diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index 06e167bfc3..0d2239386f 100644
--- a/codex-rs/core/src/client.rs
+++ b/codex-rs/core/src/client.rs
@@ -788,6 +788,7 @@ impl ModelClient {
         let tools = create_tools_json_for_responses_api(&prompt.tools)?;
         let (instructions, tools) = if model_info.use_responses_lite {
             let additional_tools = ResponseItem::AdditionalTools {
+                id: None,
                 role: "developer".to_string(),
                 tools,
             };
diff --git a/codex-rs/core/src/session/mod.rs b/codex-rs/core/src/session/mod.rs
index c85898555b..7e9569fdfd 100644
--- a/codex-rs/core/src/session/mod.rs
+++ b/codex-rs/core/src/session/mod.rs
@@ -2712,6 +2712,7 @@ impl Session {
                 continue;
             }
             let prefix = match item {
+                ResponseItem::AdditionalTools { .. } => "at",
                 ResponseItem::Message { .. } => "msg",
                 ResponseItem::Reasoning { .. } => "rs",
                 ResponseItem::LocalShellCall { .. } => "lsh",
diff --git a/codex-rs/core/src/session/tests.rs b/codex-rs/core/src/session/tests.rs
index 6f84da3c79..1245045e31 100644
--- a/codex-rs/core/src/session/tests.rs
+++ b/codex-rs/core/src/session/tests.rs
@@ -221,6 +221,19 @@ fn assign_missing_response_item_ids_skips_agent_messages() {
     assert!(items[1].id().is_some_and(|id| id.starts_with("msg_")));
 }
 
+#[test]
+fn assign_missing_response_item_ids_assigns_additional_tools_ids() {
+    let items = Cow::Owned(vec![ResponseItem::AdditionalTools {
+        id: None,
+        role: "developer".to_string(),
+        tools: Vec::new(),
+    }]);
+
+    let items = Session::assign_missing_response_item_ids(items);
+
+    assert!(items[0].id().is_some_and(|id| id.starts_with("at_")));
+}
+
 fn assistant_message(text: &str) -> ResponseItem {
     ResponseItem::Message {
         id: None,
diff --git a/codex-rs/protocol/src/models.rs b/codex-rs/protocol/src/models.rs
index 02f39ede6e..182bc90644 100644
--- a/codex-rs/protocol/src/models.rs
+++ b/codex-rs/protocol/src/models.rs
@@ -920,6 +920,8 @@ pub enum ResponseItem {
     #[schemars(skip)]
     #[ts(skip)]
     AdditionalTools {
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        id: Option<String>,
         role: String,
         tools: Vec<serde_json::Value>,
     },
@@ -1156,7 +1158,8 @@ impl ResponseItem {
     /// Returns the non-empty Responses API item ID, if present.
     pub fn id(&self) -> Option<&str> {
         match self {
-            Self::Message { id, .. }
+            Self::AdditionalTools { id, .. }
+            | Self::Message { id, .. }
             | Self::AgentMessage { id, .. }
             | Self::LocalShellCall { id, .. }
             | Self::FunctionCall { id, .. }
@@ -1177,7 +1180,8 @@ impl ResponseItem {
     /// Sets or clears the Responses API item ID for variants that carry one.
     pub fn set_id(&mut self, new_id: Option<String>) {
         match self {
-            Self::Message { id, .. }
+            Self::AdditionalTools { id, .. }
+            | Self::Message { id, .. }
             | Self::AgentMessage { id, .. }
             | Self::LocalShellCall { id, .. }
             | Self::FunctionCall { id, .. }
@@ -2145,6 +2149,14 @@ mod tests {
         item.set_id(/*new_id*/ None);
 
         assert_eq!(item.id(), None);
+
+        let mut additional_tools = ResponseItem::AdditionalTools {
+            id: None,
+            role: "developer".to_string(),
+            tools: Vec::new(),
+        };
+        additional_tools.set_id(Some("at_test".to_string()));
+        assert_eq!(additional_tools.id(), Some("at_test"));
     }
 
     fn response_item_with_metadata(metadata: Option<ResponseItemMetadata>) -> ResponseItem {