diff --git a/.changeset/gold-vans-complain.md b/.changeset/gold-vans-complain.md
new file mode 100644
index 00000000..e0a90beb
--- /dev/null
+++ b/.changeset/gold-vans-complain.md
@@ -0,0 +1,5 @@
+---
+"@openai/agents-openai": patch
+---
+
+fix: resolve #425 duplicate item error when using conversationId with tools
diff --git a/packages/agents-openai/src/openaiResponsesModel.ts b/packages/agents-openai/src/openaiResponsesModel.ts
index a8a3a001..16652d2e 100644
--- a/packages/agents-openai/src/openaiResponsesModel.ts
+++ b/packages/agents-openai/src/openaiResponsesModel.ts
@@ -424,8 +424,51 @@ function getPrompt(prompt: ModelRequest['prompt']):
   };
 }
 
+type InputArray = Exclude<ModelRequest['input'], string>;
+
+const RESPONSE_ITEM_ID_PREFIXES = ['rs_', 'resp_', 'res_', 'msg_'] as const;
+
+function hasStoredConversationMetadata(item: InputArray[number]): boolean {
+  if (!item || typeof item !== 'object') {
+    return false;
+  }
+
+  const providerData = (item as { providerData?: Record<string, unknown> })
+    .providerData;
+  if (providerData && typeof providerData === 'object') {
+    const responseId =
+      (providerData['response_id'] as string | undefined) ??
+      (providerData['responseId'] as string | undefined);
+    if (typeof responseId === 'string' && responseId.length > 0) {
+      return true;
+    }
+  }
+
+  const id = (item as { id?: unknown }).id;
+  if (typeof id === 'string') {
+    for (const prefix of RESPONSE_ITEM_ID_PREFIXES) {
+      if (id.startsWith(prefix)) {
+        return true;
+      }
+    }
+  }
+
+  return false;
+}
+
+function getLastStoredIndex(items: InputArray): number {
+  for (let i = items.length - 1; i >= 0; i--) {
+    if (hasStoredConversationMetadata(items[i])) {
+      return i;
+    }
+  }
+
+  return -1;
+}
+
 function getInputItems(
   input: ModelRequest['input'],
+  conversationId?: string,
 ): OpenAI.Responses.ResponseInputItem[] {
   if (typeof input === 'string') {
     return [
@@ -436,7 +479,15 @@ function getInputItems(
     ];
   }
 
-  return input.map((item) => {
+  let filteredInput: InputArray = input;
+  if (conversationId) {
+    const lastStoredIndex = getLastStoredIndex(input);
+    if (lastStoredIndex >= 0) {
+      filteredInput = input.slice(lastStoredIndex + 1);
+    }
+  }
+
+  return filteredInput.map((item) => {
     if (isMessageItem(item)) {
       return getMessageItem(item);
     }
@@ -847,7 +898,7 @@ export class OpenAIResponsesModel implements Model {
   ): Promise<
     Stream<OpenAI.Responses.ResponseStreamEvent> | OpenAI.Responses.Response
   > {
-    const input = getInputItems(request.input);
+    const input = getInputItems(request.input, request.conversationId);
     const { tools, include } = getTools(request.tools, request.handoffs);
     const toolChoice = getToolChoice(request.modelSettings.toolChoice);
     const { text, ...restOfProviderData } =
diff --git a/packages/agents-openai/test/openaiResponsesModel.helpers.test.ts b/packages/agents-openai/test/openaiResponsesModel.helpers.test.ts
index 012eaafd..ea9d703d 100644
--- a/packages/agents-openai/test/openaiResponsesModel.helpers.test.ts
+++ b/packages/agents-openai/test/openaiResponsesModel.helpers.test.ts
@@ -279,6 +279,49 @@ describe('getInputItems', () => {
       ] as any),
     ).toThrow(UserError);
   });
+
+  it('excludes stored items when conversationId is provided', () => {
+    const items = getInputItems(
+      [
+        { role: 'user', content: 'hi' },
+        {
+          type: 'function_call',
+          id: 'rs_123',
+          name: 'tool',
+          callId: 'call_1',
+          arguments: '{}',
+          status: 'in_progress',
+          providerData: { response_id: 'resp_1' },
+        },
+        {
+          type: 'function_call_result',
+          callId: 'call_1',
+          status: 'completed',
+          output: { type: 'text', text: 'done' },
+        },
+      ] as any,
+      'conv_123',
+    );
+
+    expect(items).toHaveLength(1);
+    expect(items[0]).toMatchObject({
+      type: 'function_call_output',
+      output: 'done',
+    });
+  });
+
+  it('retains new items when no stored metadata exists', () => {
+    const items = getInputItems(
+      [{ role: 'user', content: 'hello' }] as any,
+      'conv_123',
+    );
+
+    expect(items).toHaveLength(1);
+    expect(items[0]).toMatchObject({
+      role: 'user',
+      content: 'hello',
+    });
+  });
 });
 
 describe('convertToOutputItem', () => {