diff --git a/backend/__tests__/integration/oidcLogout.test.js b/backend/__tests__/integration/oidcLogout.test.js
index f03ca407..d18ccfdf 100644
--- a/backend/__tests__/integration/oidcLogout.test.js
+++ b/backend/__tests__/integration/oidcLogout.test.js
@@ -187,7 +187,7 @@ describe('OIDC logout-to-IdP (#798 phase 3)', () => {
     }
   });
 
-  it('stores a bare marker for oversized ID tokens; logout still round-trips, without a hint', async () => {
+  it('stores an issuer-tagged marker for oversized ID tokens; logout still round-trips, without a hint', async () => {
     idp.setNextUser({
       sub: 'logout-sub-5',
       email: 'logout5@example.com',
@@ -198,7 +198,10 @@ describe('OIDC logout-to-IdP (#798 phase 3)', () => {
     const cbRes = await ssoRoundTrip();
     const cookie = idTokenCookie(cbRes);
     expect(cookie).toBeTruthy();
-    expect(decodeURIComponent(cookie.replace('oidc_id_token=', ''))).toBe('sso');
+    // Issuer-tagged marker, not the (oversized) token itself.
+    const marker = decodeURIComponent(cookie.replace('oidc_id_token=', ''));
+    expect(marker.startsWith('sso.')).toBe(true);
+    expect(Buffer.from(marker.split('.')[1], 'base64url').toString('utf8')).toBe(idp.issuer);
 
     const res = await request(app)
       .post('/api/auth/logout')
@@ -210,6 +213,15 @@ describe('OIDC logout-to-IdP (#798 phase 3)', () => {
     expect(url.searchParams.get('client_id')).toBe(idp.clientId);
   });
 
+  it('skips the round-trip for an oversized-token marker from a DIFFERENT issuer', async () => {
+    const foreignMarker = `sso.${Buffer.from('http://other-idp.example').toString('base64url')}`;
+    const res = await request(app)
+      .post('/api/auth/logout')
+      .set('Cookie', `oidc_id_token=${foreignMarker}`)
+      .expect(200);
+    expect(res.body.ssoLogoutUrl).toBeUndefined();
+  });
+
   it('a fresh local-password login clears a stale SSO marker', async () => {
     const role = await db('roles').where({ name: 'admin' }).first();
     await db('admin_users').insert({
diff --git a/backend/src/routes/auth.js b/backend/src/routes/auth.js
index 3936a1f7..ded3c862 100644
--- a/backend/src/routes/auth.js
+++ b/backend/src/routes/auth.js
@@ -353,10 +353,10 @@ router.post('/logout', async (req, res) => {
     const oidcIdToken = req.cookies?.[OIDC_ID_TOKEN_COOKIE];
     if (oidcIdToken) {
       res.clearCookie(OIDC_ID_TOKEN_COOKIE, oidcIdTokenClearOptions());
-      // The bare 'sso' marker (oversized ID token at login) still triggers
-      // the round-trip, just without an id_token_hint.
-      const hint = oidcIdToken.split('.').length === 3 ? oidcIdToken : undefined;
-      ssoLogoutUrl = await require('../services/oidcService').buildEndSessionUrl(hint);
+      // The service interprets the marker itself: raw ID token → hint
+      // (iss/aud-validated), issuer-tagged 'sso.<b64>' marker (oversized
+      // token) → round-trip without a hint, unknown/foreign origin → null.
+      ssoLogoutUrl = await require('../services/oidcService').buildEndSessionUrl(oidcIdToken);
     }
 
     res.json({ message: 'Logged out successfully', ...(ssoLogoutUrl ? { ssoLogoutUrl } : {}) });
@@ -1065,11 +1065,14 @@ router.get('/admin/sso/callback', async (req, res) => {
 
