diff --git a/.changeset/fix-baseitem-id.md b/.changeset/fix-baseitem-id.md
new file mode 100644
index 00000000000..4e515b6ae9c
--- /dev/null
+++ b/.changeset/fix-baseitem-id.md
@@ -0,0 +1,5 @@
+----
+'@keystone-6/core': patch
+----
+
+Fixes the type of `id` on type `BaseItem`, to be type `unknown`, not `string` - `string` is not guaranteed
diff --git a/packages/auth/src/gql/getInitFirstItemSchema.ts b/packages/auth/src/gql/getInitFirstItemSchema.ts
index 5db6cfdb0e4..41334166e78 100644
--- a/packages/auth/src/gql/getInitFirstItemSchema.ts
+++ b/packages/auth/src/gql/getInitFirstItemSchema.ts
@@ -61,7 +61,10 @@ export function getInitFirstItemSchema ({
           // the input value can't round-trip like the Upload scalar here is quite low)
           const item = await sudoContext.db[listKey].createOne({ data: { ...data, ...itemData } })
           const sessionToken = (await context.sessionStrategy.start({
-            data: { listKey, itemId: item.id.toString() },
+            data: {
+              listKey,
+              itemId: item.id
+            },
             context,
           }))
 
diff --git a/packages/core/src/types/next-fields.ts b/packages/core/src/types/next-fields.ts
index fa3cadc1b8d..8e53b0679b4 100644
--- a/packages/core/src/types/next-fields.ts
+++ b/packages/core/src/types/next-fields.ts
@@ -12,7 +12,10 @@ import {
 
 export { Decimal }
 
-export type BaseItem = { id: { toString(): string }, [key: string]: unknown }
+export type BaseItem = {
+  id: unknown,
+  [key: string]: unknown
+}
 
 export type ListGraphQLTypes = { types: GraphQLTypesForList }
 
diff --git a/packages/core/src/types/type-info.ts b/packages/core/src/types/type-info.ts
index ad3daa73c6b..c661fbe115a 100644
--- a/packages/core/src/types/type-info.ts
+++ b/packages/core/src/types/type-info.ts
@@ -12,7 +12,7 @@ export type BaseListTypeInfo<Session = any> = {
     create: GraphQLInput
     update: GraphQLInput
     where: GraphQLInput
-    uniqueWhere: { readonly id?: string | number | null } & GraphQLInput
+    uniqueWhere: { readonly id?: unknown } & GraphQLInput
     orderBy: Record<string, 'asc' | 'desc' | null>
   }
 
diff --git a/packages/fields-document/src/structure-graphql-input.ts b/packages/fields-document/src/structure-graphql-input.ts
index 0b3de2630cf..87914e3fd55 100644
--- a/packages/fields-document/src/structure-graphql-input.ts
+++ b/packages/fields-document/src/structure-graphql-input.ts
@@ -340,7 +340,7 @@ export async function resolveRelateToManyForUpdateInput (
   value: _UpdateValueManyType,
   context: KeystoneContext,
   foreignListKey: string,
-  prevVal: { id: string }[]
+  prevVal: BaseItem[]
 ) {
   if (
     !Array.isArray(value.connect) &&
@@ -442,7 +442,7 @@ export async function checkUniqueItemExists (
     throw missingItem(operation, uniqueInput)
   }
 
-  return { id: item.id.toString() }
+  return { id: item.id }
 }
 
 async function handleCreateAndUpdate (
@@ -470,7 +470,7 @@ async function resolveCreateMutation (value: any, context: KeystoneContext, fore
     // it could change in the future
     {} as GraphQLResolveInfo
   )) as BaseItem
-  return { id: id.toString() }
+  return { id }
 }
 
 export function resolveRelateToOneForCreateInput (