[codex] Use input items for Responses Lite tools

codex-rs/codex-api/src/common.rs

@@ -28,7 +28,8 @@ pub struct CompactionInput<'a> {
     pub input: &'a [ResponseItem],
     #[serde(skip_serializing_if = "str::is_empty")]
     pub instructions: &'a str,
-    pub tools: Vec<Value>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub tools: Option<Vec<Value>>,
     pub parallel_tool_calls: bool,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub reasoning: Option<Reasoning>,
@@ -192,7 +193,8 @@ pub struct ResponsesApiRequest {
     #[serde(skip_serializing_if = "String::is_empty")]
     pub instructions: String,
     pub input: Vec<ResponseItem>,
-    pub tools: Vec<serde_json::Value>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub tools: Option<Vec<serde_json::Value>>,
     pub tool_choice: String,
     pub parallel_tool_calls: bool,
     pub reasoning: Option<Reasoning>,
@@ -240,7 +242,8 @@ pub struct ResponseCreateWsRequest {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub previous_response_id: Option<String>,
     pub input: Vec<ResponseItem>,
-    pub tools: Vec<Value>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub tools: Option<Vec<Value>>,
     pub tool_choice: String,
     pub parallel_tool_calls: bool,
     pub reasoning: Option<Reasoning>,

codex-rs/codex-api/src/endpoint/responses_websocket.rs

@@ -827,11 +827,11 @@ mod tests {
                 phase: None,
                 internal_chat_message_metadata_passthrough: None,
             }],
-            tools: vec![json!({
+            tools: Some(vec![json!({
                 "type": "function",
                 "name": "lookup",
                 "parameters": {"type": "object"}
-            })],
+            })]),
             tool_choice: "auto".to_string(),
             parallel_tool_calls: true,
             reasoning: None,

codex-rs/codex-api/tests/clients.rs

@@ -315,7 +315,7 @@ async fn responses_client_stream_request_preserves_item_ids() -> Result<()> {
             phase: None,
             internal_chat_message_metadata_passthrough: None,
         }],
-        tools: Vec::new(),
+        tools: Some(Vec::new()),
         tool_choice: "auto".into(),
         parallel_tool_calls: false,
         reasoning: None,
@@ -401,7 +401,7 @@ async fn streaming_client_retries_on_transport_error() -> Result<()> {
         model: "gpt-test".into(),
         instructions: "Say hi".into(),
         input: Vec::new(),
-        tools: Vec::new(),
+        tools: Some(Vec::new()),
         tool_choice: "auto".into(),
         parallel_tool_calls: false,
         reasoning: None,
@@ -520,7 +520,7 @@ async fn azure_store_sends_ids_and_headers() -> Result<()> {
             phase: None,
             internal_chat_message_metadata_passthrough: None,
         }],
-        tools: Vec::new(),
+        tools: Some(Vec::new()),
         tool_choice: "auto".into(),
         parallel_tool_calls: false,
         reasoning: None,

codex-rs/core/src/agent/control/spawn.rs

@@ -41,7 +41,8 @@ fn keep_forked_rollout_item(item: &RolloutItem, preserve_reference_context_item:
             _ => false,
         },
         RolloutItem::ResponseItem(
-            ResponseItem::AgentMessage { .. }
+            ResponseItem::AdditionalTools { .. }
+            | ResponseItem::AgentMessage { .. }
             | ResponseItem::Reasoning { .. }
             | ResponseItem::LocalShellCall { .. }
             | ResponseItem::FunctionCall { .. }

