diff --git a/codex-rs/codex-api/src/common.rs b/codex-rs/codex-api/src/common.rs
index aeea85e7fc..10eab17c69 100644
--- a/codex-rs/codex-api/src/common.rs
+++ b/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>,
diff --git a/codex-rs/codex-api/tests/clients.rs b/codex-rs/codex-api/tests/clients.rs
index 488f6dc674..3874dad7d9 100644
--- a/codex-rs/codex-api/tests/clients.rs
+++ b/codex-rs/codex-api/tests/clients.rs
@@ -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,
             metadata: None,
         }],
-        tools: Vec::new(),
+        tools: Some(Vec::new()),
         tool_choice: "auto".into(),
         parallel_tool_calls: false,
         reasoning: None,
diff --git a/codex-rs/core/src/agent/control/spawn.rs b/codex-rs/core/src/agent/control/spawn.rs
index 3f746ef790..1329f768ad 100644
--- a/codex-rs/core/src/agent/control/spawn.rs
+++ b/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 { .. }
diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index 73d7398a3d..4647668bf5 100644
--- a/codex-rs/core/src/client.rs
+++ b/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;
@@ -778,12 +779,31 @@ 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_metadata);
         }
         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 {
+                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,
+                });
+            }
+            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()]
@@ -810,7 +830,7 @@ 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(),
diff --git a/codex-rs/core/src/client_common.rs b/codex-rs/core/src/client_common.rs
index e0c2ff03c3..54ad70daaa 100644
--- a/codex-rs/core/src/client_common.rs
+++ b/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 { .. }
diff --git a/codex-rs/core/src/client_common_tests.rs b/codex-rs/core/src/client_common_tests.rs
index 8107049ab3..1f489f8e41 100644
--- a/codex-rs/core/src/client_common_tests.rs
+++ b/codex-rs/core/src/client_common_tests.rs
@@ -104,13 +104,11 @@ fn responses_lite_request_copies_strip_image_details() {
 
 #[test]
 fn serializes_text_verbosity_when_set() {
-    let input: Vec<ResponseItem> = vec![];
-    let tools: Vec<serde_json::Value> = vec![];
     let req = ResponsesApiRequest {
         model: "gpt-5.4".to_string(),
         instructions: "i".to_string(),
-        input,
-        tools,
+        input: vec![],
+        tools: Some(vec![]),
         tool_choice: "auto".to_string(),
         parallel_tool_calls: true,
         reasoning: None,
@@ -137,8 +135,6 @@ fn serializes_text_verbosity_when_set() {
 
 #[test]
 fn serializes_text_schema_with_strict_format() {
-    let input: Vec<ResponseItem> = vec![];
-    let tools: Vec<serde_json::Value> = vec![];
     let schema = serde_json::json!({
         "type": "object",
         "properties": {
@@ -156,8 +152,8 @@ fn serializes_text_schema_with_strict_format() {
     let req = ResponsesApiRequest {
         model: "gpt-5.4".to_string(),
         instructions: "i".to_string(),
-        input,
-        tools,
+        input: vec![],
+        tools: Some(vec![]),
         tool_choice: "auto".to_string(),
         parallel_tool_calls: true,
         reasoning: None,
@@ -212,13 +208,11 @@ fn serializes_text_schema_with_non_strict_format() {
 
