diff --git a/packages/ai/src/protocols/open-responses.ts b/packages/ai/src/protocols/open-responses.ts
index 1cc4c331ea2..423ba38de6f 100644
--- a/packages/ai/src/protocols/open-responses.ts
+++ b/packages/ai/src/protocols/open-responses.ts
@@ -334,14 +334,14 @@ export const StreamItem = Schema.StructWithRest(
     id: Schema.optional(Schema.String),
     call_id: Schema.optional(Schema.String),
     name: Schema.optional(Schema.String),
-    namespace: Schema.optional(Schema.String),
+    namespace: optionalNull(Schema.String),
     arguments: Schema.optional(Schema.String),
     encrypted_content: optionalNull(Schema.String),
   }),
   [Schema.Record(Schema.String, Schema.Unknown)],
 )
 export type StreamItem = Schema.Schema.Type<typeof StreamItem>
-export type OutputItem = StreamItem & { readonly id: string }
+export type OutputItem = StreamItem & { readonly id: string; readonly namespace?: string }
 
 // Responses-compatible providers put error details at the top level, under `error`, or under
 // `response.error`, and gateways reshape them freely: strings, numeric codes, extra fields. Those
@@ -946,6 +946,8 @@ const resolveItem = (
   index: number | undefined,
 ): OutputItem => ({
   ...item,
+  // Some gateways serialize an absent namespace as null.
+  namespace: item.namespace ?? undefined,
   id:
     item.id ??
     (index === undefined ? undefined : state.outputItems[index]) ??
diff --git a/packages/ai/test/provider/openai-compatible-responses.test.ts b/packages/ai/test/provider/openai-compatible-responses.test.ts
index 81860398d6d..2b1427aabc8 100644
--- a/packages/ai/test/provider/openai-compatible-responses.test.ts
+++ b/packages/ai/test/provider/openai-compatible-responses.test.ts
@@ -644,6 +644,27 @@ describe("Open Responses-compatible route", () => {
       }),
     )
 
+    it.effect("treats a null function call namespace as absent", () =>
+      Effect.gen(function* () {
+        const item = { type: "function_call", id: "fc_1", call_id: "call_1", name: "lookup", namespace: null }
+        const response = yield* LLMClient.generate(request).pipe(
+          Effect.provide(
+            fixedResponse(
+              sseEvents(
+                { type: "response.output_item.added", output_index: 0, item: { ...item, arguments: "" } },
+                { type: "response.function_call_arguments.done", item_id: "fc_1", output_index: 0, arguments: "{}" },
+                { type: "response.output_item.done", output_index: 0, item: { ...item, arguments: "{}" } },
+                { type: "response.completed", response: { id: "resp_1", output: [{ ...item, arguments: "{}" }] } },
+              ),
+            ),
+          ),
+        )
+
+        expect(response.toolCalls).toEqual([expect.objectContaining({ id: "call_1", name: "lookup", input: {} })])
+        expect(response.toolCalls[0]?.namespace).toBeUndefined()
+      }),
+    )
+
     it.effect("rejects missing, null, and non-string event IDs even with a registered output index", () =>
       Effect.gen(function* () {
         yield* Effect.forEach(