diff --git a/web/src/app/App.tsx b/web/src/app/App.tsx
new file mode 100644
index 0000000..cfb46d3
--- /dev/null
+++ b/web/src/app/App.tsx
@@ -0,0 +1,5 @@
+import { AppProviders } from "@/app/providers";
+
+export function App() {
+  return <AppProviders />;
+}
diff --git a/web/src/app/AppErrorBoundary.tsx b/web/src/app/AppErrorBoundary.tsx
new file mode 100644
index 0000000..456e24a
--- /dev/null
+++ b/web/src/app/AppErrorBoundary.tsx
@@ -0,0 +1,44 @@
+import { Component, type ErrorInfo, type ReactNode } from "react";
+
+interface Props {
+  children: ReactNode;
+}
+
+interface State {
+  hasError: boolean;
+  message?: string;
+}
+
+/** Minimal error boundary that renders a dark fallback panel. */
+export class AppErrorBoundary extends Component<Props, State> {
+  state: State = { hasError: false };
+
+  static getDerivedStateFromError(error: Error): State {
+    return { hasError: true, message: error.message };
+  }
+
+  componentDidCatch(error: Error, info: ErrorInfo) {
+    console.error("Uncaught application error:", error, info);
+  }
+
+  render() {
+    if (this.state.hasError) {
+      return (
+        <div className="flex min-h-screen flex-col items-center justify-center gap-2 bg-neutral-950 p-8 text-neutral-100">
+          <h1 className="text-lg font-semibold">Something went wrong</h1>
+          {this.state.message && (
+            <p className="text-sm text-neutral-400">{this.state.message}</p>
+          )}
+          <button
+            type="button"
+            className="mt-2 rounded-md border border-neutral-700 px-3 py-1 text-sm text-neutral-300 hover:bg-neutral-800"
+            onClick={() => window.location.reload()}
+          >
+            Reload
+          </button>
+        </div>
+      );
+    }
+    return this.props.children;
+  }
+}
diff --git a/web/src/app/providers.tsx b/web/src/app/providers.tsx
new file mode 100644
index 0000000..154413f
--- /dev/null
+++ b/web/src/app/providers.tsx
@@ -0,0 +1,29 @@
+import { useState } from "react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { RouterProvider } from "react-router";
+
+import { router } from "@/app/router";
+import { AppErrorBoundary } from "@/app/AppErrorBoundary";
+
+export function AppProviders() {
+  const [queryClient] = useState(
+    () =>
+      new QueryClient({
+        defaultOptions: {
+          queries: {
+            staleTime: 60_000,
+            retry: 1,
+            refetchOnWindowFocus: false,
+          },
+        },
+      }),
+  );
+
+  return (
+    <AppErrorBoundary>
+      <QueryClientProvider client={queryClient}>
+        <RouterProvider router={router} />
+      </QueryClientProvider>
+    </AppErrorBoundary>
+  );
+}
diff --git a/web/src/app/router.tsx b/web/src/app/router.tsx
new file mode 100644
index 0000000..f8bd529
--- /dev/null
+++ b/web/src/app/router.tsx
@@ -0,0 +1,10 @@
+import { createBrowserRouter } from "react-router";
+
+import { WorkspacePage } from "@/pages/workspace/WorkspacePage";
+
+export const router = createBrowserRouter([
+  {
+    path: "/",
+    element: <WorkspacePage />,
+  },
+]);
diff --git a/web/src/lib/api/client.ts b/web/src/lib/api/client.ts
new file mode 100644
index 0000000..78e11e8
--- /dev/null
+++ b/web/src/lib/api/client.ts
@@ -0,0 +1,11 @@
+/**
+ * Reserved API client for future backend calls.
+ *
+ * No real network requests are made in this task; later tasks will
+ * wire this helper to actual endpoints.
