diff --git a/ee/apps/den-api/src/routes/auth/index.ts b/ee/apps/den-api/src/routes/auth/index.ts
index fb3aca0174..80c2348bb3 100644
--- a/ee/apps/den-api/src/routes/auth/index.ts
+++ b/ee/apps/den-api/src/routes/auth/index.ts
@@ -273,6 +273,13 @@ export function isBetterAuthEmailPasswordRequest(request: Request) {
   return request.method.toUpperCase() === "POST" && (path === "/sign-in/email" || path === "/sign-up/email")
 }
 
+// Email OTP sign-in issues a session (and creates unknown users) just like
+// email/password, so SSO-only deployments must reject it as well.
+export function isBetterAuthEmailOtpSignInRequest(request: Request) {
+  const url = new URL(request.url)
+  return request.method.toUpperCase() === "POST" && getBetterAuthProxyPath(url.pathname) === "/sign-in/email-otp"
+}
+
 export function isBetterAuthEmailSignupRequest(request: Request) {
   const url = new URL(request.url)
   return request.method.toUpperCase() === "POST" && getBetterAuthProxyPath(url.pathname) === "/sign-up/email"
@@ -341,7 +348,7 @@ async function getSingleOrgAuthGuardResponse(request: Request, context: Context,
     }
   }
 
-  if (isBetterAuthEmailPasswordRequest(request)) {
+  if (isBetterAuthEmailPasswordRequest(request) || isBetterAuthEmailOtpSignInRequest(request)) {
     const status = await getSingletonSsoStatus()
     if (status.configured) {
       return singleOrgSsoRequiredResponse(status.signInPath)
diff --git a/ee/apps/den-api/test/single-org-route-guards.test.ts b/ee/apps/den-api/test/single-org-route-guards.test.ts
index 3f3271f0b3..592a0fba96 100644
--- a/ee/apps/den-api/test/single-org-route-guards.test.ts
+++ b/ee/apps/den-api/test/single-org-route-guards.test.ts
@@ -177,6 +177,20 @@ test("single_org SSO-only guard recognizes email/password auth requests", () =>
   }))).toBe(false)
 })
 
+test("single_org SSO-only guard recognizes email OTP sign-in requests", () => {
+  expect(authRoutesModule.isBetterAuthEmailOtpSignInRequest(new Request("http://den.local/api/auth/sign-in/email-otp", {
+    method: "POST",
+  }))).toBe(true)
+
+  expect(authRoutesModule.isBetterAuthEmailOtpSignInRequest(new Request("http://den.local/api/auth/sign-in/email", {
+    method: "POST",
+  }))).toBe(false)
+
+  expect(authRoutesModule.isBetterAuthEmailOtpSignInRequest(new Request("http://den.local/api/auth/sign-in/email-otp", {
+    method: "GET",
+  }))).toBe(false)
+})
+
 test("single_org SSO-only guard still recognizes invited email signup requests", () => {
   expect(authRoutesModule.isBetterAuthEmailPasswordRequest(new Request("http://den.local/api/auth/sign-up/email?invite=inv_example", {
     method: "POST",
diff --git a/ee/apps/den-web/app/sso/[orgSlug]/page.tsx b/ee/apps/den-web/app/sso/[orgSlug]/page.tsx
index b44ea5302a..4919349cb5 100644
--- a/ee/apps/den-web/app/sso/[orgSlug]/page.tsx
+++ b/ee/apps/den-web/app/sso/[orgSlug]/page.tsx
@@ -2,7 +2,7 @@
 
 import Link from "next/link";
 import { DenStatusScreen } from "../../../components/den-status-screen";
-import { useEffect, useMemo, useState } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
 import { useParams, useSearchParams } from "next/navigation";
 import { getSocialCallbackUrl, requestJson } from "../../(den)/_lib/den-flow";
 
@@ -12,6 +12,7 @@ export default function OrganizationSsoSignInPage() {
   const [error, setError] = useState<string | null>(null);
   const [redirectUrl, setRedirectUrl] = useState<string | null>(null);
   const [attempt, setAttempt] = useState(0);
+  const startRequests = useRef(new Map<string, ReturnType<typeof requestJson>>());
   const orgSlug = typeof params?.orgSlug === "string" ? params.orgSlug : "";
 
   const callbackURL = useMemo(() => searchParams.get("callbackURL") || getSocialCallbackUrl(), [searchParams]);
@@ -25,19 +26,28 @@ export default function OrganizationSsoSignInPage() {
 
     void (async () => {
       try {
-        const { response, payload } = await requestJson("/api/auth/sign-in/sso", {
-          method: "POST",
-          headers: {
-            "Content-Type": "application/json",
-            Accept: "application/json",
-          },
-          body: JSON.stringify({
-            organizationSlug: orgSlug,
-            callbackURL,
-            errorCallbackURL,
-            loginHint,
-          }),
-        });
+        // React StrictMode runs this effect twice in dev. A second sign-in/sso
+        // call overwrites the OAuth state cookie and the callback then fails
+        // with state_mismatch, so both runs share one request.
+        const requestKey = JSON.stringify([orgSlug, callbackURL, errorCallbackURL, loginHint, attempt]);
+        let startRequest = startRequests.current.get(requestKey);
+        if (!startRequest) {
+          startRequest = requestJson("/api/auth/sign-in/sso", {
+            method: "POST",
+            headers: {
+              "Content-Type": "application/json",
+              Accept: "application/json",
+            },
+            body: JSON.stringify({
+              organizationSlug: orgSlug,
+              callbackURL,
+              errorCallbackURL,
+              loginHint,
+            }),
+          });
+          startRequests.current.set(requestKey, startRequest);
+        }
+        const { response, payload } = await startRequest;
 
         if (!response.ok) {
           throw new Error(