codex-rs/core/src/client.rs

@@ -73,6 +73,7 @@ use codex_otel::current_span_w3c_trace_context;
 use codex_protocol::ThreadId;
 use codex_protocol::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use codex_protocol::config_types::Verbosity as VerbosityConfig;
+use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::ModelInfo;
 use codex_protocol::openai_models::ReasoningEffort as ReasoningEffortConfig;
@@ -778,14 +779,35 @@ impl ModelClient {
         service_tier: Option<String>,
         responses_metadata: &CodexResponsesMetadata,
     ) -> Result<ResponsesApiRequest> {
-        let instructions = &prompt.base_instructions.text;
         let mut input = prompt.get_formatted_input_for_request(model_info.use_responses_lite);
         if !self.state.provider.info().is_openai() {
             input
                 .iter_mut()
                 .for_each(ResponseItem::clear_internal_chat_message_metadata_passthrough);
         }
         let tools = create_tools_json_for_responses_api(&prompt.tools)?;
+        let (instructions, tools) = if model_info.use_responses_lite {
+            let mut prefix = vec![ResponseItem::AdditionalTools {
+                id: None,
+                role: "developer".to_string(),
+                tools,
+            }];
+            if !prompt.base_instructions.text.is_empty() {
+                prefix.push(ResponseItem::Message {
+                    id: None,
+                    role: "developer".to_string(),
+                    content: vec![ContentItem::InputText {
+                        text: prompt.base_instructions.text.clone(),
+                    }],
+                    phase: None,
+                    internal_chat_message_metadata_passthrough: None,
+                });
+            }
+            input.splice(0..0, prefix);
+            (String::new(), None)
+        } else {
+            (prompt.base_instructions.text.clone(), Some(tools))
+        };
         let reasoning = Self::build_reasoning(model_info, effort, summary);
         let include = if reasoning.is_some() {
             vec!["reasoning.encrypted_content".to_string()]
@@ -812,7 +834,7 @@ impl ModelClient {
         let service_tier = model_info.service_tier_for_request(service_tier);
         let request = ResponsesApiRequest {
             model: model_info.slug.clone(),
-            instructions: instructions.clone(),
+            instructions,
             input,
             tools,
             tool_choice: "auto".to_string(),

codex-rs/core/src/client_common.rs

@@ -83,7 +83,8 @@ fn strip_image_details(items: &mut [ResponseItem]) {
                     }
                 }
             }
-            ResponseItem::Reasoning { .. }
+            ResponseItem::AdditionalTools { .. }
+            | ResponseItem::Reasoning { .. }
             | ResponseItem::AgentMessage { .. }
             | ResponseItem::LocalShellCall { .. }
             | ResponseItem::FunctionCall { .. }

codex-rs/core/src/client_common_tests.rs

@@ -110,7 +110,7 @@ fn serializes_text_verbosity_when_set() {
         model: "gpt-5.4".to_string(),
         instructions: "i".to_string(),
         input,
-        tools,
+        tools: Some(tools),
         tool_choice: "auto".to_string(),
         parallel_tool_calls: true,
         reasoning: None,
@@ -157,7 +157,7 @@ fn serializes_text_schema_with_strict_format() {
         model: "gpt-5.4".to_string(),
         instructions: "i".to_string(),
         input,
-        tools,
+        tools: Some(tools),
         tool_choice: "auto".to_string(),
         parallel_tool_calls: true,
         reasoning: None,
@@ -218,7 +218,7 @@ fn omits_text_when_not_set() {
         model: "gpt-5.4".to_string(),
         instructions: "i".to_string(),
         input,
-        tools,
+        tools: Some(tools),
         tool_choice: "auto".to_string(),
         parallel_tool_calls: true,
         reasoning: None,
@@ -241,7 +241,7 @@ fn serializes_flex_service_tier_when_set() {
         model: "gpt-5.4".to_string(),
         instructions: "i".to_string(),
         input: vec![],
-        tools: vec![],
+        tools: Some(vec![]),
         tool_choice: "auto".to_string(),
         parallel_tool_calls: true,
         reasoning: None,

codex-rs/core/src/compact_remote.rs

@@ -352,7 +352,8 @@ pub(crate) fn should_keep_compacted_history_item(item: &ResponseItem) -> bool {
         ResponseItem::AgentMessage { .. } => true,
         ResponseItem::Compaction { .. } | ResponseItem::ContextCompaction { .. } => true,
         ResponseItem::CompactionTrigger { .. } => false,
-        ResponseItem::Reasoning { .. }
+        ResponseItem::AdditionalTools { .. }
+        | ResponseItem::Reasoning { .. }
         | ResponseItem::LocalShellCall { .. }
         | ResponseItem::FunctionCall { .. }
         | ResponseItem::ToolSearchCall { .. }