+ */
+const baseUrl = import.meta.env.VITE_API_BASE_URL ?? "";
+
+export function apiClient(path: string, init?: RequestInit): Promise<Response> {
+  return fetch(`${baseUrl}${path}`, init);
+}
diff --git a/web/src/lib/store/workspace-ui-store.ts b/web/src/lib/store/workspace-ui-store.ts
new file mode 100644
index 0000000..969fa02
--- /dev/null
+++ b/web/src/lib/store/workspace-ui-store.ts
@@ -0,0 +1,19 @@
+import { create } from "zustand";
+
+interface WorkspaceUiState {
+  activeFilePath: string;
+  terminalVisible: boolean;
+  sidebarCollapsed: boolean;
+  setActiveFilePath: (path: string) => void;
+  toggleTerminal: () => void;
+  toggleSidebar: () => void;
+}
+
+export const useWorkspaceUiStore = create<WorkspaceUiState>((set) => ({
+  activeFilePath: "src/main.tsx",
+  terminalVisible: true,
+  sidebarCollapsed: false,
+  setActiveFilePath: (path) => set({ activeFilePath: path }),
+  toggleTerminal: () => set((s) => ({ terminalVisible: !s.terminalVisible })),
+  toggleSidebar: () => set((s) => ({ sidebarCollapsed: !s.sidebarCollapsed })),
+}));
diff --git a/web/src/main.tsx b/web/src/main.tsx
index dfe6583..a4a1cfa 100644
--- a/web/src/main.tsx
+++ b/web/src/main.tsx
@@ -1,14 +1,12 @@
 import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import "@/styles/index.css";
+import { App } from "@/app/App";
+
+document.documentElement.classList.add("dark");
 
 createRoot(document.getElementById("root")!).render(
   <StrictMode>
-    <div className="min-h-screen bg-neutral-950 p-8 text-neutral-100">
-      <h1 className="text-2xl font-semibold">Codespace</h1>
-      <p className="mt-2 text-sm text-neutral-400">
-        Web frontend scaffold. IDE shell coming in later tasks.
-      </p>
-    </div>
+    <App />
   </StrictMode>,
 );
diff --git a/web/src/pages/workspace/WorkspacePage.tsx b/web/src/pages/workspace/WorkspacePage.tsx
new file mode 100644
index 0000000..7f0316e
--- /dev/null
+++ b/web/src/pages/workspace/WorkspacePage.tsx
@@ -0,0 +1,28 @@
+import { findFileByPath, mockWorkspaceFiles } from "@/pages/workspace/mock-data";
+import { useWorkspaceUiStore } from "@/lib/store/workspace-ui-store";
+
+export function WorkspacePage() {
+  const activeFilePath = useWorkspaceUiStore((s) => s.activeFilePath);
+  const activeFile = findFileByPath(mockWorkspaceFiles, activeFilePath);
+
+  return (
+    <div className="flex h-full flex-col bg-background text-foreground">
+      <header className="border-b border-border px-4 py-2">
+        <h1 className="text-sm font-semibold">Codespace Workspace</h1>
+      </header>
+      <div className="flex-1 overflow-auto p-4">
+        <p className="text-sm text-muted-foreground">
+          Active file: <code className="text-foreground">{activeFilePath}</code>
+        </p>
+        <p className="mt-2 text-sm text-muted-foreground">
+          Mock file match:{" "}
+          <code className="text-foreground">
+            {activeFile
+              ? `${activeFile.name} (${activeFile.language ?? activeFile.kind})`
+              : "not found"}
+          </code>
+        </p>
+      </div>
+    </div>
+  );
+}
diff --git a/web/src/pages/workspace/mock-data.ts b/web/src/pages/workspace/mock-data.ts
new file mode 100644
index 0000000..b838c99
--- /dev/null
+++ b/web/src/pages/workspace/mock-data.ts
@@ -0,0 +1,80 @@
+export type MockFileNode = {
+  name: string;
+  path: string;
+  kind: "file" | "dir";
+  language?: string;
+  content?: string;
+  children?: MockFileNode[];
+};
+
+const file = (
+  name: string,
+  path: string,
+  language: string,
+  content: string,
+): MockFileNode => ({ name, path, kind: "file", language, content });
+
+const dir = (
+  name: string,
+  path: string,
+  children: MockFileNode[],
+): MockFileNode => ({ name, path, kind: "dir", children });
+
+export const mockWorkspaceFiles: MockFileNode[] = [
+  file("package.json", "package.json", "json", `{
+  "name": "codespace",
+  "version": "0.0.0"
+}
+`),
+  file("README.md", "README.md", "markdown", `# Codespace
+
+A self-hosted development environment.
+`),
+  dir("src", "src", [
+    file(
+      "main.tsx",
+      "src/main.tsx",
+      "typescript",
+      `import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+import { App } from "@/app/App";
+import "@/styles/index.css";
+
+createRoot(document.getElementById("root")!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+);
+`,
+    ),
+  ]),
+  dir("internal", "internal", [
+    dir("api", "internal/api", [
+      file(
+        "router.go",
+        "internal/api/router.go",
+        "go",
+        `package api
+
+// Router is reserved for future backend routes.
+type Router struct{}
+`,
+      ),
+    ]),
+  ]),
+];
+
+/** Depth-first search for a node with the given slash-delimited path. */
+export function findFileByPath(
+  files: MockFileNode[],
+  path: string,
+): MockFileNode | undefined {
+  for (const node of files) {
+    if (node.path === path) return node;
+    if (node.children) {
+      const found = findFileByPath(node.children, path);
+      if (found) return found;
+    }
+  }
+  return undefined;
+}