diff --git a/apps/backend/src/lib/tokens.tsx b/apps/backend/src/lib/tokens.tsx
index 9322f746e..0092e6504 100644
--- a/apps/backend/src/lib/tokens.tsx
+++ b/apps/backend/src/lib/tokens.tsx
@@ -443,7 +443,9 @@ export async function createRefreshTokenObj(options: CreateRefreshTokenOptions)
 
   const refreshToken = generateSecureRandomString();
 
-  const scopes = options.scopes ? [...new Set(options.scopes)] : [];
+  // Dedupe so the persisted scope string never carries duplicates (the registry intersection at
+  // grant time already constrains the set, this just normalizes it).
+  const scopes = options.scopes != null ? [...new Set(options.scopes)] : [];
 
   const refreshTokenObj = await globalPrismaClient.projectUserRefreshToken.create({
     data: {
diff --git a/apps/e2e/tests/backend/endpoints/api/v1/auth/sessions/scopes.test.ts b/apps/e2e/tests/backend/endpoints/api/v1/auth/sessions/scopes.test.ts
index 55357a31d..f8309f96f 100644
--- a/apps/e2e/tests/backend/endpoints/api/v1/auth/sessions/scopes.test.ts
+++ b/apps/e2e/tests/backend/endpoints/api/v1/auth/sessions/scopes.test.ts
@@ -79,7 +79,19 @@ it("treats unrestricted (no-scope) sessions as unrestricted for scoped endpoints
     body: { display_name: "Unrestricted Test Team" },
     userAuth: { accessToken },
   });
-  expect(createRes.body.code).not.toBe("INSUFFICIENT_SCOPE");
+  expect(createRes).toMatchInlineSnapshot(`
+    NiceResponse {
+      "status": 201,
+      "body": {
+        "client_metadata": null,
+        "client_read_only_metadata": null,
+        "display_name": "Unrestricted Test Team",
+        "id": "<stripped UUID>",
+        "profile_image_url": null,
+      },
+      "headers": Headers { <some fields may have been hidden> },
+    }
+  `);
 });
 
 it("persists scopes across a token refresh", async ({ expect }) => {
@@ -106,8 +118,20 @@ it("persists scopes across a token refresh", async ({ expect }) => {
     body: { display_name: "Refreshed Scoped Team" },
     userAuth: { accessToken: refreshedAccessToken },
   });
-  expect(createRes.status).toBe(403);
-  expect(createRes.body.code).toBe("INSUFFICIENT_SCOPE");
+  expect(createRes).toMatchInlineSnapshot(`
+    NiceResponse {
+      "status": 403,
+      "body": {
+        "code": "INSUFFICIENT_SCOPE",
+        "details": { "missing_scopes": ["teams:write"] },
+        "error": "The access token is missing the following required scope(s): 'teams:write'. Mint a token that includes these scopes and try again.",
+      },
+      "headers": Headers {
+        "x-stack-known-error": "INSUFFICIENT_SCOPE",
+        <some fields may have been hidden>,
+      },
+    }
+  `);
 });
 
 it("rejects creating a session with an unknown scope", async ({ expect }) => {
diff --git a/packages/stack-shared/src/crud.tsx b/packages/stack-shared/src/crud.tsx
index 7df0b9a2b..1bfad8361 100644
--- a/packages/stack-shared/src/crud.tsx
+++ b/packages/stack-shared/src/crud.tsx
@@ -1,6 +1,6 @@
 import * as yup from 'yup';
 import { yupObject, yupString } from './schema-fields';
-import { Scope } from './scopes';
+import type { Scope } from './scopes';
 import { filterUndefined } from './utils/objects';
 import { NullishCoalesce } from './utils/types';
 
diff --git a/packages/stack-shared/src/known-errors.tsx b/packages/stack-shared/src/known-errors.tsx
index 3453736d4..ee490730b 100644
--- a/packages/stack-shared/src/known-errors.tsx
+++ b/packages/stack-shared/src/known-errors.tsx
@@ -1389,7 +1389,11 @@ const InsufficientScope = createKnownErrorConstructor(
       missing_scopes: missingScopes,
     },
   ] as const,
-  (json: any) => [json.missing_scopes] as const,
+  (json: any) => [
+    Array.isArray(json?.missing_scopes)
+      ? json.missing_scopes
+      : throwErr("missing_scopes not found (or not an array) in InsufficientScope details"),
+  ] as const,
 );
 
 const InvalidSharedOAuthProviderId = createKnownErrorConstructor(