diff --git a/packages/ai/src/protocols/open-responses.ts b/packages/ai/src/protocols/open-responses.ts
index 24dcfb3db9b..33594399697 100644
--- a/packages/ai/src/protocols/open-responses.ts
+++ b/packages/ai/src/protocols/open-responses.ts
@@ -288,6 +288,7 @@ export const Event = Schema.StructWithRest(
     arguments: Schema.optional(Schema.String),
     text: Schema.optional(Schema.String),
     item_id: Schema.optional(Schema.String),
+    output_index: Schema.optional(Schema.Number),
     summary_index: Schema.optional(Schema.Number),
     item: Schema.optional(StreamItem),
     response: Schema.optional(
@@ -341,6 +342,7 @@ export interface ParserState {
   readonly tools: ToolStream.State<string>
   readonly hasFunctionCall: boolean
   readonly lifecycle: Lifecycle.State
+  readonly outputItems: Readonly<Record<number, string>>
   readonly messageItems: ReadonlySet<string>
   readonly messagePhases: Readonly<Record<string, MessagePhase | null>>
   readonly reasoningItems: Readonly<Record<string, ReasoningStreamItem>>
@@ -818,6 +820,9 @@ const onOutputTextDone = (state: ParserState, event: Event, id: string): StepRes
   return [{ ...state, lifecycle: Lifecycle.textEnd(state.lifecycle, events, id) }, events]
 }
 
+export const outputItemID = (state: ParserState, event: Event) =>
+  event.output_index === undefined ? event.item_id : (state.outputItems[event.output_index] ?? event.item_id)
+
 export const onReasoningDelta = (state: ParserState, event: Event, itemID: string): StepResult => {
   const item = state.reasoningItems[itemID]
   if (!event.delta || !item) return [state, NO_EVENTS]
@@ -1201,7 +1206,11 @@ export const providerFailure = (id: string, event: Event, fallback: string) => {
 
 const providerError = (state: ParserState, event: Event, fallback: string) => providerFailure(state.id, event, fallback)
 
-export const step = (state: ParserState, event: Event) => {
+export const step = (state: ParserState, input: Event) => {
+  const event =
+    input.item_id && outputItemID(state, input) !== input.item_id
+      ? { ...input, item_id: outputItemID(state, input) }
+      : input
   if (event.type === "response.output_text.delta" || event.type === "response.output_text.done") {
     if (!event.item_id) return ProviderShared.eventError(state.id, `${event.type} is missing item_id`)
     return Effect.succeed(
@@ -1243,7 +1252,14 @@ export const step = (state: ParserState, event: Event) => {
   if (event.type === "response.output_item.added") {
     if (event.item?.type === "message" && !event.item.id)
       return ProviderShared.eventError(state.id, `${event.type} message is missing id`)
-    return Effect.succeed(onOutputItemAdded(state, event))
+    return Effect.succeed(
+      onOutputItemAdded(
+        event.output_index !== undefined && event.item?.id
+          ? { ...state, outputItems: { ...state.outputItems, [event.output_index]: event.item.id } }
+          : state,
+        event,
+      ),
+    )
   }
   if (event.type === "response.function_call_arguments.delta" || event.type === "response.function_call_arguments.done")
     return event.item_id
@@ -1278,6 +1294,7 @@ export const initial = (request: LLMRequest, extension: Extension = BASE): Parse
   hasFunctionCall: false,
   tools: ToolStream.empty<string>(),
   lifecycle: Lifecycle.initial(),
+  outputItems: {},
   messageItems: new Set<string>(),
   messagePhases: {},
   reasoningItems: {},
diff --git a/packages/ai/src/protocols/openai-responses.ts b/packages/ai/src/protocols/openai-responses.ts
index e63e6da1bb6..b230e8f1f5f 100644
--- a/packages/ai/src/protocols/openai-responses.ts
+++ b/packages/ai/src/protocols/openai-responses.ts
@@ -166,7 +166,9 @@ const HOSTED_TOOLS = {
 const step = (state: OpenResponses.ParserState, event: OpenResponses.Event) => {
   if (event.type === "response.reasoning_text.delta")
     return event.item_id
-      ? Effect.succeed(OpenResponses.onReasoningDelta(state, event, event.item_id))
+      ? Effect.succeed(
+          OpenResponses.onReasoningDelta(state, event, OpenResponses.outputItemID(state, event) ?? event.item_id),
+        )
       : ProviderShared.eventError(ADAPTER, `${event.type} is missing item_id`)
   if (event.type === "response.output_item.done" && event.item && ResponsesHostedTools.isItem(event.item, HOSTED_TOOLS))
     return ResponsesHostedTools.onDone(state, event.item, HOSTED_TOOLS)
diff --git a/packages/ai/test/provider/openai-compatible-responses.test.ts b/packages/ai/test/provider/openai-compatible-responses.test.ts
index 93ade26eb71..066a01894be 100644
--- a/packages/ai/test/provider/openai-compatible-responses.test.ts
+++ b/packages/ai/test/provider/openai-compatible-responses.test.ts
@@ -225,6 +225,31 @@ describe("Open Responses-compatible route", () => {
     }),
   )
 
