diff --git a/codex-rs/tui/src/bottom_pane/chat_composer.rs b/codex-rs/tui/src/bottom_pane/chat_composer.rs
index 30727c298b..6b42e5134c 100644
--- a/codex-rs/tui/src/bottom_pane/chat_composer.rs
+++ b/codex-rs/tui/src/bottom_pane/chat_composer.rs
@@ -954,21 +954,17 @@ impl ChatComposer {
                         return (InputResult::None, true);
                     }
                 }
-                // If we have pending placeholder pastes, submit immediately to expand them.
+                // If we have pending placeholder pastes, replace them in the textarea text
+                // and continue to the normal submission flow to handle slash commands.
                 if !self.pending_pastes.is_empty() {
                     let mut text = self.textarea.text().to_string();
-                    self.textarea.set_text("");
                     for (placeholder, actual) in &self.pending_pastes {
                         if text.contains(placeholder) {
                             text = text.replace(placeholder, actual);
                         }
                     }
+                    self.textarea.set_text(&text);
                     self.pending_pastes.clear();
-                    if text.is_empty() {
-                        return (InputResult::None, true);
-                    }
-                    self.history.record_local_submission(&text);
-                    return (InputResult::Submitted(text), true);
                 }
 
                 // During a paste-like burst, treat Enter as a newline instead of submit.
@@ -3030,6 +3026,70 @@ mod tests {
         assert!(composer.textarea.is_empty());
     }
 
+    #[test]
+    fn custom_prompt_with_large_paste_expands_correctly() {
+        use crossterm::event::KeyCode;
+        use crossterm::event::KeyEvent;
+        use crossterm::event::KeyModifiers;
+
+        let (tx, _rx) = unbounded_channel::<AppEvent>();
+        let sender = AppEventSender::new(tx);
+        let mut composer = ChatComposer::new(
+            true,
+            sender,
+            false,
+            "Ask Codex to do anything".to_string(),
+            false,
+        );
+
+        // Create a custom prompt with positional args (no named args like $USER)
+        composer.set_custom_prompts(vec![CustomPrompt {
+            name: "code-review".to_string(),
+            path: "/tmp/code-review.md".to_string().into(),
+            content: "Please review the following code:\n\n$1".to_string(),
+            description: None,
+            argument_hint: None,
+        }]);
+
+        // Type the slash command
+        let command_text = "/prompts:code-review ";
+        composer.textarea.set_text(command_text);
+        composer.textarea.set_cursor(command_text.len());
+
+        // Paste large content (>3000 chars) to trigger placeholder
+        let large_content = "x".repeat(LARGE_PASTE_CHAR_THRESHOLD + 3000);
+        composer.handle_paste(large_content.clone());
+
+        // Verify placeholder was created
+        let placeholder = format!("[Pasted Content {} chars]", large_content.chars().count());
+        assert_eq!(
+            composer.textarea.text(),
+            format!("/prompts:code-review {}", placeholder)
+        );
+        assert_eq!(composer.pending_pastes.len(), 1);
+        assert_eq!(composer.pending_pastes[0].0, placeholder);
+        assert_eq!(composer.pending_pastes[0].1, large_content);
+
+        // Submit by pressing Enter
+        let (result, _needs_redraw) =
+            composer.handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+        // Verify the custom prompt was expanded with the large content as positional arg
+        match result {
+            InputResult::Submitted(text) => {
+                // The prompt should be expanded, with the large content replacing $1
+                assert_eq!(
+                    text,
+                    format!("Please review the following code:\n\n{}", large_content),
+                    "Expected prompt expansion with large content as $1"
+                );
+            }
+            _ => panic!("expected Submitted, got: {result:?}"),
+        }
+        assert!(composer.textarea.is_empty());
+        assert!(composer.pending_pastes.is_empty());
+    }
+
     #[test]
     fn slash_path_input_submits_without_command_error() {
         use crossterm::event::KeyCode;