diff --git a/apps/desktop/src/main.ts b/apps/desktop/src/main.ts
index c5507c6fb0..015ce234eb 100644
--- a/apps/desktop/src/main.ts
+++ b/apps/desktop/src/main.ts
@@ -102,6 +102,7 @@ const SET_SAVED_ENVIRONMENT_SECRET_CHANNEL = "desktop:set-saved-environment-secr
 const REMOVE_SAVED_ENVIRONMENT_SECRET_CHANNEL = "desktop:remove-saved-environment-secret";
 const GET_SERVER_EXPOSURE_STATE_CHANNEL = "desktop:get-server-exposure-state";
 const SET_SERVER_EXPOSURE_MODE_CHANNEL = "desktop:set-server-exposure-mode";
+const OPEN_THREAD_CHANNEL = "desktop:open-thread";
 const BASE_DIR = process.env.T3CODE_HOME?.trim() || Path.join(OS.homedir(), ".t3");
 const STATE_DIR = Path.join(BASE_DIR, "userdata");
 const DESKTOP_SETTINGS_PATH = Path.join(STATE_DIR, "desktop-settings.json");
@@ -216,6 +217,7 @@ let backendListeningDetector: ServerListeningDetector | null = null;
 let restartAttempt = 0;
 let restartTimer: ReturnType<typeof setTimeout> | null = null;
 let isQuitting = false;
+let pendingDeepLinkThreadId: string | null = null;
 let desktopProtocolRegistered = false;
 let aboutCommitHashCache: string | null | undefined;
 let desktopLogSink: RotatingFileSink | null = null;
@@ -1106,6 +1108,35 @@ function revealWindow(window: BrowserWindow): void {
   window.focus();
 }
 
+function handleDeepLink(url: string): void {
+  let parsed: URL;
+  try {
+    parsed = new URL(url);
+  } catch {
+    return;
+  }
+
+  if (parsed.protocol !== `${DESKTOP_SCHEME}:` || parsed.hostname !== "thread") {
+    return;
+  }
+
+  const threadId = parsed.pathname.slice(1); // strip leading "/"
+  if (!threadId) {
+    return;
+  }
+
+  writeDesktopLogHeader(`deep link open-thread threadId=${threadId}`);
+
+  const window = mainWindow ?? BrowserWindow.getAllWindows()[0] ?? null;
+  if (window && !window.isDestroyed()) {
+    revealWindow(window);
+    window.webContents.send(OPEN_THREAD_CHANNEL, threadId);
+  } else {
+    // App is still launching; dispatch once the window finishes loading.
+    pendingDeepLinkThreadId = threadId;
+  }
+}
+
 function emitUpdateState(): void {
   for (const window of BrowserWindow.getAllWindows()) {
     if (window.isDestroyed()) continue;
@@ -1997,6 +2028,10 @@ function createWindow(): BrowserWindow {
   window.webContents.on("did-finish-load", () => {
     window.setTitle(APP_DISPLAY_NAME);
     emitUpdateState();
+    if (pendingDeepLinkThreadId !== null) {
+      window.webContents.send(OPEN_THREAD_CHANNEL, pendingDeepLinkThreadId);
+      pendingDeepLinkThreadId = null;
+    }
   });
 
   // On Linux/Wayland with `show: false`, Electron's `ready-to-show` only
@@ -2113,12 +2148,37 @@ app.on("before-quit", () => {
   restoreStdIoCapture?.();
 });
 
+// macOS: fired when the OS routes a t3:// URL to this process (already running or cold-start).
+// Must be registered before whenReady() to avoid missing early events.
+app.on("open-url", (event, url) => {
+  event.preventDefault();
+  handleDeepLink(url);
+});
+
+// Windows / Linux: a second instance is launched with the URL in argv.
+// requestSingleInstanceLock redirects that second instance here instead.
+if (!app.requestSingleInstanceLock()) {
+  app.quit();
+} else {
+  app.on("second-instance", (_event, argv) => {
+    const url = argv.find((arg) => arg.startsWith(`${DESKTOP_SCHEME}://`));
+    if (url) {
+      handleDeepLink(url);
+    }
+    const window = mainWindow ?? BrowserWindow.getAllWindows()[0] ?? null;
+    if (window) {
+      revealWindow(window);
+    }
+  });
+}
+
 app
   .whenReady()
   .then(() => {
     writeDesktopLogHeader("app ready");
     configureAppIdentity();
     configureApplicationMenu();
+    app.setAsDefaultProtocolClient(DESKTOP_SCHEME);
     registerDesktopProtocol();
     configureAutoUpdater();
     void bootstrap().catch((error) => {
diff --git a/apps/desktop/src/preload.ts b/apps/desktop/src/preload.ts
index a675604872..8867a299b9 100644
--- a/apps/desktop/src/preload.ts
+++ b/apps/desktop/src/preload.ts
@@ -7,6 +7,7 @@ const SET_THEME_CHANNEL = "desktop:set-theme";
 const CONTEXT_MENU_CHANNEL = "desktop:context-menu";
 const OPEN_EXTERNAL_CHANNEL = "desktop:open-external";
 const MENU_ACTION_CHANNEL = "desktop:menu-action";
+const OPEN_THREAD_CHANNEL = "desktop:open-thread";
 const UPDATE_STATE_CHANNEL = "desktop:update-state";
 const UPDATE_GET_STATE_CHANNEL = "desktop:update-get-state";
 const UPDATE_SET_CHANNEL_CHANNEL = "desktop:update-set-channel";
@@ -85,4 +86,15 @@ contextBridge.exposeInMainWorld("desktopBridge", {
       ipcRenderer.removeListener(UPDATE_STATE_CHANNEL, wrappedListener);
     };
   },
+  onOpenThread: (listener) => {
+    const wrappedListener = (_event: Electron.IpcRendererEvent, threadId: unknown) => {
+      if (typeof threadId !== "string") return;
+      listener(threadId);
+    };
+
+    ipcRenderer.on(OPEN_THREAD_CHANNEL, wrappedListener);
+    return () => {
+      ipcRenderer.removeListener(OPEN_THREAD_CHANNEL, wrappedListener);
+    };
+  },
 } satisfies DesktopBridge);
diff --git a/apps/web/src/components/AppSidebarLayout.tsx b/apps/web/src/components/AppSidebarLayout.tsx
index b1ce57235a..7af06e5ac6 100644
--- a/apps/web/src/components/AppSidebarLayout.tsx
+++ b/apps/web/src/components/AppSidebarLayout.tsx
@@ -7,12 +7,14 @@ import {
   clearShortcutModifierState,
   syncShortcutModifierStateFromKeyboardEvent,
 } from "../shortcutModifierState";
+import { useStore } from "../store";
 