     // Cookie limit is 4KB — an oversized ID token (huge group lists) can't
     // be stored, but the SSO-session marker must survive or logout-to-IdP
-    // silently turns off for exactly those users. Store the bare marker
-    // 'sso' instead; logout then sends the end-session request without an
-    // id_token_hint (costs one IdP confirmation click).
+    // silently turns off for exactly those users. Store an issuer-tagged
+    // marker instead; logout then sends the end-session request without an
+    // id_token_hint (costs one IdP confirmation click), and the tag still
+    // lets buildEndSessionUrl refuse the round-trip after an issuer change.
     if (idToken) {
-      const cookieValue = idToken.length <= 3900 ? idToken : 'sso';
+      const cookieValue = idToken.length <= 3900
+        ? idToken
+        : oidcService.buildOversizeSsoMarker(claims.iss);
       res.cookie(OIDC_ID_TOKEN_COOKIE, cookieValue, oidcIdTokenCookieOptions(res));
     }
 
diff --git a/backend/src/services/oidcService.js b/backend/src/services/oidcService.js
index 2d770895..11d02146 100644
--- a/backend/src/services/oidcService.js
+++ b/backend/src/services/oidcService.js
@@ -374,28 +374,48 @@ function decodeJwtPayload(token) {
   }
 }
 
-async function buildEndSessionUrl(idTokenHint) {
+/**
+ * Marker for SSO sessions whose ID token is too large for a cookie:
+ * `sso.<base64url(issuer)>`. Keeps the issuer identity so /logout can still
+ * validate the marker against the current config — a bare marker would
+ * round-trip to the wrong IdP after an issuer change.
+ */
+function buildOversizeSsoMarker(issuer) {
+  return `sso.${Buffer.from(String(issuer || '')).toString('base64url')}`;
+}
+
+async function buildEndSessionUrl(sessionMarker) {
   try {
     const cfg = await getOidcConfig();
     if (!cfg.enabled || !cfg.logoutFromIdp || !isConfigured(cfg)) return null;
     const { client, issuerMetadata } = await getClient(cfg);
     if (!issuerMetadata.end_session_endpoint) return null;
 
-    // The hint comes from a cookie minted at login — the OIDC config may
-    // have changed since. A token from a DIFFERENT issuer means the session
-    // belongs to another IdP entirely: skip the round-trip (ending the new
-    // IdP's session would be wrong, and providers that validate the hint
-    // would strand the user on an error page). Same issuer but a different
-    // client (aud): the hint is unusable, but the browser's IdP session is
-    // real — round-trip without the hint.
-    let hint = idTokenHint;
-    if (hint) {
-      const payload = decodeJwtPayload(hint);
-      if (!payload || payload.iss !== issuerMetadata.issuer) return null;
-      const audMatches = Array.isArray(payload.aud)
-        ? payload.aud.includes(cfg.clientId)
-        : payload.aud === cfg.clientId;
-      if (!audMatches) hint = undefined;
+    // The marker comes from a cookie minted at login — the OIDC config may
+    // have changed since. A session from a DIFFERENT issuer must not be
+    // round-tripped: ending the newly configured IdP's session would be
+    // wrong, and providers that validate the hint would strand the user on
+    // an error page. Same issuer but a different client (aud): the hint is
+    // unusable, but the browser's IdP session is real — round-trip without
+    // the hint. A marker whose origin can't be determined is skipped.
+    let hint;
+    if (sessionMarker) {
+      const parts = String(sessionMarker).split('.');
+      if (parts.length === 3) {
+        // Raw ID token.
+        const payload = decodeJwtPayload(sessionMarker);
+        if (!payload || payload.iss !== issuerMetadata.issuer) return null;
+        const audMatches = Array.isArray(payload.aud)
+          ? payload.aud.includes(cfg.clientId)
+          : payload.aud === cfg.clientId;
+        if (audMatches) hint = sessionMarker;
+      } else if (parts.length === 2 && parts[0] === 'sso') {
+        // Oversized-token marker — issuer identity only, never a hint.
+        const iss = Buffer.from(parts[1], 'base64url').toString('utf8');
+        if (iss !== issuerMetadata.issuer) return null;
+      } else {
+        return null;
+      }
     }
 
     const postLogoutRedirectUri = await getPostLogoutRedirectUri();
@@ -732,6 +752,7 @@ module.exports = {
   buildAuthorizationRequest,
   handleCallback,
   buildEndSessionUrl,
+  buildOversizeSsoMarker,
   resolveAdminFromClaims,
   saveOidcSettings,
   invalidateDiscoveryCache,