diff --git a/web/.env.example b/web/.env.example
index af4b6d4a9bc7..dc7c5fbd543d 100644
--- a/web/.env.example
+++ b/web/.env.example
@@ -37,6 +37,9 @@ CMUX_PRO_FROM_EMAIL=
 # checkout flow. Price ids are optional overrides; otherwise the app resolves
 # prices by lookup key in Stripe test/live mode.
 STRIPE_SECRET_KEY=
+# Set to 1 only after the Stripe account has tax registrations configured.
+# Leave at 0 (or unset) to keep Checkout tax calculation disabled.
+STRIPE_AUTOMATIC_TAX=0
 STRIPE_WEBHOOK_SECRET=
 STRIPE_PRO_MONTHLY_PRICE_ID=
 # Retired. Keep unset; grandfathered $240 subscriptions remain in Stripe, while
diff --git a/web/app/api/billing/checkout/route.ts b/web/app/api/billing/checkout/route.ts
index 05f86fd78572..0dabf82adc76 100644
--- a/web/app/api/billing/checkout/route.ts
+++ b/web/app/api/billing/checkout/route.ts
@@ -189,6 +189,7 @@ async function stripeProCheckout(
       client_reference_id: stackUserId,
       metadata,
       subscription_data: { metadata },
+      ...stripeCheckoutTaxOptions(),
       customer: stripeBillingStatus.customerId ?? undefined,
       customer_email: stripeBillingStatus.customerId
         ? undefined
@@ -278,6 +279,7 @@ async function stripeTeamCheckout(
       client_reference_id: resolvedTeamId,
       metadata,
       subscription_data: { metadata },
+      ...stripeCheckoutTaxOptions(),
       allow_promotion_codes: true,
       success_url: successUrl,
       cancel_url: cancelUrl.toString(),
@@ -437,6 +439,19 @@ function checkoutBillingInterval(raw: string | null): BillingInterval | null {
   return raw === "month" || raw === "year" ? raw : null;
 }
 
+function stripeCheckoutTaxOptions(): {
+  readonly automatic_tax?: { readonly enabled: true };
+  readonly tax_id_collection?: { readonly enabled: true };
+} {
+  // The env schema validates this opt-in, while reading process.env here keeps
+  // the route's test seam dynamic when a test toggles the flag after imports.
+  if (process.env.STRIPE_AUTOMATIC_TAX?.trim() !== "1") return {};
+  return {
+    automatic_tax: { enabled: true },
+    tax_id_collection: { enabled: true },
+  };
+}
+
 async function checkoutStackServerApp(): Promise<CheckoutStackServerApp | null> {
   const { getStackServerApp, isStackConfigured } = await import("../../../lib/stack");
   if (!isStackConfigured()) return null;
diff --git a/web/app/env.ts b/web/app/env.ts
index e35c3ecaea48..60637a7431c5 100644
--- a/web/app/env.ts
+++ b/web/app/env.ts
@@ -198,6 +198,8 @@ export const env = createEnv({
     // Direct Stripe billing for cmux Pro. Optional: when unset, checkout is
     // unavailable.
     STRIPE_SECRET_KEY: z.string().min(1).optional(),
+    // Deliberately opt in only after Stripe tax registrations are configured.
+    STRIPE_AUTOMATIC_TAX: z.enum(["0", "1"]).optional(),
     STRIPE_WEBHOOK_SECRET: z.string().min(1).optional(),
     STRIPE_PRO_MONTHLY_PRICE_ID: z.string().min(1).optional(),
     // Deliberately distinct from the legacy STRIPE_PRO_YEARLY_PRICE_ID,
@@ -376,6 +378,7 @@ export const env = createEnv({
     CMUX_FOUNDERS_FROM_EMAIL: trimEnv(process.env.CMUX_FOUNDERS_FROM_EMAIL),
     CMUX_PRO_FROM_EMAIL: trimEnv(process.env.CMUX_PRO_FROM_EMAIL),
     STRIPE_SECRET_KEY: trimEnv(process.env.STRIPE_SECRET_KEY),
+    STRIPE_AUTOMATIC_TAX: trimEnv(process.env.STRIPE_AUTOMATIC_TAX),
     STRIPE_WEBHOOK_SECRET: trimEnv(process.env.STRIPE_WEBHOOK_SECRET),
     STRIPE_PRO_MONTHLY_PRICE_ID: trimEnv(process.env.STRIPE_PRO_MONTHLY_PRICE_ID),
     STRIPE_PRO_YEARLY_PRICE_ID: trimEnv(process.env.STRIPE_PRO_YEARLY_PRICE_ID),
diff --git a/web/tests/billing-checkout-route.test.ts b/web/tests/billing-checkout-route.test.ts
index 0cd1f6bcf5ac..959ab5f6ef0e 100644
--- a/web/tests/billing-checkout-route.test.ts
+++ b/web/tests/billing-checkout-route.test.ts
@@ -10,6 +10,7 @@ const dbClientModule = await import("../db/client");
 const realCloudDb = dbClientModule.cloudDb;
 const realCloseCloudDbForTests = dbClientModule.closeCloudDbForTests;
 const realCreateAwsRdsIamPool = dbClientModule.createAwsRdsIamPool;
+const originalStripeAutomaticTax = process.env.STRIPE_AUTOMATIC_TAX;
 
