feat: add /undo to remove the latest exchange

docs/slash-commands.md

@@ -13,6 +13,7 @@
 | `/export [path]`       | Write a Markdown transcript        |
 | `/retry`               | Retry a failed or cancelled reply  |
 | `/regenerate`          | Replace the latest assistant reply |
+| `/undo`                | Remove the latest exchange         |
 | `/theme <relay\|mono>` | Change the terminal palette        |
 | `/stats`               | Show message and token estimates   |
 | `/quit`                | Close the interface                |

src/commands/catalog.ts

@@ -16,6 +16,7 @@ export const commandCatalog: readonly CommandDescription[] = [
   {name: 'export', usage: '/export [path]', description: 'Export the conversation as Markdown.'},
   {name: 'retry', usage: '/retry', description: 'Retry the last failed or cancelled reply.'},
   {name: 'regenerate', usage: '/regenerate', description: 'Replace the latest assistant reply.'},
+  {name: 'undo', usage: '/undo', description: 'Remove the latest message and its reply.'},
   {name: 'theme', usage: '/theme <relay|mono>', description: 'Change the terminal palette.'},
   {name: 'stats', usage: '/stats', description: 'Show message and token estimates.'},
   {name: 'quit', usage: '/quit', description: 'Close Relay Chat.'},

src/commands/execute.ts

@@ -95,6 +95,10 @@ export const executeSlashCommand = async (
     case 'regenerate':
       await session.regenerate();
       return;
+    case 'undo':
+      session.undo();
+      store.setNotice('Removed the latest message and its reply.');
+      return;
     case 'theme': {
       const name = chosenPath(command);
       if (name === undefined || !chatThemes.has(name))

src/session/chat-session.ts

@@ -66,6 +66,14 @@ export class ChatSession {
     await this.regenerate();
   }
 
+  undo(): string {
+    if (this.isStreaming) throw new Error('Cancel the active reply before undoing a turn.');
+    const message = lastUserMessage(this.#store.getSnapshot().conversation);
+    if (message === undefined) throw new Error('There is no user message to undo.');
+    this.#store.truncateFrom(message.id);
+    return message.content;
+  }
+
   async editAndResend(userMessageId: string, content: string): Promise<void> {
     if (this.isStreaming) throw new Error('A reply is already streaming.');
     const message = this.#store

tests/session.test.ts

@@ -20,6 +20,27 @@ describe('ChatSession', () => {
     expect(store.getSnapshot().conversation.title).toBe('question');
   });
 
+  it('undoes the latest user turn and its reply', async () => {
+    const store = new ChatStore(createConversation({id: 'c1'}));
+    const session = new ChatSession(
+      store,
+      new EchoTransport({
+        delayMs: 0,
+        response: (request) => `re: ${String(request.messages.length)}`,
+      }),
+    );
+    await session.send('first');
+    await session.send('second');
+    expect(session.undo()).toBe('second');
+    expect(store.getSnapshot().conversation.messages.map(({content}) => content)).toEqual([
+      'first',
+      're: 1',
+    ]);
+    session.undo();
+    expect(store.getSnapshot().conversation.messages).toEqual([]);
+    expect(() => session.undo()).toThrow('There is no user message to undo.');
+  });
+
   it('regenerates without duplicating the user turn', async () => {
     let count = 0;
     const store = new ChatStore(createConversation({id: 'c1'}));