 #[test]
 fn omits_text_when_not_set() {
-    let input: Vec<ResponseItem> = vec![];
-    let tools: Vec<serde_json::Value> = vec![];
     let req = ResponsesApiRequest {
         model: "gpt-5.4".to_string(),
         instructions: "i".to_string(),
-        input,
-        tools,
+        input: vec![],
+        tools: Some(vec![]),
         tool_choice: "auto".to_string(),
         parallel_tool_calls: true,
         reasoning: None,
@@ -241,7 +235,7 @@ 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,
diff --git a/codex-rs/core/src/compact_remote.rs b/codex-rs/core/src/compact_remote.rs
index 7a47881e47..99f1f175ee 100644
--- a/codex-rs/core/src/compact_remote.rs
+++ b/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 { .. }
diff --git a/codex-rs/core/src/context_manager/history.rs b/codex-rs/core/src/context_manager/history.rs
index 6ce1c6148f..b21122ffb5 100644
--- a/codex-rs/core/src/context_manager/history.rs
+++ b/codex-rs/core/src/context_manager/history.rs
@@ -362,7 +362,8 @@ impl ContextManager {
                 output: truncate_function_output_payload(output, policy_with_serialization_budget),
                 metadata: metadata.clone(),
             },
-            ResponseItem::Message { .. }
+            ResponseItem::AdditionalTools { .. }
+            | ResponseItem::Message { .. }
             | ResponseItem::AgentMessage { .. }
             | ResponseItem::Reasoning { .. }
             | ResponseItem::LocalShellCall { .. }
@@ -452,7 +453,8 @@ 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 { .. }
@@ -698,7 +700,8 @@ 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 { .. }
diff --git a/codex-rs/core/src/image_preparation.rs b/codex-rs/core/src/image_preparation.rs
index d30dfbb5ca..0210ff96ed 100644
--- a/codex-rs/core/src/image_preparation.rs
+++ b/codex-rs/core/src/image_preparation.rs
@@ -52,7 +52,8 @@ 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 { .. }
diff --git a/codex-rs/core/src/session/turn.rs b/codex-rs/core/src/session/turn.rs
index 041da729ea..d7e64dc5d2 100644
--- a/codex-rs/core/src/session/turn.rs
+++ b/codex-rs/core/src/session/turn.rs
@@ -2072,7 +2072,8 @@ async fn try_run_sampling_request(
                     }
                     ResponseItem::Reasoning { .. } => true,
                     ResponseItem::AgentMessage { .. } => false,
-                    ResponseItem::LocalShellCall { .. }
+                    ResponseItem::AdditionalTools { .. }
+                    | ResponseItem::LocalShellCall { .. }
                     | ResponseItem::FunctionCall { .. }
                     | ResponseItem::ToolSearchCall { .. }
                     | ResponseItem::FunctionCallOutput { .. }
diff --git a/codex-rs/core/src/turn_timing.rs b/codex-rs/core/src/turn_timing.rs
index e7a90800b0..04f7aeb69b 100644
--- a/codex-rs/core/src/turn_timing.rs
+++ b/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,
diff --git a/codex-rs/core/tests/suite/responses_lite.rs b/codex-rs/core/tests/suite/responses_lite.rs
index b137d5bc73..d4ea6d6f4c 100644
--- a/codex-rs/core/tests/suite/responses_lite.rs
+++ b/codex-rs/core/tests/suite/responses_lite.rs
@@ -54,6 +54,70 @@ fn has_hosted_tool(tools: &[Value], tool_type: &str) -> bool {
         .any(|tool| tool.get("type").and_then(Value::as_str) == Some(tool_type))
 }
 
+fn additional_tools(body: &Value) -> Result<&[Value]> {
+    body["input"]
+        .as_array()
+        .context("Responses request input should be an array")?
+        .iter()
+        .find(|item| item.get("type").and_then(Value::as_str) == Some("additional_tools"))
+        .context("Responses request should contain additional_tools")?["tools"]
+        .as_array()
+        .map(Vec::as_slice)
+        .context("additional_tools tools should be an array")
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn responses_lite_uses_input_items_for_instructions_and_tools() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = responses::start_mock_server().await;
+    let response_mock = responses::mount_sse_once(
+        &server,
+        responses::sse(vec![
+            responses::ev_response_created("resp-1"),
+            responses::ev_completed("resp-1"),
+        ]),
+    )
+    .await;
+
+    let mut builder = test_codex()
+        .with_model_info_override("gpt-5.4", |model_info| {
+            model_info.use_responses_lite = true;
+        })
+        .with_config(|config| {
+            config.base_instructions = Some("test instructions".to_string());
+        });
+    let test = builder.build(&server).await?;
+
+    test.submit_turn("hello").await?;
+
+    let body = response_mock.single_request().body_json();
+    assert!(body.get("instructions").is_none());
+    assert!(body.get("tools").is_none());
+
+    let input = body["input"]
+        .as_array()
+        .context("Responses request input should be an array")?;
+    assert_eq!(input[0]["type"], "additional_tools");
+    assert_eq!(input[0]["role"], "developer");
+    assert_eq!(
+        input[1],
+        serde_json::json!({
+            "type": "message",
+            "role": "developer",
+            "content": [{
+                "type": "input_text",
+                "text": "test instructions",
+            }],
+        })
+    );
+
+    let tools = additional_tools(&body)?;
+    assert!(!tools.is_empty());
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn responses_lite_strips_data_image_detail_without_resize_all_images() -> Result<()> {
     skip_if_no_network!(Ok(()));
@@ -144,17 +208,10 @@ async fn responses_lite_uses_standalone_web_search_and_image_generation() -> Res
         request.header(RESPONSES_LITE_HEADER).as_deref(),
         Some("true")
     );
-    request
-        .tool_by_name("web", "run")
-        .context("Responses Lite should expose standalone web search")?;
-    request
-        .tool_by_name("image_gen", "imagegen")
-        .context("Responses Lite should expose standalone image generation")?;
-
     let body = request.body_json();
-    let tools = body["tools"]
-        .as_array()
-        .context("Responses request tools should be an array")?;
+    assert!(body.get("tools").is_none());
+    let tools = additional_tools(&body)?;
+    assert!(!tools.is_empty());
     assert!(!has_hosted_tool(tools, "web_search"));
     assert!(!has_hosted_tool(tools, "image_generation"));
 
