diff --git a/hive-c0re/src/dashboard.rs b/hive-c0re/src/dashboard.rs
index 17662b48..c1bb2158 100644
--- a/hive-c0re/src/dashboard.rs
+++ b/hive-c0re/src/dashboard.rs
@@ -51,6 +51,17 @@ pub async fn serve(port: u16, coord: Arc<Coordinator>) -> Result<()> {
         );
     }
     tracing::info!(static_dir = %static_dir.display(), "dashboard static dir resolved");
+    // Optional matrix-GUI static dist (#607). When `HIVE_MATRIX_GUI_DIR`
+    // is set and the dir exists, mount it at `/matrix/` as a sibling
+    // ServeDir before the SPA fallback. Pre-#15 (nginx front) this
+    // is the simplest way to put fluffychat-web on the same origin as
+    // the dashboard so the operator has matrix chat one tab away.
+    let matrix_gui_dir: Option<PathBuf> = std::env::var_os("HIVE_MATRIX_GUI_DIR")
+        .map(PathBuf::from)
+        .filter(|p| p.is_dir());
+    if let Some(dir) = &matrix_gui_dir {
+        tracing::info!(matrix_gui_dir = %dir.display(), "matrix GUI mount enabled at /matrix");
+    }
     let app = Router::new()
         .route("/api/state", get(api_state))
         .route("/approve/{id}", post(post_approve))
@@ -92,6 +103,14 @@ pub async fn serve(port: u16, coord: Arc<Coordinator>) -> Result<()> {
         // /static/dashboard.css → dist/static/dashboard.css, etc.).
         .fallback_service(ServeDir::new(&static_dir))
         .with_state(AppState { coord });
+    // Sibling static mount for the optional matrix GUI dist. Layered
+    // before the SPA fallback so `/matrix/...` resolves there instead
+    // of falling through to the dashboard.
+    let app = if let Some(dir) = matrix_gui_dir {
+        app.nest_service("/matrix", ServeDir::new(dir))
+    } else {
+        app
+    };
     let addr = SocketAddr::from(([0, 0, 0, 0], port));
     let listener = bind_with_retry(addr).await?;
     tracing::info!(%port, "dashboard listening");
@@ -223,6 +242,11 @@ struct StateSnapshot {
     /// links each container's config + each approval's commit into the
     /// forge's `agent-configs` repos.
     forge_present: bool,
+    /// Whether the matrix GUI static dist is mounted at `/matrix/`
+    /// (`HIVE_MATRIX_GUI_DIR` set + dir exists). Drives the dashboard's
+    /// `M4TR1X →` tab-strip entry so it doesn't flash a dead link when
+    /// the operator hasn't opted in (#607).
+    matrix_gui_enabled: bool,
 }
 
 /// `OpQuestion` + computed `question_refs` / `answer_refs`. Built
@@ -415,6 +439,9 @@ async fn api_state(headers: HeaderMap, State(state): State<AppState>) -> axum::J
         port_conflicts,
         rebuild_queue: state.coord.rebuild_queue.snapshot(),
         forge_present: crate::forge::is_present().await,
+        matrix_gui_enabled: std::env::var_os("HIVE_MATRIX_GUI_DIR")
+            .map(PathBuf::from)
+            .is_some_and(|p| p.is_dir()),
     })
 }
 
diff --git a/nix/modules/hive-c0re.nix b/nix/modules/hive-c0re.nix
index 20e2e1bd..ae288a34 100644
--- a/nix/modules/hive-c0re.nix
+++ b/nix/modules/hive-c0re.nix
@@ -220,6 +220,13 @@ in
         # Agents poll this URL for Forgejo notifications. Derived from
         # hyperhive.forge.{domain,httpPort} so it tracks forge config changes.
         HIVE_FORGE_URL = "http://${config.hyperhive.forge.domain}:${toString config.hyperhive.forge.httpPort}";
+      }
+      // lib.optionalAttrs config.hyperhive.matrix.gui.enable {
+        # Optional matrix-GUI static dist mounted at /matrix/ by the
+        # dashboard router (#607 v0). Pre-#15 / pre-nginx-front: this is
+        # the simplest same-origin shape — fluffychat-web ships as a
+        # static dist, no runtime daemon needed.
+        HIVE_MATRIX_GUI_DIR = "${config.hyperhive.matrix.gui.package}";
       };
       serviceConfig = {
         ExecStart = "${cfg.package}/bin/hive-c0re --socket /run/hyperhive/host.sock serve --hyperhive-flake ${cfg.hyperhiveFlake} --dashboard-port ${toString cfg.dashboardPort} --operator-pronouns ${lib.escapeShellArg cfg.operatorPronouns} --context-window-tokens ${lib.escapeShellArg (builtins.toJSON cfg.contextWindowTokens)}";
diff --git a/nix/modules/hive-matrix.nix b/nix/modules/hive-matrix.nix
index d029728d..fb5244e2 100644
--- a/nix/modules/hive-matrix.nix
+++ b/nix/modules/hive-matrix.nix
@@ -155,6 +155,41 @@ in
         registration tokens.
       '';
     };
+
+    gui = {
+      enable = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = ''
+          Mount a matrix web client (default `pkgs.fluffychat-web`) as
+          a static sibling of the dashboard at `/matrix/`, served by
+          hive-c0re's own axum router via the `HIVE_MATRIX_GUI_DIR`
+          env var (#607 v0). Off by default; flip to `true` once the
+          operator wants matrix chat one tab away from the dashboard
+          without standing up a separate gateway.
+
+          Same-origin via hive-c0re is the simplest single-host
+          shape; the post-#15 nginx-front re-root (`https://matrix.''${hyperhive.domain}`)
+          is tracked separately in #609. fluffychat-web supports
+          per-login server pick — point it at the in-host tuwunel URL
+          (`http://localhost:8008` by default) the first time.
+        '';
+      };
+
+      package = lib.mkOption {
+        type = lib.types.package;
+        default = pkgs.fluffychat-web;
+        defaultText = lib.literalExpression "pkgs.fluffychat-web";
+        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.
+        '';
+      };
+    };
   };
 
   config = lib.mkIf cfg.enable {