diff --git a/backend/app/services/sso_service.py b/backend/app/services/sso_service.py
new file mode 100644
index 00000000..33226a9d
--- /dev/null
+++ b/backend/app/services/sso_service.py
@@ -0,0 +1,26 @@
+"""SSO service stub. Full SAML/OIDC implementation planned for Phase 5."""
+from __future__ import annotations
+
+
+async def initiate_sso_login(account_slug: str) -> str:
+    """Return SSO redirect URL for the given account slug.
+
+    Not yet implemented — SSO is an enterprise Phase 5 feature.
+    """
+    raise NotImplementedError("SSO coming soon")
+
+
+async def process_sso_callback(saml_response: str) -> None:
+    """Process an SSO callback (SAML assertion or OIDC token exchange).
+
+    Not yet implemented — SSO is an enterprise Phase 5 feature.
+    """
+    raise NotImplementedError("SSO coming soon")
+
+
+async def validate_sso_config(config: dict) -> bool:
+    """Validate an SSO configuration dict before storing it on the account.
+
+    Not yet implemented — SSO is an enterprise Phase 5 feature.
+    """
+    raise NotImplementedError("SSO coming soon")