@@ -242,9 +299,8 @@ async fn responses_lite_omits_hosted_tools_without_standalone_extensions() -> Re
     test.submit_turn("Do not use hosted tools").await?;
 
     let body = response_mock.single_request().body_json();
-    let tools = body["tools"]
-        .as_array()
-        .context("Responses request tools should be an array")?;
+    assert!(body.get("tools").is_none());
+    let tools = additional_tools(&body)?;
     assert!(!has_hosted_tool(tools, "web_search"));
     assert!(!has_hosted_tool(tools, "image_generation"));
 
diff --git a/codex-rs/ext/image-generation/src/tool.rs b/codex-rs/ext/image-generation/src/tool.rs
index 3e2cfd1792..d4451a8926 100644
--- a/codex-rs/ext/image-generation/src/tool.rs
+++ b/codex-rs/ext/image-generation/src/tool.rs
@@ -256,7 +256,8 @@ fn recent_images(history: &[ResponseItem], count: usize) -> Vec<ImageUrl> {
             ResponseItem::CustomToolCall { call_id, .. } => {
                 custom_tool_call_ids.insert(call_id.as_str());
             }
-            ResponseItem::Message { .. }
+            ResponseItem::AdditionalTools { .. }
+            | ResponseItem::Message { .. }
             | ResponseItem::AgentMessage { .. }
             | ResponseItem::Reasoning { .. }
             | ResponseItem::LocalShellCall { .. }
@@ -296,7 +297,8 @@ fn recent_images(history: &[ResponseItem], count: usize) -> Vec<ImageUrl> {
             ResponseItem::ImageGenerationCall { result, .. } if !result.is_empty() => {
                 image_urls.push(format!("data:image/png;base64,{result}"));
             }
-            ResponseItem::Reasoning { .. }
+            ResponseItem::AdditionalTools { .. }
+            | ResponseItem::Reasoning { .. }
             | ResponseItem::AgentMessage { .. }
             | ResponseItem::LocalShellCall { .. }
             | ResponseItem::FunctionCall { .. }
diff --git a/codex-rs/otel/src/events/session_telemetry.rs b/codex-rs/otel/src/events/session_telemetry.rs
index c8e35b9f7d..66fab81ef7 100644
--- a/codex-rs/otel/src/events/session_telemetry.rs
+++ b/codex-rs/otel/src/events/session_telemetry.rs
@@ -1218,6 +1218,7 @@ impl SessionTelemetry {
 
     fn responses_item_type(item: &ResponseItem) -> String {
         match item {
+            ResponseItem::AdditionalTools { .. } => "additional_tools".into(),
             ResponseItem::Message { role, .. } => format!("message_from_{role}"),
             ResponseItem::AgentMessage { .. } => "agent_message".into(),
             ResponseItem::Reasoning { .. } => "reasoning".into(),
diff --git a/codex-rs/protocol/src/models.rs b/codex-rs/protocol/src/models.rs
index b0d373cfb0..0196709ad4 100644
--- a/codex-rs/protocol/src/models.rs
+++ b/codex-rs/protocol/src/models.rs
@@ -917,6 +917,12 @@ pub struct ResponseItemMetadata {
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, JsonSchema, TS)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum ResponseItem {
+    #[schemars(skip)]
+    #[ts(skip)]
+    AdditionalTools {
+        role: String,
+        tools: Vec<serde_json::Value>,
+    },
     Message {
         #[serde(default, skip_serializing_if = "Option::is_none")]
         #[ts(optional)]
diff --git a/codex-rs/rollout/src/policy.rs b/codex-rs/rollout/src/policy.rs
index b342d699b7..be72d622fc 100644
--- a/codex-rs/rollout/src/policy.rs
+++ b/codex-rs/rollout/src/policy.rs
@@ -30,7 +30,8 @@ pub fn persisted_rollout_items(items: &[RolloutItem]) -> Vec<RolloutItem> {
 #[inline]
 pub fn should_persist_response_item(item: &ResponseItem) -> bool {
     match item {
-        ResponseItem::Message { .. }
+        ResponseItem::AdditionalTools { .. }
+        | ResponseItem::Message { .. }
         | ResponseItem::AgentMessage { .. }
         | ResponseItem::Reasoning { .. }
         | ResponseItem::LocalShellCall { .. }
@@ -62,7 +63,8 @@ pub fn should_persist_response_item_for_memories(item: &ResponseItem) -> bool {
         | ResponseItem::CustomToolCall { .. }
         | ResponseItem::CustomToolCallOutput { .. }
         | ResponseItem::WebSearchCall { .. } => true,
-        ResponseItem::AgentMessage { .. }
+        ResponseItem::AdditionalTools { .. }
+        | ResponseItem::AgentMessage { .. }
         | ResponseItem::Reasoning { .. }
         | ResponseItem::ImageGenerationCall { .. }
         | ResponseItem::Compaction { .. }