diff --git a/hive-c0re/src/meta.rs b/hive-c0re/src/meta.rs
index fb803936..3ec17332 100644
--- a/hive-c0re/src/meta.rs
+++ b/hive-c0re/src/meta.rs
@@ -505,6 +505,7 @@ const CANONICAL_INPUTS: &[&str] = &["nixpkgs", "nixpkgs-unstable"];
 const FORWARDED_VARS: &[&str] = &[
     "HIVE_FORGE_URL",
     "HIVE_FORGE_PUBLIC_URL",
+    "HIVE_MATRIX_URL",
     "HYPERHIVE_PEERS",
     "HYPERHIVE_HIVE_DOMAIN",
     "HYPERHIVE_HIVE_NAME",
diff --git a/nix/modules/hive-c0re.nix b/nix/modules/hive-c0re.nix
index 3fc87ab3..ef3f9e98 100644
--- a/nix/modules/hive-c0re.nix
+++ b/nix/modules/hive-c0re.nix
@@ -764,6 +764,27 @@ in
           else
             "http://127.0.0.1:${toString config.services.hyperhive.forge.httpPort}";
       }
+      // lib.optionalAttrs config.services.hyperhive.matrix.enable {
+        # In-cluster matrix homeserver URL for each agent's
+        # hive-matrix-daemon. Same shape + rationale as HIVE_FORGE_URL:
+        # - Isolated (private netns): reach tuwunel via the gateway vhost
+        #   (`matrix.<domain>`) on plain http:80 — host loopback is dead.
+        # - Shared netns: direct host loopback on the tuwunel port.
+        # gatewayHost null-guard falls back to loopback so a domain-less
+        # config doesn't break eval (it just won't work under isolation,
+        # which needs a gateway anyway). Forwarded to agents by meta.rs
+        # alongside HIVE_FORGE_URL; shares the same env-forwarding ordering
+        # caveat (value baked at config-generation time).
+        HIVE_MATRIX_URL =
+          if
+            config.services.hyperhive.network.enable
+            && config.services.hyperhive.network.isolateContainers
+            && config.services.hyperhive.matrix.gatewayHost != null
+          then
+            "http://${config.services.hyperhive.matrix.gatewayHost}"
+          else
+            "http://127.0.0.1:${toString config.services.hyperhive.matrix.httpPort}";
+      }
       // lib.optionalAttrs config.services.hyperhive.matrix.gui.enable {
         # Availability flags read by the dashboard's `/api/state`.
         # Matrix GUI lives entirely on the gateway nginx (matrix tab
diff --git a/nix/templates/harness-base.nix b/nix/templates/harness-base.nix
index 8d3b91ee..edd43c25 100644
--- a/nix/templates/harness-base.nix
+++ b/nix/templates/harness-base.nix
@@ -17,6 +17,12 @@ let
   # from `userName` to keep them coupled.
   userName = config.hyperhive.user.name;
   homeDir = "/home/${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
@@ -1327,10 +1333,20 @@ in
       after = [ "network-online.target" ];
       wants = [ "network-online.target" ];
       environment = {
-        HIVE_MATRIX_URL = config.hyperhive.matrix.url;
         HIVE_MATRIX_SOCKET = "/run/hive-matrix/socket";
         RUST_LOG = "info";
       }
+      # Homeserver URL: by default the daemon inherits the host-forwarded
+      # HIVE_MATRIX_URL (set isolation-aware by hive-c0re: `matrix.<domain>`
+      # via the gateway under private-netns isolation, loopback otherwise),
+      # falling back to the daemon's built-in localhost default if the
+      # forward is absent. A per-agent `hyperhive.matrix.url` override
+      # (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 != matrixUrlDefault) {
+        HIVE_MATRIX_URL = config.hyperhive.matrix.url;
+      }
       # Multi-account: serialize the *extra* accounts to the JSON the
       # daemon parses (`accounts::configured`). Only set when extras are
       # declared; the daemon always synthesizes the primary `main`