diff --git a/codex-rs/core/src/codex.rs b/codex-rs/core/src/codex.rs
index c82c50e45d..f77e6a40c3 100644
--- a/codex-rs/core/src/codex.rs
+++ b/codex-rs/core/src/codex.rs
@@ -505,6 +505,7 @@ impl Session {
             msg: EventMsg::SessionConfigured(SessionConfiguredEvent {
                 session_id: conversation_id,
                 model,
+                reasoning_effort: model_reasoning_effort,
                 history_log_id,
                 history_entry_count,
                 initial_messages,
diff --git a/codex-rs/exec/src/event_processor_with_human_output.rs b/codex-rs/exec/src/event_processor_with_human_output.rs
index c80117706b..3856e05bf5 100644
--- a/codex-rs/exec/src/event_processor_with_human_output.rs
+++ b/codex-rs/exec/src/event_processor_with_human_output.rs
@@ -520,6 +520,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 let SessionConfiguredEvent {
                     session_id: conversation_id,
                     model,
+                    reasoning_effort: _,
                     history_log_id: _,
                     history_entry_count: _,
                     initial_messages: _,
diff --git a/codex-rs/mcp-server/src/codex_message_processor.rs b/codex-rs/mcp-server/src/codex_message_processor.rs
index 858bb99c24..b7b2b0f95f 100644
--- a/codex-rs/mcp-server/src/codex_message_processor.rs
+++ b/codex-rs/mcp-server/src/codex_message_processor.rs
@@ -593,6 +593,7 @@ impl CodexMessageProcessor {
                 let response = NewConversationResponse {
                     conversation_id,
                     model: session_configured.model,
+                    reasoning_effort: session_configured.reasoning_effort,
                     rollout_path: session_configured.rollout_path,
                 };
                 self.outgoing.send_response(request_id, response).await;
diff --git a/codex-rs/mcp-server/src/outgoing_message.rs b/codex-rs/mcp-server/src/outgoing_message.rs
index 939d986aec..04e99516ce 100644
--- a/codex-rs/mcp-server/src/outgoing_message.rs
+++ b/codex-rs/mcp-server/src/outgoing_message.rs
@@ -258,6 +258,7 @@ pub(crate) struct OutgoingError {
 mod tests {
     use codex_core::protocol::EventMsg;
     use codex_core::protocol::SessionConfiguredEvent;
+    use codex_protocol::config_types::ReasoningEffort;
     use codex_protocol::mcp_protocol::ConversationId;
     use codex_protocol::mcp_protocol::LoginChatGptCompleteNotification;
     use pretty_assertions::assert_eq;
@@ -279,6 +280,7 @@ mod tests {
             msg: EventMsg::SessionConfigured(SessionConfiguredEvent {
                 session_id: conversation_id,
                 model: "gpt-4o".to_string(),
+                reasoning_effort: ReasoningEffort::default(),
                 history_log_id: 1,
                 history_entry_count: 1000,
                 initial_messages: None,
@@ -312,6 +314,7 @@ mod tests {
         let session_configured_event = SessionConfiguredEvent {
             session_id: conversation_id,
             model: "gpt-4o".to_string(),
+            reasoning_effort: ReasoningEffort::default(),
             history_log_id: 1,
             history_entry_count: 1000,
             initial_messages: None,
@@ -342,6 +345,7 @@ mod tests {
             "msg": {
                 "session_id": session_configured_event.session_id,
                 "model": session_configured_event.model,
+                "reasoning_effort": session_configured_event.reasoning_effort,
                 "history_log_id": session_configured_event.history_log_id,
                 "history_entry_count": session_configured_event.history_entry_count,
                 "type": "session_configured",
diff --git a/codex-rs/mcp-server/tests/suite/codex_message_processor_flow.rs b/codex-rs/mcp-server/tests/suite/codex_message_processor_flow.rs
index 7737c49af3..49d9ab1807 100644
--- a/codex-rs/mcp-server/tests/suite/codex_message_processor_flow.rs
+++ b/codex-rs/mcp-server/tests/suite/codex_message_processor_flow.rs
@@ -90,6 +90,7 @@ async fn test_codex_jsonrpc_conversation_flow() {
     let NewConversationResponse {
         conversation_id,
         model,
+        reasoning_effort: _,
         rollout_path: _,
     } = new_conv_resp;
     assert_eq!(model, "mock-model");
diff --git a/codex-rs/mcp-server/tests/suite/create_conversation.rs b/codex-rs/mcp-server/tests/suite/create_conversation.rs
index 5071868c85..1b62d01d46 100644
--- a/codex-rs/mcp-server/tests/suite/create_conversation.rs
+++ b/codex-rs/mcp-server/tests/suite/create_conversation.rs
@@ -59,6 +59,7 @@ async fn test_conversation_create_and_send_message_ok() {
     let NewConversationResponse {
         conversation_id,
         model,
+        reasoning_effort: _,
         rollout_path: _,
     } = to_response::<NewConversationResponse>(new_conv_resp)
         .expect("deserialize newConversation response");
diff --git a/codex-rs/protocol/src/mcp_protocol.rs b/codex-rs/protocol/src/mcp_protocol.rs
index 26887aa59b..a382b8bc7a 100644
--- a/codex-rs/protocol/src/mcp_protocol.rs
+++ b/codex-rs/protocol/src/mcp_protocol.rs
@@ -217,6 +217,8 @@ pub struct NewConversationParams {
 pub struct NewConversationResponse {
     pub conversation_id: ConversationId,
     pub model: String,
+    /// Note this could be ignored by the model.
+    pub reasoning_effort: ReasoningEffort,
     pub rollout_path: PathBuf,
 }
 