+  it.effect("routes response deltas by output index", () =>
+    Effect.gen(function* () {
+      const model = configure({
+        apiKey: "test-key",
+        baseURL: "https://responses.example.test/v1",
+      }).model("example-model")
+      const response = yield* LLMClient.generate(LLM.request({ model, prompt: "Say hello." })).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              { type: "response.output_item.added", output_index: 2, item: { type: "message", id: "msg_1" } },
+              { type: "response.output_text.delta", output_index: 2, item_id: "wrong_message", delta: "Indexed" },
+              { type: "response.output_item.done", output_index: 2, item: { type: "message", id: "msg_1" } },
+              { type: "response.completed", response: { id: "resp_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.message.content).toEqual([
+        { type: "text", text: "Indexed", providerMetadata: { openresponses: { itemId: "msg_1" } } },
+      ])
+    }),
+  )
+
   it.effect("finalizes pending function calls from completed response output", () =>
     Effect.gen(function* () {
       const model = configure({
diff --git a/packages/ai/test/provider/openai-responses.test.ts b/packages/ai/test/provider/openai-responses.test.ts
index 0bcf24a26be..e3eba0b3e02 100644
--- a/packages/ai/test/provider/openai-responses.test.ts
+++ b/packages/ai/test/provider/openai-responses.test.ts
@@ -830,7 +830,9 @@ describe("OpenAI Responses route", () => {
           webSocket: {
             execute: (exchange) =>
               Effect.gen(function* () {
-                yield* exchange.driver.create(undefined).pipe(Effect.flatMap((create) => Ref.set(message, create.message)))
+                yield* exchange.driver
+                  .create(undefined)
+                  .pipe(Effect.flatMap((create) => Ref.set(message, create.message)))
                 return { frames: exchange.fallback(), complete: Effect.void }
               }),
           },
@@ -2051,6 +2053,163 @@ describe("OpenAI Responses route", () => {
     }),
   )
 
+  it.effect("routes assistant text by output index when its item id disagrees", () =>
+    Effect.gen(function* () {
+      const response = yield* LLMClient.generate(request).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              { type: "response.output_item.added", output_index: 2, item: { type: "message", id: "msg_1" } },
+              { type: "response.output_text.delta", output_index: 2, item_id: "wrong_message", delta: "Indexed" },
+              { type: "response.output_item.done", output_index: 2, item: { type: "message", id: "msg_1" } },
+              { type: "response.completed", response: { id: "resp_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.text).toBe("Indexed")
+      expect(response.message.content).toEqual([
+        { type: "text", text: "Indexed", providerMetadata: { openai: { itemId: "msg_1" } } },
+      ])
+    }),
+  )
+
+  it.effect("routes interleaved function calls by output index", () =>
+    Effect.gen(function* () {
+      const first = { type: "function_call", id: "fc_1", call_id: "call_1", name: "first", arguments: "" }
+      const second = { type: "function_call", id: "fc_2", call_id: "call_2", name: "second", arguments: "" }
+      const response = yield* LLMClient.generate(request).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              { type: "response.output_item.added", output_index: 1, item: first },
+              { type: "response.output_item.added", output_index: 3, item: second },
+              { type: "response.function_call_arguments.delta", output_index: 1, item_id: "fc_2", delta: '{"a":' },
+              { type: "response.function_call_arguments.delta", output_index: 3, item_id: "fc_1", delta: '{"b":' },
+              {
+                type: "response.function_call_arguments.done",
+                output_index: 3,
+                item_id: "fc_1",
+                arguments: '{"b":2}',
+              },
+              {
+                type: "response.function_call_arguments.done",
+                output_index: 1,
+                item_id: "fc_2",
+                arguments: '{"a":1}',
+              },
+              { type: "response.output_item.done", output_index: 1, item: { ...first, arguments: '{"a":1}' } },
+              { type: "response.output_item.done", output_index: 3, item: { ...second, arguments: '{"b":2}' } },
+              { type: "response.completed", response: { id: "resp_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.events.filter((event) => event.type === "tool-input-delta")).toMatchObject([
+        { id: "call_1", text: '{"a":' },
+        { id: "call_2", text: '{"b":' },
+        { id: "call_2", text: "2}" },
+        { id: "call_1", text: "1}" },
+      ])
+      expect(response.events.filter(LLMEvent.is.toolCall)).toEqual([
+        expect.objectContaining({ id: "call_1", name: "first", input: { a: 1 } }),
+        expect.objectContaining({ id: "call_2", name: "second", input: { b: 2 } }),
+      ])
+    }),
+  )
+
+  it.effect("routes reasoning summary events by output index", () =>
+    Effect.gen(function* () {
+      const response = yield* LLMClient.generate(request).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              {
+                type: "response.output_item.added",
+                output_index: 4,
+                item: { type: "reasoning", id: "rs_1" },
+              },
+              {
+                type: "response.reasoning_summary_part.added",
+                output_index: 4,
+                item_id: "wrong_reasoning",
+                summary_index: 0,
+              },
+              {
+                type: "response.reasoning_summary_text.delta",
+                output_index: 4,
+                item_id: "wrong_reasoning",
+                summary_index: 0,
+                delta: "Thinking",
+              },
+              {
+                type: "response.reasoning_summary_part.done",
+                output_index: 4,
+                item_id: "wrong_reasoning",
+                summary_index: 0,
+              },
+              {
+                type: "response.output_item.done",
+                output_index: 4,
+                item: { type: "reasoning", id: "rs_1", encrypted_content: "state" },
+              },
+              { type: "response.completed", response: { id: "resp_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.reasoning).toBe("Thinking")
+      expect(response.message.content).toEqual([
+        {
+          type: "reasoning",
+          text: "Thinking",
+          providerMetadata: { openai: { itemId: "rs_1", reasoningEncryptedContent: "state" } },
+        },
+      ])
+    }),
+  )
+
+  it.effect("routes native reasoning text deltas by output index", () =>
+    Effect.gen(function* () {
+      const response = yield* LLMClient.generate(request).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              { type: "response.output_item.added", output_index: 1, item: { type: "reasoning", id: "rs_1" } },
+              { type: "response.reasoning_text.delta", output_index: 1, item_id: "wrong_reasoning", delta: "Raw" },
+              { type: "response.output_item.done", output_index: 1, item: { type: "reasoning", id: "rs_1" } },
+              { type: "response.completed", response: { id: "resp_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.reasoning).toBe("Raw")
+    }),
+  )
+
+  it.effect("falls back to item ids when an output index was not registered", () =>
+    Effect.gen(function* () {
+      const response = yield* LLMClient.generate(request).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              { type: "response.output_item.added", item: { type: "message", id: "msg_1" } },
+              { type: "response.output_text.delta", output_index: 9, item_id: "msg_1", delta: "Fallback" },
+              { type: "response.output_item.done", item: { type: "message", id: "msg_1" } },
+              { type: "response.completed", response: { id: "resp_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.text).toBe("Fallback")
+    }),
+  )
+
   it.effect("rejects output text events without the spec-required item id", () =>
     Effect.gen(function* () {
       const error = yield* LLMClient.generate(request).pipe(
@@ -2070,6 +2229,25 @@ describe("OpenAI Responses route", () => {
     }),
   )
 
