diff --git a/codex-rs/tui/src/bottom_pane/slash_commands.rs b/codex-rs/tui/src/bottom_pane/slash_commands.rs
index e907b6c5e554..a185970f4ef0 100644
--- a/codex-rs/tui/src/bottom_pane/slash_commands.rs
+++ b/codex-rs/tui/src/bottom_pane/slash_commands.rs
@@ -294,6 +294,7 @@ mod tests {
                 SlashCommand::Ide,
                 SlashCommand::Copy,
                 SlashCommand::Raw,
+                SlashCommand::Lcs,
                 SlashCommand::Diff,
                 SlashCommand::Mention,
                 SlashCommand::Status,
diff --git a/codex-rs/tui/src/chatwidget.rs b/codex-rs/tui/src/chatwidget.rs
index f8a33e987576..152553964a15 100644
--- a/codex-rs/tui/src/chatwidget.rs
+++ b/codex-rs/tui/src/chatwidget.rs
@@ -535,6 +535,9 @@ pub(crate) struct ChatWidget {
     transcript: TranscriptState,
     config: Config,
     raw_output_mode: bool,
+    /// Whether the LCS substrate (MCP tool calls) is rendered as it runs.
+    /// True by default; `/lcs hidden` sets it false.
+    show_lcs_substrate: bool,
     /// Runtime value resolved by core. `config.service_tier` remains the explicit user choice.
     effective_service_tier: Option<String>,
     /// The unmasked collaboration mode settings (always Default mode).
@@ -1664,6 +1667,38 @@ impl ChatWidget {
         enabled
     }
 
+    /// Whether MCP tool calls are rendered as they run.
+    pub(crate) fn show_lcs_substrate(&self) -> bool {
+        self.show_lcs_substrate
+    }
+
+    pub(crate) fn set_lcs_substrate(&mut self, enabled: bool) {
+        self.show_lcs_substrate = enabled;
+        self.refresh_status_surfaces();
+    }
+
+    pub(crate) fn lcs_substrate_notice(enabled: bool) -> &'static str {
+        if enabled {
+            "LCS substrate visible: MCP tool calls are shown as they run."
+        } else {
+            "LCS substrate hidden: MCP tool calls run silently."
+        }
+    }
+
+    pub(crate) fn set_lcs_substrate_and_notify(&mut self, enabled: bool) {
+        self.set_lcs_substrate(enabled);
+        self.add_info_message(
+            Self::lcs_substrate_notice(enabled).to_string(),
+            /*hint*/ None,
+        );
+    }
+
+    pub(crate) fn toggle_lcs_substrate_and_notify(&mut self) -> bool {
+        let enabled = !self.show_lcs_substrate;
+        self.set_lcs_substrate_and_notify(enabled);
+        enabled
+    }
+
     /// Update resize-sensitive chat widget state after the terminal width changes.
     ///
     /// The app calls this even when terminal resize reflow is disabled so live stream wrapping
diff --git a/codex-rs/tui/src/chatwidget/constructor.rs b/codex-rs/tui/src/chatwidget/constructor.rs
index 7e86814b92be..15d08a329d3f 100644
--- a/codex-rs/tui/src/chatwidget/constructor.rs
+++ b/codex-rs/tui/src/chatwidget/constructor.rs
@@ -106,6 +106,9 @@ impl ChatWidget {
             }),
             transcript: TranscriptState::new(active_cell),
             raw_output_mode: config.tui_raw_output_mode,
+            // The LCS substrate (MCP tool calls) is visible by default.
+            // `/lcs hidden` opts out.
+            show_lcs_substrate: true,
             config,
             effective_service_tier,
             skills_all: Vec::new(),
diff --git a/codex-rs/tui/src/chatwidget/slash_dispatch.rs b/codex-rs/tui/src/chatwidget/slash_dispatch.rs
index a6fd68326cce..2459f30ebdbc 100644
--- a/codex-rs/tui/src/chatwidget/slash_dispatch.rs
+++ b/codex-rs/tui/src/chatwidget/slash_dispatch.rs
@@ -35,6 +35,7 @@ const SIDE_SLASH_COMMAND_UNAVAILABLE_HINT: &str =
     "Press Ctrl+C to return to the main thread first.";
 const GOAL_USAGE_HINT: &str = "Example: /goal improve benchmark coverage";
 const RAW_USAGE: &str = "Usage: /raw [on|off]";
+const LCS_USAGE: &str = "Usage: /lcs [hidden|visible]";
 
