diff --git a/nix/templates/harness-base.nix b/nix/templates/harness-base.nix
index 230d3968..edd43c25 100644
--- a/nix/templates/harness-base.nix
+++ b/nix/templates/harness-base.nix
@@ -17,6 +17,12 @@ let
   # from `userName` to keep them coupled.
   userName = config.hyperhive.user.name;
   homeDir = "/home/${userName}";
+  # Single source of truth for the default matrix homeserver URL, shared
+  # by the `hyperhive.matrix.url` option default and the daemon-unit guard
+  # that decides whether to set a unit-level HIVE_MATRIX_URL (so the two
+  # cannot drift). Matches the daemon's own built-in default
+  # (`paths::DEFAULT_HOMESERVER`).
+  matrixUrlDefault = "http://localhost:8008";
 in
 {
   # Shared scaffolding for every hyperhive harness container.
@@ -288,7 +294,7 @@ in
 
   options.hyperhive.matrix.url = lib.mkOption {
     type = lib.types.str;
-    default = "http://localhost:8008";
+    default = matrixUrlDefault;
     example = "https://matrix.darkest.space";
     description = ''
       Matrix homeserver URL the agent's `hive-matrix-daemon` connects
@@ -1338,7 +1344,7 @@ in
       # (non-default) is set unit-level so it wins over the forwarded value;
       # at the default we deliberately DON'T set it so the forwarded
       # isolation-aware value isn't shadowed.
-      // lib.optionalAttrs (config.hyperhive.matrix.url != "http://localhost:8008") {
+      // lib.optionalAttrs (config.hyperhive.matrix.url != matrixUrlDefault) {
         HIVE_MATRIX_URL = config.hyperhive.matrix.url;
       }
       # Multi-account: serialize the *extra* accounts to the JSON the