diff --git a/.changeset/fix-structured-output-stream-sse-assignability.md b/.changeset/fix-structured-output-stream-sse-assignability.md
new file mode 100644
index 000000000..70fa7ad9b
--- /dev/null
+++ b/.changeset/fix-structured-output-stream-sse-assignability.md
@@ -0,0 +1,9 @@
+---
+'@tanstack/ai': patch
+---
+
+fix(ai): restore `StructuredOutputStream` assignability to `AsyncIterable<StreamChunk>` so it can be passed to `toServerSentEventsResponse`
+
+`StructuredOutputStartEvent`, `StructuredOutputCompleteEvent`, `ApprovalRequestedEvent`, and `ToolInputAvailableEvent` declared their shape with `extends Omit<CustomEvent, 'name' | 'value'>`. Because `CustomEvent` is inferred from a zod `passthrough` schema, it carries a `[k: string]: unknown` index signature, and `Omit` on a type with a `string` index signature collapses every surviving property to `unknown` — including `type: 'CUSTOM'`. That broke union assignability against `AGUIEvent`/`StreamChunk`, so `toServerSentEventsResponse(stream)` failed to typecheck against streams returned by `chat({ outputSchema, stream: true })`.
+
+Switched to `extends CustomEvent` with refined `name`/`value` (allowed: narrower types of declared properties), which keeps `type: 'CUSTOM'` intact and preserves the existing discriminated-narrowing patterns.
diff --git a/packages/typescript/ai/src/types.ts b/packages/typescript/ai/src/types.ts
index 3ffe72ffe..fa8d3d005 100644
--- a/packages/typescript/ai/src/types.ts
+++ b/packages/typescript/ai/src/types.ts
@@ -1146,10 +1146,9 @@ export interface CustomEvent extends AGUICustomEvent {
  * }
  * ```
  */
-export interface StructuredOutputCompleteEvent<T = unknown> extends Omit<
-  CustomEvent,
-  'name' | 'value'
-> {
+export interface StructuredOutputCompleteEvent<
+  T = unknown,
+> extends CustomEvent {
   name: 'structured-output.complete'
   value: { object: T; raw: string; reasoning?: string }
 }
@@ -1162,10 +1161,7 @@ export interface StructuredOutputCompleteEvent<T = unknown> extends Omit<
  * `messageId` the deltas will be tagged with so the routing decision can be
  * made per-message rather than globally.
  */
-export interface StructuredOutputStartEvent extends Omit<
-  CustomEvent,
-  'name' | 'value'
-> {
+export interface StructuredOutputStartEvent extends CustomEvent {
   name: 'structured-output.start'
   value: { messageId: string }
 }
@@ -1177,10 +1173,7 @@ export interface StructuredOutputStartEvent extends Omit<
  * (the agent-loop branch of `runStreamingStructuredOutputImpl` in
  * `activities/chat/index.ts` forwards CUSTOM events from `TextEngine.run()`).
  */
-export interface ApprovalRequestedEvent extends Omit<
-  CustomEvent,
-  'name' | 'value'
-> {
+export interface ApprovalRequestedEvent extends CustomEvent {
   name: 'approval-requested'
   value: {
     toolCallId: string
@@ -1196,10 +1189,7 @@ export interface ApprovalRequestedEvent extends Omit<
  * will not fire for that run. Shape fixed by the agent-loop forwarding in
  * `runStreamingStructuredOutputImpl` in `activities/chat/index.ts`.
  */
-export interface ToolInputAvailableEvent extends Omit<
-  CustomEvent,
-  'name' | 'value'
-> {
+export interface ToolInputAvailableEvent extends CustomEvent {
   name: 'tool-input-available'
   value: {
     toolCallId: string
diff --git a/packages/typescript/ai/tests/chat-result-types.test.ts b/packages/typescript/ai/tests/chat-result-types.test.ts
index 5dd9d1eb9..2e8ad1aeb 100644
--- a/packages/typescript/ai/tests/chat-result-types.test.ts
+++ b/packages/typescript/ai/tests/chat-result-types.test.ts
@@ -79,6 +79,14 @@ describe('chat() return type', () => {
     })
   })
 
+  describe('StructuredOutputStream assignability', () => {
+    it('is assignable to AsyncIterable<StreamChunk> (toServerSentEventsResponse input)', () => {
+      expectTypeOf<StructuredOutputStream<Person>>().toMatchTypeOf<
+        AsyncIterable<StreamChunk>
+      >()
+    })
+  })
+
   describe('without outputSchema', () => {
     it('stream: true → AsyncIterable<StreamChunk>', () => {
       expectTypeOf<TextActivityResult<undefined, true>>().toEqualTypeOf<