diff --git a/nix/modules/hive-matrix.nix b/nix/modules/hive-matrix.nix
index c1575d9e..f748fa28 100644
--- a/nix/modules/hive-matrix.nix
+++ b/nix/modules/hive-matrix.nix
@@ -8,6 +8,82 @@ let
   cfg = config.services.hyperhive.matrix;
   hyperhiveDomain = config.services.hyperhive.domain;
   effectiveServerName = if cfg.serverName != null then cfg.serverName else hyperhiveDomain;
+
+  # Two files are missing from nixpkgs's `pkgs.fluffychat-web` dist
+  # because `flutter341.buildFlutterApplication` doesn't run the
+  # native_imaging package's emscripten build (#685):
+  #
+  #   - Imaging.js / Imaging.wasm  ← emscripten-compiled C library
+  #     from the native_imaging dart package (vendored by Famedly).
+  #     The package ships only C source + a Makefile that builds
+  #     them via emcc; pulling in `pkgs.emscripten` to run that
+  #     mid-build adds ~600 MB of closure for two files we can pin
+  #     to the upstream release tarball's prebuilt copies (same
+  #     fluffychat release version → byte-identical output).
+  #
+  # `native_executor.js` is the other missing file (web worker entry
+  # point), but that one compiles cleanly from source via
+  # `dart compile js` — handled in `fluffychat-web-fixed` below
+  # rather than vendored, since the dart SDK is already in the
+  # flutter341 closure.
+  fluffychat-web-imaging-prebuilt =
+    pkgs.runCommandLocal "fluffychat-web-imaging-prebuilt-${pkgs.fluffychat-web.version}"
+      {
+        src = pkgs.fetchurl {
+          url = "https://github.com/krille-chan/fluffychat/releases/download/v${pkgs.fluffychat-web.version}/fluffychat-web.tar.gz";
+          # v2.6.0 prebuilt tarball; bump in lockstep with
+          # `pkgs.fluffychat-web.version` on any flutter upgrade.
+          hash = "sha256-P3Bt5FZ5TPeE0k05gYxLw7FdEyyi8g5A/r7tGZMezQk=";
+        };
+      }
+      ''
+        mkdir -p $out
+        tar -xzf $src --strip-components=2 -C $out \
+          build/web/Imaging.js build/web/Imaging.wasm
+      '';
+
+  # `pkgs.fluffychat-web` with #685's three missing files patched
+  # in via postInstall, plus the existing `--base-href "/matrix/"`
+  # override (#634) for the sub-path mount. The patches sit in our
+  # tree rather than as a nixpkgs upstream PR because:
+  #   - `dart compile js` for web workers is a `buildFlutterApplication`
+  #     gap that needs an upstream patch, not a per-package one
+  #   - vendoring emscripten output for `native_imaging` is a
+  #     workaround for the same builder gap
+  # When `flutter341.buildFlutterApplication` grows worker / emcc
+  # support, drop the postInstall entirely.
+  fluffychat-web-fixed = pkgs.fluffychat-web.overrideAttrs (old: {
+    # `--base-href "/matrix/"` so relative asset paths resolve
+    # under the sub-path mount (#634). Upstream default is `/`,
+    # wrong for hyperhive's `/matrix/` location.
+    flutterBuildFlags = (old.flutterBuildFlags or [ ]) ++ [
+      "--base-href"
+      "/matrix/"
+    ];
+
+    # `dart` from the flutter341 closure (already pulled, no
+    # incremental closure cost) so we can compile the web-worker
+    # entry point that buildFlutterApplication skips.
+    nativeBuildInputs = (old.nativeBuildInputs or [ ]) ++ [ pkgs.flutter341.dart ];
+
+    postInstall =
+      (old.postInstall or "")
+      + ''
+        # #685: compile web/native_executor.dart → native_executor.js.
+        # The flutter web bootstrap loads this from /matrix/native_executor.js
+        # at startup; without it, main.dart.js logs a network-error and
+        # the SPA renders blank (see #643 for the symptom).
+        ${pkgs.flutter341.dart}/bin/dart compile js \
+          -o $out/native_executor.js \
+          $src/web/native_executor.dart
+
+        # #685: vendor Imaging.{js,wasm} from upstream prebuilt — see
+        # comment on `fluffychat-web-imaging-prebuilt` above for the
+        # closure-size rationale vs running emcc here.
+        install -m 644 ${fluffychat-web-imaging-prebuilt}/Imaging.js  $out/Imaging.js
+        install -m 644 ${fluffychat-web-imaging-prebuilt}/Imaging.wasm $out/Imaging.wasm
+      '';
+  });
 in
 {
   # Private Matrix homeserver (matrix-tuwunel — the official conduwuit
@@ -202,32 +278,24 @@ in
 
       package = lib.mkOption {
         type = lib.types.package;
-        default = pkgs.fluffychat-web.overrideAttrs (old: {
-          # fluffychat-web ships with `<base href="/">` baked into its
-          # index.html — flutter's `--base-href` build flag replaces
-          # that placeholder. Default upstream build is `--base-href "/"`
-          # which is wrong for hyperhive's `/matrix/` sub-path mount:
-          # 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 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 `flutterBuildFlags`.";
+        default = fluffychat-web-fixed;
+        defaultText = lib.literalMD ''
+          `pkgs.fluffychat-web` rebuilt with `--base-href /matrix/` (#634)
+          and patched via `postInstall` to add the three files
+          `flutter341.buildFlutterApplication` skips: `native_executor.js`
+          (compiled via `dart compile js`), plus `Imaging.js` and
+          `Imaging.wasm` (vendored from the upstream release tarball;
+          they're emscripten output from the `native_imaging` dart
+          package and the alternative is +~600 MB of `pkgs.emscripten` in
+          the build closure for two files). See the `let` block in
+          `nix/modules/hive-matrix.nix` for the full rationale (#685).
+        '';
         description = ''
           Static web client dist to serve at `/matrix/`. Defaults to
           `pkgs.fluffychat-web` rebuilt with `--base-href "/matrix/"`
           so relative asset paths resolve under the sub-path mount
-          (#634). Override to swap for `hydrogen-web` (lightest),
+          (#634), plus a `postInstall` patch for #685's three missing
+          files. 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.