codex-rs/core/src/context/world_state/environment_tests.rs

@@ -251,6 +251,6 @@ fn user_message(text: &str) -> ResponseItem {
             text: text.to_string(),
         }],
         phase: None,
-        metadata: None,
+        internal_chat_message_metadata_passthrough: None,
     }
 }

codex-rs/core/src/context_manager/history.rs

@@ -386,7 +386,8 @@ impl ContextManager {
                 output: truncate_function_output_payload(output, policy_with_serialization_budget),
                 internal_chat_message_metadata_passthrough: metadata.clone(),
             },
-            ResponseItem::Message { .. }
+            ResponseItem::AdditionalTools { .. }
+            | ResponseItem::Message { .. }
             | ResponseItem::AgentMessage { .. }
             | ResponseItem::Reasoning { .. }
             | ResponseItem::LocalShellCall { .. }
@@ -476,7 +477,8 @@ pub(crate) fn truncate_function_output_payload(
 fn is_api_message(message: &ResponseItem) -> bool {
     match message {
         ResponseItem::Message { role, .. } => role.as_str() != "system",
-        ResponseItem::AgentMessage { .. }
+        ResponseItem::AdditionalTools { .. }
+        | ResponseItem::AgentMessage { .. }
         | ResponseItem::FunctionCallOutput { .. }
         | ResponseItem::FunctionCall { .. }
         | ResponseItem::ToolSearchCall { .. }
@@ -722,7 +724,8 @@ fn is_model_generated_item(item: &ResponseItem) -> bool {
         | ResponseItem::Compaction { .. }
         | ResponseItem::ContextCompaction { .. } => true,
         ResponseItem::CompactionTrigger { .. } => false,
-        ResponseItem::FunctionCallOutput { .. }
+        ResponseItem::AdditionalTools { .. }
+        | ResponseItem::FunctionCallOutput { .. }
         | ResponseItem::ToolSearchOutput { .. }
         | ResponseItem::CustomToolCallOutput { .. }
         | ResponseItem::AgentMessage { .. }

codex-rs/core/src/image_preparation.rs

@@ -57,7 +57,8 @@ pub(crate) fn prepare_response_items(items: &mut [ResponseItem]) {
                     prepare_tool_output_content(content);
                 }
             }
-            ResponseItem::Reasoning { .. }
+            ResponseItem::AdditionalTools { .. }
+            | ResponseItem::Reasoning { .. }
             | ResponseItem::AgentMessage { .. }
             | ResponseItem::LocalShellCall { .. }
             | ResponseItem::FunctionCall { .. }

codex-rs/core/src/session/mod.rs

@@ -2714,6 +2714,7 @@ impl Session {
                 continue;
             }
             let prefix = match item {
+                ResponseItem::AdditionalTools { .. } => "at",
                 ResponseItem::Message { .. } => "msg",
                 ResponseItem::Reasoning { .. } => "rs",
                 ResponseItem::LocalShellCall { .. } => "lsh",

codex-rs/core/src/session/tests.rs

@@ -222,6 +222,19 @@ 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,

codex-rs/core/src/session/turn.rs

@@ -2091,7 +2091,8 @@ async fn try_run_sampling_request(
                     }
                     ResponseItem::Reasoning { .. } => true,
                     ResponseItem::AgentMessage { .. } => false,
-                    ResponseItem::LocalShellCall { .. }
+                    ResponseItem::AdditionalTools { .. }
+                    | ResponseItem::LocalShellCall { .. }
                     | ResponseItem::FunctionCall { .. }
                     | ResponseItem::ToolSearchCall { .. }
                     | ResponseItem::FunctionCallOutput { .. }

