From 3d25f32a0c11334b2ccce07c029bd7aae2823daa Mon Sep 17 00:00:00 2001
From: Henry Kwon <henry@lyra.so>
Date: Wed, 25 Mar 2026 18:29:48 -0400
Subject: [PATCH] fix: serialize undefined array elements as null

---
 .../standard/rpc-json-serializer.test.ts      | 24 +++++++++++++++++++
 .../adapters/standard/rpc-json-serializer.ts  |  2 +-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/packages/client/src/adapters/standard/rpc-json-serializer.test.ts b/packages/client/src/adapters/standard/rpc-json-serializer.test.ts
index 98979e450..cc77c8fca 100644
--- a/packages/client/src/adapters/standard/rpc-json-serializer.test.ts
+++ b/packages/client/src/adapters/standard/rpc-json-serializer.test.ts
@@ -136,6 +136,30 @@ describe.each([
   })
 })
 
+describe('standardRPCJsonSerializer: undefined in arrays produces JSON-safe output', () => {
+  const serializer = new StandardRPCJsonSerializer()
+
+  it('serialize uses null as placeholder for undefined array elements', () => {
+    const [json] = serializer.serialize([undefined, 'a', undefined])
+    expect(json).toEqual([null, 'a', null])
+  })
+
+  it('round-trips undefined array elements through JSON.parse(JSON.stringify(...))', () => {
+    const [json, meta, maps, blobs] = serializer.serialize([undefined, 'a', undefined])
+    const result = JSON.parse(JSON.stringify({ json, meta, maps }))
+    const deserialized = serializer.deserialize(result.json, result.meta, result.maps, (i: number) => blobs[i]!)
+    expect(deserialized).toEqual([undefined, 'a', undefined])
+  })
+
+  it('round-trips nested undefined array elements (e.g. TanStack Query pageParams)', () => {
+    const data = { pageParams: [undefined, 'cursor_abc'], pages: [{ items: [1, 2] }] }
+    const [json, meta, maps, blobs] = serializer.serialize(data)
+    const result = JSON.parse(JSON.stringify({ json, meta, maps }))
+    const deserialized = serializer.deserialize(result.json, result.meta, result.maps, (i: number) => blobs[i]!)
+    expect(deserialized).toEqual(data)
+  })
+})
+
 describe('standardRPCJsonSerializer: custom serializers', () => {
   it('should throw when type is duplicated', () => {
     expect(() => {
diff --git a/packages/client/src/adapters/standard/rpc-json-serializer.ts b/packages/client/src/adapters/standard/rpc-json-serializer.ts
index 30f16bd50..c644b4d08 100644
--- a/packages/client/src/adapters/standard/rpc-json-serializer.ts
+++ b/packages/client/src/adapters/standard/rpc-json-serializer.ts
@@ -100,7 +100,7 @@ export class StandardRPCJsonSerializer {
       const json = data.map((v, i) => {
         if (v === undefined) {
           meta.push([STANDARD_RPC_JSON_SERIALIZER_BUILT_IN_TYPES.UNDEFINED, ...segments, i])
-          return v
+          return null
         }
 
         return this.serialize(v, [...segments, i], meta, maps, blobs)[0]