 const THREAD_SIDEBAR_WIDTH_STORAGE_KEY = "chat_thread_sidebar_width";
 const THREAD_SIDEBAR_MIN_WIDTH = 13 * 16;
 const THREAD_MAIN_CONTENT_MIN_WIDTH = 40 * 16;
 export function AppSidebarLayout({ children }: { children: ReactNode }) {
   const navigate = useNavigate();
+  const activeEnvironmentId = useStore((state) => state.activeEnvironmentId);
 
   useEffect(() => {
     const onWindowKeyDown = (event: KeyboardEvent) => {
@@ -36,6 +38,27 @@ export function AppSidebarLayout({ children }: { children: ReactNode }) {
     };
   }, []);
 
+  useEffect(() => {
+    const onOpenThread = window.desktopBridge?.onOpenThread;
+    if (typeof onOpenThread !== "function") {
+      return;
+    }
+
+    const unsubscribe = onOpenThread((threadId) => {
+      if (!activeEnvironmentId) {
+        return;
+      }
+      void navigate({
+        to: "/$environmentId/$threadId",
+        params: { environmentId: activeEnvironmentId, threadId },
+      });
+    });
+
+    return () => {
+      unsubscribe?.();
+    };
+  }, [navigate, activeEnvironmentId]);
+
   useEffect(() => {
     const onMenuAction = window.desktopBridge?.onMenuAction;
     if (typeof onMenuAction !== "function") {
diff --git a/apps/web/src/components/settings/SettingsPanels.browser.tsx b/apps/web/src/components/settings/SettingsPanels.browser.tsx
index b508b29b77..fa29a12f7f 100644
--- a/apps/web/src/components/settings/SettingsPanels.browser.tsx
+++ b/apps/web/src/components/settings/SettingsPanels.browser.tsx
@@ -313,6 +313,7 @@ const createDesktopBridgeStub = (overrides?: {
     showContextMenu: vi.fn().mockResolvedValue(null),
     openExternal: vi.fn().mockResolvedValue(true),
     onMenuAction: () => () => {},
+    onOpenThread: () => () => {},
     getUpdateState: vi.fn().mockResolvedValue(idleUpdateState),
     setUpdateChannel:
       overrides?.setUpdateChannel ??
diff --git a/apps/web/src/localApi.test.ts b/apps/web/src/localApi.test.ts
index c361cbd787..e35318ff89 100644
--- a/apps/web/src/localApi.test.ts
+++ b/apps/web/src/localApi.test.ts
@@ -185,6 +185,7 @@ function makeDesktopBridge(overrides: Partial<DesktopBridge> = {}): DesktopBridg
     showContextMenu: async () => null,
     openExternal: async () => true,
     onMenuAction: () => () => undefined,
+    onOpenThread: () => () => undefined,
     getUpdateState: async () => {
       throw new Error("getUpdateState not implemented in test");
     },
diff --git a/packages/contracts/src/ipc.ts b/packages/contracts/src/ipc.ts
index a1abc0fa4a..0cb4f29801 100644
--- a/packages/contracts/src/ipc.ts
+++ b/packages/contracts/src/ipc.ts
@@ -169,6 +169,7 @@ export interface DesktopBridge {
   ) => Promise<T | null>;
   openExternal: (url: string) => Promise<boolean>;
   onMenuAction: (listener: (action: string) => void) => () => void;
+  onOpenThread: (listener: (threadId: string) => void) => () => void;
   getUpdateState: () => Promise<DesktopUpdateState>;
   setUpdateChannel: (channel: DesktopUpdateChannel) => Promise<DesktopUpdateState>;
   checkForUpdate: () => Promise<DesktopUpdateCheckResult>;
diff --git a/scripts/build-desktop-artifact.ts b/scripts/build-desktop-artifact.ts
index 74e8bed0cb..55b4e4ded5 100644
--- a/scripts/build-desktop-artifact.ts
+++ b/scripts/build-desktop-artifact.ts
@@ -592,6 +592,7 @@ const createBuildConfig = Effect.fn("createBuildConfig")(function* (
       target: target === "dmg" ? [target, "zip"] : [target],
       icon: "icon.icns",
       category: "public.app-category.developer-tools",
+      protocols: [{ name: "T3 Code", schemes: ["t3"] }],
     };
   }