codex-rs/core/src/turn_timing.rs

@@ -380,7 +380,8 @@ fn response_item_records_turn_ttft(item: &ResponseItem) -> bool {
         | ResponseItem::Compaction { .. }
         | ResponseItem::ContextCompaction { .. } => true,
         ResponseItem::CompactionTrigger { .. } => false,
-        ResponseItem::FunctionCallOutput { .. }
+        ResponseItem::AdditionalTools { .. }
+        | ResponseItem::FunctionCallOutput { .. }
         | ResponseItem::CustomToolCallOutput { .. }
         | ResponseItem::ToolSearchOutput { .. }
         | ResponseItem::Other => false,

codex-rs/core/tests/suite/agent_websocket.rs

@@ -1,6 +1,10 @@
 use anyhow::Result;
 use codex_features::Feature;
 use codex_protocol::config_types::ServiceTier;
+use codex_protocol::protocol::EventMsg;
+use codex_protocol::protocol::Op;
+use codex_protocol::protocol::ThreadSettingsOverrides;
+use codex_protocol::user_input::UserInput;
 use core_test_support::responses::WebSocketConnectionConfig;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
@@ -10,12 +14,89 @@ use core_test_support::responses::start_websocket_server;
 use core_test_support::responses::start_websocket_server_with_headers;
 use core_test_support::skip_if_no_network;
 use core_test_support::test_codex::test_codex;
+use core_test_support::wait_for_event;
 use pretty_assertions::assert_eq;
 use serde_json::Value;
 use std::time::Duration;
 
 const WS_V2_BETA_HEADER_VALUE: &str = "responses_websockets=2026-02-06";
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn websocket_model_switch_to_responses_lite_omits_top_level_tools() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_websocket_server(vec![vec![
+        vec![ev_response_created("warm-1"), ev_completed("warm-1")],
+        vec![ev_response_created("resp-1"), ev_completed("resp-1")],
+        vec![ev_response_created("resp-2"), ev_completed("resp-2")],
+    ]])
+    .await;
+
+    let mut builder = test_codex()
+        .with_model_info_override("gpt-5.4", |model_info| {
+            model_info.use_responses_lite = true;
+        })
+        .with_model("gpt-5.3-codex");
+    let test = builder.build_with_websocket_server(&server).await?;
+
+    test.submit_turn("non-lite turn").await?;
+    test.codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "lite turn".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            responsesapi_client_metadata: None,
+            additional_context: Default::default(),
+            thread_settings: ThreadSettingsOverrides {
+                model: Some("gpt-5.4".to_string()),
+                ..Default::default()
+            },
+        })
+        .await?;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    assert_eq!(server.handshakes().len(), 1);
+    let connection = server.single_connection();
+    assert_eq!(connection.len(), 3);
+    let non_lite_turn = connection
+        .get(1)
+        .expect("missing non-lite turn request")
+        .body_json();
+    let lite_turn = connection
+        .get(2)
+        .expect("missing lite turn request")
+        .body_json();
+
+    assert_eq!(non_lite_turn["model"].as_str(), Some("gpt-5.3-codex"));
+    assert_eq!(lite_turn["model"].as_str(), Some("gpt-5.4"));
+    assert!(
+        non_lite_turn
+            .get("tools")
+            .and_then(Value::as_array)
+            .is_some_and(|tools| !tools.is_empty())
+    );
+    assert_eq!(lite_turn.get("previous_response_id"), None);
+    assert_eq!(lite_turn.get("tools"), None);
+    assert_eq!(lite_turn.get("instructions"), None);
+    let additional_tools = lite_turn
+        .get("input")
+        .and_then(Value::as_array)
+        .and_then(|input| input.first())
+        .filter(|item| item.get("type").and_then(Value::as_str) == Some("additional_tools"))
+        .and_then(|item| item.get("tools"))
+        .and_then(Value::as_array)
+        .expect("lite turn should start with an additional_tools item");
+    assert!(!additional_tools.is_empty());
+
+    server.shutdown().await;
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn websocket_test_codex_shell_chain() -> Result<()> {
     skip_if_no_network!(Ok(()));