 const SIGNED_IN_USER_ID = "7f5e4e80-3d96-4f6a-8f2e-3c1e4a4d0d01";
 const ANONYMOUS_USER_ID = "5a0f6f7a-7d9f-4bc5-a3be-2d11f7a6c902";
@@ -159,6 +160,11 @@ beforeAll(() => {
 
 afterAll(() => {
   useStubDb = false;
+  if (originalStripeAutomaticTax === undefined) {
+    delete process.env.STRIPE_AUTOMATIC_TAX;
+  } else {
+    process.env.STRIPE_AUTOMATIC_TAX = originalStripeAutomaticTax;
+  }
 });
 
 describe("billing checkout route", () => {
@@ -174,6 +180,7 @@ describe("billing checkout route", () => {
     userResponses = [];
     stackAuthUnavailable = false;
     stripeConfigured = false;
+    delete process.env.STRIPE_AUTOMATIC_TAX;
     createdStripeSessions.length = 0;
     createdStripeCustomers.length = 0;
     insertedStripeCustomers.length = 0;
@@ -454,6 +461,8 @@ describe("billing checkout route", () => {
         "https://cmux.test/api/billing/complete?session_id={CHECKOUT_SESSION_ID}&cmux_scheme=cmux",
       cancel_url: "https://cmux.test/pricing?billing=cancelled&interval=month",
     });
+    expect(createdStripeSessions[0]).not.toHaveProperty("automatic_tax");
+    expect(createdStripeSessions[0]).not.toHaveProperty("tax_id_collection");
     expect(captureBillingCheckoutStarted).toHaveBeenCalledTimes(1);
     expect(captureBillingCheckoutStarted).toHaveBeenCalledWith({
       sessionId: CHECKOUT_SESSION_ID,
@@ -463,6 +472,22 @@ describe("billing checkout route", () => {
     });
   });
 
+  test("enables Stripe Tax and tax-id collection only when opted in", async () => {
+    process.env.STRIPE_AUTOMATIC_TAX = "1";
+    stripeConfigured = true;
+    userResponses = [null, anonymousUser];
+
+    const response = await GET(
+      new NextRequest("https://cmux.test/api/billing/checkout"),
+    );
+
+    expect(response.headers.get("location")).toBe("https://checkout.stripe.com/c/session");
+    expect(createdStripeSessions[0]).toMatchObject({
+      automatic_tax: { enabled: true },
+      tax_id_collection: { enabled: true },
+    });
+  });
+
   test("does not capture checkout analytics when Stripe returns no session id", async () => {
     stripeConfigured = true;
     stripeSessionResponse = {