fix(ai): replay function call item ids

packages/ai/src/protocols/open-responses.ts

@@ -335,12 +335,16 @@ const metadataItemID = (
     : undefined
 }
 
-const lowerToolCall = (part: ToolCallPart): OpenResponsesInputItem => ({
-  type: "function_call",
-  call_id: part.id,
-  name: part.name,
-  arguments: ProviderShared.encodeJson(part.input),
-})
+const lowerToolCall = (part: ToolCallPart, providerMetadataKey: string): OpenResponsesInputItem => {
+  const itemId = metadataItemID(part, providerMetadataKey)
+  return {
+    type: "function_call",
+    ...(itemId === undefined ? {} : { id: itemId }),
+    call_id: part.id,
+    name: part.name,
+    arguments: ProviderShared.encodeJson(part.input),
+  }
+}
 
 const lowerReasoning = (part: ReasoningPart, providerMetadataKey: string): OpenResponsesReasoningInput | undefined => {
   const metadata = part.providerMetadata?.[providerMetadataKey]
@@ -503,7 +507,7 @@ const lowerMessages = Effect.fn("OpenResponses.lowerMessages")(function* (reques
         if (part.type === "tool-call") {
           flushText()
           if (part.providerExecuted === true) continue
-          input.push(lowerToolCall(part))
+          input.push(lowerToolCall(part, providerMetadataKey))
           continue
         }
         if (part.type === "tool-result" && part.providerExecuted === true) {

packages/ai/test/provider/openai-responses.test.ts

@@ -458,14 +458,19 @@ describe("OpenAI Responses route", () => {
     }),
   )
 
-  it.effect("preserves opaque assistant item ids without assigning ids to function items", () =>
+  it.effect("replays provider function call item ids without assigning output ids", () =>
     Effect.gen(function* () {
       const canonical = LLM.request({
         model,
         messages: [
           Message.assistant([
             { type: "text", text: "Calling.", itemId: "plain-text" },
-            ToolCallPart.make({ id: "call_1", itemId: "plain-call", name: "lookup", input: {} }),
+            ToolCallPart.make({
+              id: "call_1",
+              name: "lookup",
+              input: {},
+              providerMetadata: { openai: { itemId: "plain-call" } },
+            }),
           ]),
           Message.tool({ id: "call_1", itemId: "plain-output", name: "lookup", result: "done" }),
         ],
@@ -474,12 +479,12 @@ describe("OpenAI Responses route", () => {
 
       expect(canonical.messages[0]?.content.map((part) => (part.type === "media" ? undefined : part.itemId))).toEqual([
         "plain-text",
-        "plain-call",
+        undefined,
       ])
       expect(canonical.messages[1]?.content[0]).toMatchObject({ itemId: "plain-output" })
       expect(prepared.body.input).toEqual([
         { role: "assistant", id: "plain-text", content: [{ type: "output_text", text: "Calling." }] },
-        { type: "function_call", call_id: "call_1", name: "lookup", arguments: "{}" },
+        { type: "function_call", id: "plain-call", call_id: "call_1", name: "lookup", arguments: "{}" },
         { type: "function_call_output", call_id: "call_1", output: '"done"' },
       ])
     }),