diff --git a/nix/modules/hive-matrix.nix b/nix/modules/hive-matrix.nix
index a2ef7abe..3b0a820d 100644
--- a/nix/modules/hive-matrix.nix
+++ b/nix/modules/hive-matrix.nix
@@ -159,14 +159,17 @@ in
     gui = {
       enable = lib.mkOption {
         type = lib.types.bool;
-        default = false;
+        default = cfg.enable;
+        defaultText = lib.literalExpression "config.services.hyperhive.matrix.enable";
         description = ''
           Mount a matrix web client (default `pkgs.fluffychat-web`) as
           a static sibling of the dashboard at `/matrix/`, served by
           hive-c0re's own axum router via the `HIVE_MATRIX_GUI_DIR`
-          env var (#607 v0). Off by default; flip to `true` once the
-          operator wants matrix chat one tab away from the dashboard
-          without standing up a separate gateway.
+          env var (#607). Defaults to whatever
+          `services.hyperhive.matrix.enable` is — turning on the
+          homeserver gives you the web client by default; set to
+          `false` explicitly to opt out of the GUI while keeping
+          the homeserver running for agents.
 
           Same-origin via hive-c0re is the simplest single-host
           shape; the post-#15 nginx-front re-root (`https://matrix.''${services.hyperhive.domain}`)
@@ -178,15 +181,36 @@ in
 
       package = lib.mkOption {
         type = lib.types.package;
-        default = pkgs.fluffychat-web;
-        defaultText = lib.literalExpression "pkgs.fluffychat-web";
+        default = pkgs.fluffychat-web.overrideAttrs (old: {
+          # fluffychat-web ships with `<base href="/">` baked into its
+          # index.html — that's the placeholder flutter would replace
+          # via `--base-href` at build time. Since hyperhive serves the
+          # dist at `/matrix/` (not the document root), the literal
+          # `<base href="/">` would resolve every relative asset path
+          # (`Imaging.js`, `flutter.js`, `splash/*`, etc.) against the
+          # ROOT of the origin, leading to 404s on every load (#634).
+          # Patch the placeholder to `<base href="/matrix/">` at install
+          # time so the SPA boots correctly under the sub-path mount.
+          # If/when we move to subdomain routing (#609 −
+          # `matrix.${hyperhiveDomain}`), drop this override and revert
+          # to upstream `pkgs.fluffychat-web` — it'll be at the root
+          # again.
+          postInstall = (old.postInstall or "") + ''
+            ${pkgs.gnused}/bin/sed -i \
+              's|<base href="/">|<base href="/matrix/">|' \
+              $out/index.html
+          '';
+        });
+        defaultText = lib.literalExpression "pkgs.fluffychat-web (patched for /matrix/ base href)";
         description = ''
           Static web client dist to serve at `/matrix/`. Defaults to
-          `pkgs.fluffychat-web` — first-class threads support, e2ee
-          + voice + spaces, full feature surface in the flutter→web
-          build. Override to swap for `hydrogen-web` (lightest),
-          `cinny` (no threads), `element-web` (heaviest, full
-          features), or an out-of-tree client dist.
+          `pkgs.fluffychat-web` (with the `<base href>` patched to
+          `/matrix/` so relative asset paths resolve under the
+          sub-path mount, #634). Override to swap for `hydrogen-web`
+          (lightest), `cinny` (no threads), `element-web` (heaviest,
+          full features), or an out-of-tree client dist — any
+          replacement also needs its `<base href>` aligned with the
+          mount path.
         '';
       };
     };