assertions` message beat re-adding
+        `readOnly` on top once the option had a real definition either way.)
       '';
     };
 
@@ -507,6 +534,13 @@ in
   };
 
   config = lib.mkIf deployCfg.matrix.enable {
+    # The option's own value, defined explicitly rather than left on its
+    # bare `default` — `mkDefault` so a real override (an operator's, or
+    # the pre-rename shim's) still resolves cleanly rather than crashing
+    # eval; the `assertions` entry below is what actually rejects it, with
+    # a message naming the reason. See that option's own comment above.
+    services.hyperhive.deploy.matrix.registrationTokenFile = lib.mkDefault registrationTokenPath;
+
     # Matrix's own gateway surface: the sub-domain vhost, the name the
     # hive resolver answers for, and the Accept-header map that vhost's
     # SPA fallback reads. All three are matrix knowledge and none of
@@ -676,6 +710,27 @@ in
           httpPort and has no such name.
         '';
       }
+      {
+        # mkDefault above lets an override resolve instead of crashing
+        # eval — this is the actual rejection, with a message that names
+        # the real fix instead of nixpkgs' generic conflicting-definition
+        # text. Covers both the current path and the pre-rename
+        # `swarm.matrix.registrationTokenFile` shim in one check, since
+        # both land on the same merged option.
+        assertion = deployCfg.matrix.registrationTokenFile == registrationTokenPath;
+        message = ''
+          services.hyperhive.deploy.matrix.registrationTokenFile is fixed at
+          ${registrationTokenPath} and cannot be moved — hive-c0re's Rust
+          side derives this same path independently and has no way to learn
+          an override, so moving it desyncs the two silently instead of
+          loudly.
+
+          Integrating an externally-managed registration token? Deliver it
+          by writing into ${registrationTokenPath} instead of pointing this
+          option elsewhere — see glue-matrix-bao-token.nix, which does
+          exactly that from the swarm secret store.
+        '';
+      }
     ];
 
     # One declaration, two readers. The homeserver knows its own callback
diff --git a/nix/module-eval.nix b/nix/module-eval.nix
index 29e23a1b..e61db7b4 100644
--- a/nix/module-eval.nix
+++ b/nix/module-eval.nix
@@ -296,6 +296,21 @@ let
         builtins.elem httpPort ports
         && matrixOldPath.containers.hive-matrix.bindMounts ? "/etc/matrix/register.token";
     }
+    {
+      # registrationTokenFile lost its override capability entirely
+      # (bao-delivered secrets don't need one — glue-matrix-bao-token.nix
+      # already writes into the fixed path instead of moving it), unlike its
+      # five siblings in the same rename. `matrixOldPath` above proves the
+      # override still *resolves* (mkDefault, not a crash) — this proves it
+      # also gets *rejected*, by a named assertion rather than nixpkgs' generic
+      # conflicting-definition text. Reads `.assertions` directly (cheap: a
+      # list of `{assertion; message;}`, not `system.build.toplevel`) rather
+      # than forcing a real build just to observe a boolean.
+      name = "overriding registrationTokenFile (even via the pre-rename shim) trips a named assertion, not a silent desync";
+      ok = lib.any (
+        a: !a.assertion && lib.hasInfix "registrationTokenFile" a.message
+      ) matrixOldPath.assertions;
+    }
     {
       # Reads the DELIVERY UNIT, not the options: `responderConfigured` gates
       # whether it exists at all, and the seed path is interpolated into its