diff --git a/crates/bindings-typescript/src/lib/type_util.test-d.ts b/crates/bindings-typescript/src/lib/type_util.test-d.ts
new file mode 100644
index 00000000000..80824b999d3
--- /dev/null
+++ b/crates/bindings-typescript/src/lib/type_util.test-d.ts
@@ -0,0 +1,56 @@
+import type { ConnectionId } from './connection_id';
+import type { Identity } from './identity';
+import type { Prettify } from './type_util';
+import type { TimeDuration } from './time_duration';
+import type { Timestamp } from './timestamp';
+import type { Uuid } from './uuid';
+
+declare const brand: unique symbol;
+
+/**
+ * A branded primitive: an intersection of a primitive with a marker object.
+ * `Prettify` must leave it alone — mapping it produces a structural record of
+ * `String`'s members that no longer satisfies `string`.
+ */
+type UserId = string & { readonly [brand]: 'UserId' };
+
+declare const userId: Prettify<UserId>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _brandedIsStillAString: string = userId;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _brandedKeepsItsBrand: UserId = userId;
+
+// Plain primitives round-trip unchanged.
+declare const str: Prettify<string>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _str: string = str;
+declare const num: Prettify<number>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _num: number = num;
+declare const big: Prettify<bigint>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _big: bigint = big;
+
+// Every SATS wrapper class is passed through as the class, not as a
+// structural record of its members.
+declare const identity: Prettify<Identity>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _identity: Identity = identity;
+declare const connectionId: Prettify<ConnectionId>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _connectionId: ConnectionId = connectionId;
+declare const timestamp: Prettify<Timestamp>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _timestamp: Timestamp = timestamp;
+declare const timeDuration: Prettify<TimeDuration>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _timeDuration: TimeDuration = timeDuration;
+declare const uuid: Prettify<Uuid>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _uuid: Uuid = uuid;
+
+// Object types are still flattened.
+type Intersected = { a: string } & { b: number };
+declare const flattened: Prettify<Intersected>;
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const _flattened: { a: string; b: number } = flattened;
diff --git a/crates/bindings-typescript/src/lib/type_util.ts b/crates/bindings-typescript/src/lib/type_util.ts
index 942b76a1424..789595b24e8 100644
--- a/crates/bindings-typescript/src/lib/type_util.ts
+++ b/crates/bindings-typescript/src/lib/type_util.ts
@@ -3,20 +3,28 @@ import type { Identity } from './identity';
 import type { ScheduleAt } from './schedule_at';
 import type { TimeDuration } from './time_duration';
 import type { Timestamp } from './timestamp';
+import type { Uuid } from './uuid';
 
 type DoNotPrettify =
   | Identity
   | ConnectionId
   | Timestamp
   | TimeDuration
-  | ScheduleAt;
+  | ScheduleAt
+  | Uuid;
 
 /**
  * Utility to make TS show cleaner types by flattening intersections.
+ *
+ * Only object types are flattened. A non-object has no intersection to
+ * flatten, and mapping one is not always identity: `string & Brand` becomes a
+ * structural record of `String`'s members that no longer satisfies `string`.
  */
 export type Prettify<T> = T extends DoNotPrettify
   ? T
-  : { [K in keyof T]: T[K] } & {};
+  : T extends string | number | boolean | bigint | symbol | null | undefined
+    ? T
+    : { [K in keyof T]: T[K] } & {};
 
 /**
  * Helper function to sets a field in an object