 impl ChatWidget {
     /// Dispatch a bare slash command and record its staged local-history entry.
@@ -374,6 +375,9 @@ impl ChatWidget {
                 let enabled = self.toggle_raw_output_mode_and_notify();
                 self.emit_raw_output_mode_changed(enabled);
             }
+            SlashCommand::Lcs => {
+                self.toggle_lcs_substrate_and_notify();
+            }
             SlashCommand::Diff => {
                 self.add_diff_in_progress();
                 let tx = self.app_event_tx.clone();
@@ -669,6 +673,11 @@ impl ChatWidget {
                 }
                 _ => self.add_error_message(RAW_USAGE.to_string()),
             },
+            SlashCommand::Lcs => match trimmed.to_ascii_lowercase().as_str() {
+                "hidden" => self.set_lcs_substrate_and_notify(/*enabled*/ false),
+                "visible" => self.set_lcs_substrate_and_notify(/*enabled*/ true),
+                _ => self.add_error_message(LCS_USAGE.to_string()),
+            },
             SlashCommand::Rename if !trimmed.is_empty() => {
                 if !self.ensure_thread_rename_allowed() {
                     return;
@@ -996,6 +1005,7 @@ impl ChatWidget {
             | SlashCommand::Rollout
             | SlashCommand::Copy
             | SlashCommand::Raw
+            | SlashCommand::Lcs
             | SlashCommand::Vim
             | SlashCommand::Diff
             | SlashCommand::App
diff --git a/codex-rs/tui/src/chatwidget/tests/slash_commands.rs b/codex-rs/tui/src/chatwidget/tests/slash_commands.rs
index 9c2802fb6041..5c310d3d704b 100644
--- a/codex-rs/tui/src/chatwidget/tests/slash_commands.rs
+++ b/codex-rs/tui/src/chatwidget/tests/slash_commands.rs
@@ -2419,6 +2419,44 @@ async fn raw_slash_command_reports_usage_for_invalid_arg() {
     );
 }
 
+#[tokio::test]
+async fn lcs_slash_command_toggles_and_accepts_hidden_visible_args() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+
+    // The LCS substrate (MCP tool calls) is visible by default.
+    assert!(chat.show_lcs_substrate());
+
+    chat.dispatch_command_with_args(SlashCommand::Lcs, "hidden".to_string(), Vec::new());
+    assert!(!chat.show_lcs_substrate());
+
+    chat.dispatch_command_with_args(SlashCommand::Lcs, "visible".to_string(), Vec::new());
+    assert!(chat.show_lcs_substrate());
+
+    // Bare `/lcs` toggles.
+    chat.dispatch_command(SlashCommand::Lcs);
+    assert!(!chat.show_lcs_substrate());
+}
+
+#[tokio::test]
+async fn lcs_slash_command_reports_usage_for_invalid_arg() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+
+    chat.dispatch_command_with_args(SlashCommand::Lcs, "status".to_string(), Vec::new());
+
+    // Unchanged.
+    assert!(chat.show_lcs_substrate());
+    let cells = drain_insert_history(&mut rx);
+    let rendered = cells
+        .iter()
+        .map(|lines| lines_to_single_string(lines))
+        .collect::<Vec<_>>()
+        .join("\n");
+    assert!(
+        rendered.contains("Usage: /lcs [hidden|visible]"),
+        "expected lcs usage error, got {rendered:?}"
+    );
+}
+
 #[tokio::test]
 async fn compact_queues_user_messages_snapshot() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
diff --git a/codex-rs/tui/src/chatwidget/tool_lifecycle.rs b/codex-rs/tui/src/chatwidget/tool_lifecycle.rs
index 5df5e7bc124a..2922766df42a 100644
--- a/codex-rs/tui/src/chatwidget/tool_lifecycle.rs
+++ b/codex-rs/tui/src/chatwidget/tool_lifecycle.rs
@@ -50,19 +50,16 @@ impl ChatWidget {
     }
 