+  it.effect("requires item ids even when their output index is known", () =>
+    Effect.gen(function* () {
+      const error = yield* LLMClient.generate(request).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              { type: "response.output_item.added", output_index: 0, item: { type: "message", id: "msg_1" } },
+              { type: "response.output_text.delta", output_index: 0, delta: "Missing item ID" },
+            ),
+          ),
+        ),
+        Effect.flip,
+      )
+
+      expect(error.reason._tag).toBe("InvalidProviderOutput")
+      expect(error.message).toContain("response.output_text.delta is missing item_id")
+    }),
+  )
+
   it.effect("ignores deltas without a matching output item", () =>
     Effect.gen(function* () {
       const response = yield* LLMClient.generate(request).pipe(
diff --git a/packages/ai/test/provider/xai-responses.test.ts b/packages/ai/test/provider/xai-responses.test.ts
index eeb1d1e8de6..a09ea69d49b 100644
--- a/packages/ai/test/provider/xai-responses.test.ts
+++ b/packages/ai/test/provider/xai-responses.test.ts
@@ -70,6 +70,42 @@ describe("xAI Responses route", () => {
     }),
   )
 
+  it.effect("routes xAI reasoning summaries by output index", () =>
+    Effect.gen(function* () {
+      const response = yield* LLMClient.generate(LLM.request({ model, prompt: "Think" })).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              {
+                type: "response.output_item.added",
+                output_index: 3,
+                item: { type: "reasoning", id: "reasoning_1" },
+              },
+              {
+                type: "response.reasoning_summary_text.delta",
+                output_index: 3,
+                item_id: "wrong_reasoning",
+                summary_index: 0,
+                delta: "Considering.",
+              },
+              {
+                type: "response.output_item.done",
+                output_index: 3,
+                item: { type: "reasoning", id: "reasoning_1", encrypted_content: "opaque" },
+              },
+              { type: "response.completed", response: { id: "response_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.reasoning).toBe("Considering.")
+      expect(response.message.content.find((part) => part.type === "reasoning")).toMatchObject({
+        providerMetadata: { xai: { itemId: "reasoning_1", reasoningEncryptedContent: "opaque" } },
+      })
+    }),
+  )
+
   it.effect("parses xAI hosted tool items", () =>
     Effect.gen(function* () {
       const response = yield* LLMClient.generate(LLM.request({ model, prompt: "Search X" })).pipe(