diff --git a/nanobot/providers/openai_responses/converters.py b/nanobot/providers/openai_responses/converters.py
index e0bfe832d..27c59ab58 100644
--- a/nanobot/providers/openai_responses/converters.py
+++ b/nanobot/providers/openai_responses/converters.py
@@ -15,6 +15,7 @@ def convert_messages(messages: list[dict[str, Any]]) -> tuple[str, list[dict[str
     """
     system_prompt = ""
     input_items: list[dict[str, Any]] = []
+    used_item_ids: set[str] = set()
 
     for idx, msg in enumerate(messages):
         role = msg.get("role")
@@ -30,17 +31,19 @@ def convert_messages(messages: list[dict[str, Any]]) -> tuple[str, list[dict[str
 
         if role == "assistant":
             if isinstance(content, str) and content:
+                message_id = _unique_item_id(f"msg_{idx}", used_item_ids)
                 input_items.append({
                     "type": "message", "role": "assistant",
                     "content": [{"type": "output_text", "text": content}],
-                    "status": "completed", "id": f"msg_{idx}",
+                    "status": "completed", "id": message_id,
                 })
             for tool_call in msg.get("tool_calls", []) or []:
                 fn = tool_call.get("function") or {}
                 call_id, item_id = split_tool_call_id(tool_call.get("id"))
+                response_item_id = _unique_item_id(item_id or f"fc_{idx}", used_item_ids)
                 input_items.append({
                     "type": "function_call",
-                    "id": item_id or f"fc_{idx}",
+                    "id": response_item_id,
                     "call_id": call_id or f"call_{idx}",
                     "name": fn.get("name"),
                     "arguments": fn.get("arguments") or "{}",
@@ -97,6 +100,20 @@ def convert_tools(tools: list[dict[str, Any]]) -> list[dict[str, Any]]:
     return converted
 
 
+def _unique_item_id(item_id: str, used: set[str]) -> str:
+    """Return a Responses input item id that is unique within one request."""
+    if item_id not in used:
+        used.add(item_id)
+        return item_id
+
+    suffix = 2
+    while f"{item_id}_{suffix}" in used:
+        suffix += 1
+    unique = f"{item_id}_{suffix}"
+    used.add(unique)
+    return unique
+
+
 def split_tool_call_id(tool_call_id: Any) -> tuple[str, str | None]:
     """Split a compound ``call_id|item_id`` string.
 
diff --git a/tests/providers/test_openai_responses.py b/tests/providers/test_openai_responses.py
index 74a934f85..36040db58 100644
--- a/tests/providers/test_openai_responses.py
+++ b/tests/providers/test_openai_responses.py
@@ -155,6 +155,49 @@ class TestConvertMessages:
         assert items[0]["id"] == "fc_1"
         assert items[0]["name"] == "get_weather"
 
+    def test_duplicate_response_item_ids_are_made_unique(self):
+        """Codex rejects replayed Responses input items with duplicate ids."""
+        _, items = convert_messages([
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [{
+                    "id": "call_a|rs_same",
+                    "function": {"name": "first", "arguments": "{}"},
+                }],
+            },
+            {"role": "tool", "tool_call_id": "call_a|rs_same", "content": "ok"},
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [{
+                    "id": "call_b|rs_same",
+                    "function": {"name": "second", "arguments": "{}"},
+                }],
+            },
+            {"role": "tool", "tool_call_id": "call_b|rs_same", "content": "ok"},
+        ])
+        function_call_ids = [
+            item["id"] for item in items if item.get("type") == "function_call"
+        ]
+        assert function_call_ids == ["rs_same", "rs_same_2"]
+        assert len(function_call_ids) == len(set(function_call_ids))
+
+    def test_fallback_response_item_ids_are_unique_with_multiple_tool_calls(self):
+        _, items = convert_messages([{
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {"id": "call_a", "function": {"name": "first", "arguments": "{}"}},
+                {"id": "call_b", "function": {"name": "second", "arguments": "{}"}},
+            ],
+        }])
+        function_call_ids = [
+            item["id"] for item in items if item.get("type") == "function_call"
+        ]
+        assert function_call_ids == ["fc_0", "fc_0_2"]
+        assert len(function_call_ids) == len(set(function_call_ids))
+
     def test_assistant_with_tool_calls_no_id(self):
         """Fallback IDs when tool_call.id is missing."""
         _, items = convert_messages([{