     pub(super) fn on_mcp_tool_call_started(&mut self, item: ThreadItem) {
-        let item2 = item.clone();
-        self.defer_or_handle(
-            |q| q.push_item_started(item),
-            |s| s.handle_mcp_tool_call_started_now(item2),
-        );
+        // Render MCP tool calls as they start, rather than batching them into
+        // the interrupt queue until the stream cycle ends. Flush pending
+        // interrupts first to preserve ordering, then handle immediately.
+        self.flush_interrupt_queue();
+        self.handle_mcp_tool_call_started_now(item);
     }
 
     pub(super) fn on_mcp_tool_call_completed(&mut self, item: ThreadItem) {
-        let item2 = item.clone();
-        self.defer_or_handle(
-            |q| q.push_item_completed(item),
-            |s| s.handle_mcp_tool_call_completed_now(item2),
-        );
+        self.flush_interrupt_queue();
+        self.handle_mcp_tool_call_completed_now(item);
     }
 
     pub(super) fn on_web_search_begin(&mut self, call_id: String) {
@@ -159,6 +156,9 @@ impl ChatWidget {
 
     pub(crate) fn handle_mcp_tool_call_started_now(&mut self, item: ThreadItem) {
         self.record_visible_turn_activity();
+        if !self.show_lcs_substrate {
+            return;
+        }
         let ThreadItem::McpToolCall {
             id,
             server,
@@ -186,6 +186,10 @@ impl ChatWidget {
 
     pub(crate) fn handle_mcp_tool_call_completed_now(&mut self, item: ThreadItem) {
         self.flush_answer_stream_with_separator();
+        self.transcript.had_work_activity = true;
+        if !self.show_lcs_substrate {
+            return;
+        }
 
         let ThreadItem::McpToolCall {
             id,
diff --git a/codex-rs/tui/src/slash_command.rs b/codex-rs/tui/src/slash_command.rs
index 2481af21ede6..cb59e4056f99 100644
--- a/codex-rs/tui/src/slash_command.rs
+++ b/codex-rs/tui/src/slash_command.rs
@@ -43,6 +43,8 @@ pub enum SlashCommand {
     Btw,
     Copy,
     Raw,
+    /// Toggle whether the LCS substrate (MCP tool calls) is shown as it runs.
+    Lcs,
     Diff,
     Mention,
     Status,
@@ -97,6 +99,9 @@ impl SlashCommand {
             SlashCommand::Quit | SlashCommand::Exit => "exit Codex",
             SlashCommand::Copy => "copy last response as markdown",
             SlashCommand::Raw => "toggle raw scrollback mode for copy-friendly terminal selection",
+            SlashCommand::Lcs => {
+                "toggle LCS substrate visibility (MCP tool calls): /lcs hidden|visible"
+            }
             SlashCommand::Diff => "show git diff (including untracked files)",
             SlashCommand::Mention => "mention a file",
             SlashCommand::Skills => "use skills to improve how Codex performs specific tasks",
@@ -163,6 +168,7 @@ impl SlashCommand {
                 | SlashCommand::Keymap
                 | SlashCommand::Mcp
                 | SlashCommand::Raw
+                | SlashCommand::Lcs
                 | SlashCommand::Pets
                 | SlashCommand::Side
                 | SlashCommand::Btw
@@ -177,6 +183,7 @@ impl SlashCommand {
             self,
             SlashCommand::Copy
                 | SlashCommand::Raw
+                | SlashCommand::Lcs
                 | SlashCommand::Diff
                 | SlashCommand::Mention
                 | SlashCommand::Status
@@ -211,6 +218,7 @@ impl SlashCommand {
             SlashCommand::Diff
             | SlashCommand::Copy
             | SlashCommand::Raw
+            | SlashCommand::Lcs
             | SlashCommand::Rename
             | SlashCommand::Mention
             | SlashCommand::Skills