diff --git a/openspec/changes/code-review-fix/tasks.md b/openspec/changes/code-review-fix/tasks.md
index 9de7ef5..cbe64bf 100644
--- a/openspec/changes/code-review-fix/tasks.md
+++ b/openspec/changes/code-review-fix/tasks.md
@@ -19,7 +19,7 @@
 
 ## 3. Authentication
 
-- [ ] 3.1 `[sonnet]` Create `src/middleware.ts` with API key auth middleware: read `API_KEY` env var, check `X-API-Key` header on all `/api/*` routes except `/api/health`, return 401 if invalid
+- [x] 3.1 `[sonnet]` Create `src/middleware.ts` with API key auth middleware: read `API_KEY` env var, check `X-API-Key` header on all `/api/*` routes except `/api/health`, return 401 if invalid
 - [ ] 3.2 `[sonnet]` Add FastAPI `Depends()` API key dependency in `services/ml/app/main.py`: read `API_KEY` env var, check `X-API-Key` header, exempt `/health` endpoint
 - [ ] 3.3 `[sonnet]` Update all Next.js proxy routes to forward `X-API-Key` header to ML service
 - [ ] 3.4 `[haiku]` Add `API_KEY` to `.env.example` with placeholder value and instructions
diff --git a/src/middleware.ts b/src/middleware.ts
new file mode 100644
index 0000000..dda9010
--- /dev/null
+++ b/src/middleware.ts
@@ -0,0 +1,33 @@
+import { NextResponse } from "next/server";
+import type { NextRequest } from "next/server";
+
+export function middleware(request: NextRequest) {
+  const { pathname } = request.nextUrl;
+
+  // Skip auth check for the health endpoint
+  if (pathname === "/api/health") {
+    return NextResponse.next();
+  }
+
+  const apiKey = process.env.API_KEY;
+
+  // If API_KEY is not configured, skip auth check (fail-open for development)
+  if (!apiKey) {
+    console.warn(
+      "Warning: API_KEY environment variable is not set. API authentication is disabled."
+    );
+    return NextResponse.next();
+  }
+
+  const requestApiKey = request.headers.get("X-API-Key");
+
+  if (!requestApiKey || requestApiKey !== apiKey) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  return NextResponse.next();
+}
+
+export const config = {
+  matcher: ["/api/:path*"],
+};