diff --git a/nix/modules/hive-matrix.nix b/nix/modules/hive-matrix.nix
index 3c17d271..85716052 100644
--- a/nix/modules/hive-matrix.nix
+++ b/nix/modules/hive-matrix.nix
@@ -191,16 +191,17 @@ in
           # the browser resolves relative asset paths (`Imaging.js`,
           # `flutter.js`, `splash/*`) against the document ROOT,
           # producing 404s for every asset (#634). Inject the right
-          # base-href into the build via nixpkgs's `buildFlutterApplication`
-          # targetFlags. When subdomain routing lands (#609 —
-          # `matrix.${hyperhiveDomain}`), drop this override; upstream's
-          # `/` base-href is correct at the root of a dedicated subdomain.
-          targetFlags = (old.targetFlags or [ ]) ++ [
-            "--base-href"
-            "/matrix/"
-          ];
+          # base-href into the actual `flutter build web` invocation
+          # via the upstream derivation's `flutterBuildFlags` string
+          # (the buildPhase is literally
+          # `flutter build web -v $flutterBuildFlags`). When subdomain
+          # routing lands (#609 — `matrix.${hyperhiveDomain}`), drop
+          # this override; upstream's `/` base-href is correct at the
+          # root of a dedicated subdomain.
+          flutterBuildFlags =
+            (old.flutterBuildFlags or "") + " --base-href /matrix/";
         });
-        defaultText = lib.literalMD "`pkgs.fluffychat-web` rebuilt with `--base-href /matrix/` via `targetFlags`.";
+        defaultText = lib.literalMD "`pkgs.fluffychat-web` rebuilt with `--base-href /matrix/` via `flutterBuildFlags`.";
         description = ''
           Static web client dist to serve at `/matrix/`. Defaults to
           `pkgs.fluffychat-web` rebuilt with `--base-href "/matrix/"`