diff --git a/codex-rs/protocol/src/protocol.rs b/codex-rs/protocol/src/protocol.rs
index 2eb3d1e808..e1a9920e78 100644
--- a/codex-rs/protocol/src/protocol.rs
+++ b/codex-rs/protocol/src/protocol.rs
@@ -1081,6 +1081,9 @@ pub struct SessionConfiguredEvent {
     /// Tell the client what model is being queried.
     pub model: String,
 
+    /// The effort the model is putting into reasoning about the user's request.
+    pub reasoning_effort: ReasoningEffortConfig,
+
     /// Identifier of the history log file (inode on Unix, 0 otherwise).
     pub history_log_id: u64,
 
@@ -1169,6 +1172,7 @@ mod tests {
             msg: EventMsg::SessionConfigured(SessionConfiguredEvent {
                 session_id: conversation_id,
                 model: "codex-mini-latest".to_string(),
+                reasoning_effort: ReasoningEffortConfig::default(),
                 history_log_id: 0,
                 history_entry_count: 0,
                 initial_messages: None,
diff --git a/codex-rs/tui/src/chatwidget/tests.rs b/codex-rs/tui/src/chatwidget/tests.rs
index c148b8670b..dc1da26eb4 100644
--- a/codex-rs/tui/src/chatwidget/tests.rs
+++ b/codex-rs/tui/src/chatwidget/tests.rs
@@ -138,6 +138,7 @@ fn resumed_initial_messages_render_history() {
     let configured = codex_core::protocol::SessionConfiguredEvent {
         session_id: conversation_id,
         model: "test-model".to_string(),
+        reasoning_effort: ReasoningEffortConfig::default(),
         history_log_id: 0,
         history_entry_count: 0,
         initial_messages: Some(vec![
diff --git a/codex-rs/tui/src/history_cell.rs b/codex-rs/tui/src/history_cell.rs
index b3b45774c1..8bfe9c277f 100644
--- a/codex-rs/tui/src/history_cell.rs
+++ b/codex-rs/tui/src/history_cell.rs
@@ -601,6 +601,7 @@ pub(crate) fn new_session_info(
 ) -> PlainHistoryCell {
     let SessionConfiguredEvent {
         model,
+        reasoning_effort: _,
         session_id: _,
         history_log_id: _,
         history_entry_count: _,