diff --git a/env.example b/env.example
index c812e00f..ffdc0f0b 100644
--- a/env.example
+++ b/env.example
@@ -163,6 +163,16 @@ AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 # Set to "false" to block private IPs, localhost, and internal hostnames
 # ALLOW_PRIVATE_URLS=false
 
+# Behind a CDN such as Cloudflare (Optional)
+# The daily quota is counted per IP. By default the IP is the first
+# X-Forwarded-For entry, which visitors can set to anything. Name the header
+# your CDN fills with the visitor's real IP instead:
+# CLIENT_IP_HEADER=cf-connecting-ip
+# Then have the CDN add an X-Origin-Secret header with this value to every
+# request. API calls without it get 403, so nobody can skip the CDN and fake
+# the IP header above.
+# ORIGIN_SECRET=a-long-random-string
+
 # Self-hosted deployment (Optional)
 # Self-hosted users may implement custom quota-management solutions,
 # which triggers the client UI to display messages suggesting self-hosting or sponsorship.
diff --git a/lib/user-id.ts b/lib/user-id.ts
index 3d7d7181..2a347afa 100644
--- a/lib/user-id.ts
+++ b/lib/user-id.ts
@@ -2,10 +2,19 @@
  * Generate a userId from request for tracking purposes.
  * Uses base64url encoding of IP for URL-safe identifier.
  * Note: base64 is reversible - this is NOT privacy protection.
+ *
+ * The first X-Forwarded-For entry is whatever the visitor sent, so behind a
+ * CDN set CLIENT_IP_HEADER to the header it fills with the real IP (e.g.
+ * cf-connecting-ip), and ORIGIN_SECRET so requests that skip the CDN are
+ * refused (see proxy.ts).
  */
 export function getUserIdFromRequest(req: Request): string {
-    const forwardedFor = req.headers.get("x-forwarded-for")
-    const rawIp = forwardedFor?.split(",")[0]?.trim() || "anonymous"
+    const ipHeader = process.env.CLIENT_IP_HEADER
+    const rawIp =
+        (ipHeader
+            ? req.headers.get(ipHeader)?.trim()
+            : req.headers.get("x-forwarded-for")?.split(",")[0]?.trim()) ||
+        "anonymous"
     return rawIp === "anonymous"
         ? rawIp
         : `user-${Buffer.from(rawIp).toString("base64url")}`
diff --git a/proxy.ts b/proxy.ts
index fdf43a43..7616d2e5 100644
--- a/proxy.ts
+++ b/proxy.ts
@@ -27,9 +27,19 @@ function getLocale(request: NextRequest): string | undefined {
 export function proxy(request: NextRequest) {
     const pathname = request.nextUrl.pathname
 
-    // Skip API routes, static files, and Next.js internals
+    if (pathname.startsWith("/api/")) {
+        // With ORIGIN_SECRET set, API calls must come through the CDN that
+        // adds this header. A call straight to the origin could fake the
+        // CLIENT_IP_HEADER and get a fresh quota for every made-up IP.
+        const secret = process.env.ORIGIN_SECRET
+        if (secret && request.headers.get("x-origin-secret") !== secret) {
+            return NextResponse.json({ error: "Forbidden" }, { status: 403 })
+        }
+        return
+    }
+
+    // Skip static files and Next.js internals
     if (
-        pathname.startsWith("/api/") ||
         pathname.startsWith("/_next/") ||
         pathname.startsWith("/drawio") ||
         pathname.includes("/favicon") ||
@@ -58,6 +68,9 @@ export function proxy(request: NextRequest) {
 }
 
 export const config = {
-    // Matcher ignoring `/_next/` and `/api/`
-    matcher: ["/((?!api|_next/static|_next/image|favicon.ico).*)"],
+    // API routes (for ORIGIN_SECRET), and pages without `/_next/` assets
+    matcher: [
+        "/api/:path*",
+        "/((?!api|_next/static|_next/image|favicon.ico).*)",
+    ],
 }
diff --git a/tests/unit/client-ip.test.ts b/tests/unit/client-ip.test.ts
new file mode 100644
index 00000000..caec89b3
--- /dev/null
+++ b/tests/unit/client-ip.test.ts
@@ -0,0 +1,63 @@
+import { NextRequest } from "next/server"
+import { afterEach, describe, expect, it } from "vitest"
+import { getUserIdFromRequest } from "@/lib/user-id"
+import { proxy } from "@/proxy"
+
+const idFor = (ip: string) => `user-${Buffer.from(ip).toString("base64url")}`
+
+afterEach(() => {
+    delete process.env.CLIENT_IP_HEADER
+    delete process.env.ORIGIN_SECRET
+})
+
+describe("getUserIdFromRequest", () => {
+    const req = new Request("http://localhost/api/chat", {
+        headers: {
+            "x-forwarded-for": "203.0.113.9, 198.51.100.7",
+            "cf-connecting-ip": "198.51.100.7",
+        },
+    })
+
+    it("uses the first X-Forwarded-For entry by default", () => {
+        expect(getUserIdFromRequest(req)).toBe(idFor("203.0.113.9"))
+    })
+
+    it("uses CLIENT_IP_HEADER when set", () => {
+        process.env.CLIENT_IP_HEADER = "cf-connecting-ip"
+        expect(getUserIdFromRequest(req)).toBe(idFor("198.51.100.7"))
+    })
+
+    it("is anonymous when the configured header is missing", () => {
+        process.env.CLIENT_IP_HEADER = "cf-connecting-ip"
+        expect(
+            getUserIdFromRequest(new Request("http://localhost/api/chat")),
+        ).toBe("anonymous")
+    })
+})
+
+describe("ORIGIN_SECRET", () => {
+    const call = (path: string, secret?: string) =>
+        proxy(
+            new NextRequest(`http://localhost${path}`, {
+                headers: secret ? { "x-origin-secret": secret } : {},
+            }),
+        )
+
+    it("lets every API call through when unset", () => {
+        expect(call("/api/chat")).toBeUndefined()
+    })
+
+    it("refuses API calls without the right header", async () => {
+        process.env.ORIGIN_SECRET = "s3cret"
+        for (const res of [call("/api/chat"), call("/api/chat", "wrong")]) {
+            expect(res?.status).toBe(403)
+        }
+        expect(call("/api/chat", "s3cret")).toBeUndefined()
+    })
+
+    it("leaves pages alone, such as the health check on /", () => {
+        process.env.ORIGIN_SECRET = "s3cret"
+        expect(call("/")?.status).toBe(307)
+        expect(call("/en")).toBeUndefined()
+    })
+})