diff --git a/packages/openai-adapters/src/apis/openaiResponses.ts b/packages/openai-adapters/src/apis/openaiResponses.ts
index 7ba62228451..1d060fad9bb 100644
--- a/packages/openai-adapters/src/apis/openaiResponses.ts
+++ b/packages/openai-adapters/src/apis/openaiResponses.ts
@@ -26,7 +26,6 @@ import {
   ResponseInputImage,
   ResponseInputText,
   ResponseOutputItem,
-  ResponseOutputMessage,
   ResponseOutputRefusal,
   ResponseOutputText,
   ResponseReasoningSummaryTextDeltaEvent,
@@ -288,7 +287,6 @@ export function toResponsesInput(
   messages: ChatCompletionMessageParam[],
 ): ResponseInput {
   const inputItems: ResponseInput = [];
-  let assistantMessageCounter = 0;
 
   for (const message of messages) {
     if (message.role === "tool") {
@@ -337,18 +335,19 @@ export function toResponsesInput(
         assistantMessage.refusal ?? null,
       );
       if (assistantContentParts.length > 0) {
-        const providedId = (message as any).id;
-        const assistantId =
-          typeof providedId === "string" && providedId.startsWith("msg_")
-            ? providedId
-            : `msg_${(assistantMessageCounter++).toString().padStart(4, "0")}`;
+        // Chat Completions history does not carry the Responses reasoning item
+        // associated with an output message. Replay it as an easy input message
+        // without inventing a msg_ ID that would require that missing reasoning.
+        const content = assistantContentParts
+          .map((part) =>
+            part.type === "output_text" ? part.text : part.refusal,
+          )
+          .join("");
         inputItems.push({
           type: "message",
           role: "assistant",
-          content: assistantContentParts,
-          id: assistantId,
-          status: "completed",
-        } as ResponseOutputMessage as any);
+          content,
+        });
       }
       if (assistantMessage.tool_calls?.length) {
         assistantMessage.tool_calls.forEach((toolCall, index) => {
@@ -360,12 +359,9 @@ export function toResponsesInput(
               name: toolCall.function.name ?? "",
               arguments: toolCall.function.arguments ?? "{}",
             };
-            if (
-              typeof toolCall.id === "string" &&
-              toolCall.id.startsWith("fc_")
-            ) {
-              functionCall.id = toolCall.id;
-            }
+            // Chat Completions exposes only the call ID, not the Responses
+            // output-item ID. Reusing it as `id` can make the Responses API
+            // require a reasoning item that this representation cannot carry.
             inputItems.push(functionCall);
           }
         });
diff --git a/packages/openai-adapters/src/test/openai-responses.vitest.ts b/packages/openai-adapters/src/test/openai-responses.vitest.ts
index 68173da898b..c6f77b9ef70 100644
--- a/packages/openai-adapters/src/test/openai-responses.vitest.ts
+++ b/packages/openai-adapters/src/test/openai-responses.vitest.ts
@@ -25,7 +25,7 @@ import {
 } from "../apis/openaiResponses.js";
 
 describe("toResponsesInput", () => {
-  it("maps assistant text content to output_text with generated msg ids", () => {
+  it("does not invent Responses output item ids for assistant history", () => {
     const messages: ChatCompletionMessageParam[] = [
       {
         role: "assistant",
@@ -40,14 +40,9 @@ describe("toResponsesInput", () => {
     expect(assistant).toMatchObject({
       type: "message",
       role: "assistant",
-      id: "msg_0000",
     });
-    expect(assistant.content).toMatchObject([
-      {
-        type: "output_text",
-        text: "Hello there!",
-      },
-    ]);
+    expect(assistant).not.toHaveProperty("id");
+    expect(assistant.content).toBe("Hello there!");
   });
 
   it("maps assistant refusal content to refusal output items", () => {
@@ -63,12 +58,7 @@ describe("toResponsesInput", () => {
 
     expect(inputItems).toHaveLength(1);
     const assistant = inputItems[0] as any;
-    expect(assistant.content).toEqual([
-      {
-        type: "refusal",
-        refusal: "I must decline.",
-      },
-    ]);
+    expect(assistant.content).toBe("I must decline.");
   });
 
   it("converts assistant structured content into output_text items", () => {
@@ -82,12 +72,7 @@ describe("toResponsesInput", () => {
     const inputItems = toResponsesInput(messages);
 
     const assistant = inputItems[0] as any;
-    expect(assistant.content).toMatchObject([
-      {
-        type: "output_text",
-        text: "Structured hello.",
-      },
-    ]);
+    expect(assistant.content).toBe("Structured hello.");
   });
 
   it("converts chat messages, multimodal content, and tool interactions into Responses input items", () => {
@@ -107,7 +92,7 @@ describe("toResponsesInput", () => {
         role: "assistant",
         tool_calls: [
           {
-            id: "fc_call_1",
+            id: "call_1",
             type: "function",
             function: {
               name: "searchDocs",
@@ -146,8 +131,7 @@ describe("toResponsesInput", () => {
       },
       {
         type: "function_call",
-        call_id: "fc_call_1",
-        id: "fc_call_1",
+        call_id: "call_1",
         name: "searchDocs",
         arguments: '{"query":"vitest expectations"}',
       },
@@ -157,16 +141,17 @@ describe("toResponsesInput", () => {
         output: "Found 3 relevant documents.",
       },
     ]);
+    expect(inputItems[2]).not.toHaveProperty("id");
   });
 });
 
-it("omits function_call id when tool call id lacks fc_ prefix", () => {
+it("does not treat a Chat Completions tool call id as a Responses item id", () => {
   const messages: ChatCompletionMessageParam[] = [
     {
       role: "assistant",
       tool_calls: [
         {
-          id: "call_custom",
+          id: "fc_custom",
           type: "function",
           function: {
             name: "lookup",
@@ -184,7 +169,7 @@ it("omits function_call id when tool call id lacks fc_ prefix", () => {
   ) as any;
 
   expect(functionCall).toBeTruthy();
-  expect(functionCall.call_id).toBe("call_custom");
+  expect(functionCall.call_id).toBe("fc_custom");
   expect(functionCall).not.toHaveProperty("id");
 });