Proposal: Add a /reply command (#2680)

* Add /reply command to quote last assistant message in editor

- Made open_editor public in editor.rs for reuse
- Added ReplyArgs and reply module to CLI structure
- Quotes most recent assistant message with > prefixes
- Handles edge cases: no message found, empty content
- Provides colored user feedback throughout flow

* Add /reply to COMMANDS array for autocompletion

Fixes missing tab completion for the /reply command by adding it
to the COMMANDS list used by the rustyline completer.

* Fix /reply to detect unchanged content

When user opens editor with quoted message and quits without changes,
detect that content is unchanged from initial text and don't submit.
Updates message to 'No changes made in editor' for clarity.

* missing docs

Author: nikomatsakis

crates/chat-cli/src/cli/chat/cli/mod.rs

@@ -12,6 +12,7 @@ pub mod model;
 pub mod persist;
 pub mod profile;
 pub mod prompts;
+pub mod reply;
 pub mod subscribe;
 pub mod tangent;
 pub mod todos;
@@ -32,6 +33,7 @@ use model::ModelArgs;
 use persist::PersistSubcommand;
 use profile::AgentSubcommand;
 use prompts::PromptsArgs;
+use reply::ReplyArgs;
 use tangent::TangentArgs;
 use todos::TodoSubcommand;
 use tools::ToolsArgs;
@@ -73,6 +75,8 @@ pub enum SlashCommand {
     /// Open $EDITOR (defaults to vi) to compose a prompt
     #[command(name = "editor")]
     PromptEditor(EditorArgs),
+    /// Open $EDITOR with the most recent assistant message quoted for reply
+    Reply(ReplyArgs),
     /// Summarize the conversation to free up context space
     Compact(CompactArgs),
     /// View tools and permissions
@@ -143,6 +147,7 @@ impl SlashCommand {
             Self::Context(args) => args.execute(os, session).await,
             Self::Knowledge(subcommand) => subcommand.execute(os, session).await,
             Self::PromptEditor(args) => args.execute(session).await,
+            Self::Reply(args) => args.execute(session).await,
             Self::Compact(args) => args.execute(os, session).await,
             Self::Tools(args) => args.execute(session).await,
             Self::Issue(args) => {
@@ -187,6 +192,7 @@ impl SlashCommand {
             Self::Context(_) => "context",
             Self::Knowledge(_) => "knowledge",
             Self::PromptEditor(_) => "editor",
+            Self::Reply(_) => "reply",
             Self::Compact(_) => "compact",
             Self::Tools(_) => "tools",
             Self::Issue(_) => "issue",

crates/chat-cli/src/cli/chat/cli/reply.rs

@@ -0,0 +1,101 @@
+use clap::Args;
+use crossterm::execute;
+use crossterm::style::{self, Color};
+
+use super::editor::open_editor;
+use crate::cli::chat::{ChatError, ChatSession, ChatState};
+
+/// Arguments to the `/reply` command.
+#[deny(missing_docs)]
+#[derive(Debug, PartialEq, Args)]
+pub struct ReplyArgs {}
+
+impl ReplyArgs {
+    pub async fn execute(self, session: &mut ChatSession) -> Result<ChatState, ChatError> {
+        // Get the most recent assistant message from transcript
+        let last_assistant_message = session
+            .conversation
+            .transcript
+            .iter()
+            .rev()
+            .find(|msg| !msg.starts_with("> "))
+            .cloned();
+
+        let initial_text = match last_assistant_message {
+            Some(msg) => {
+                // Format with > prefix for each line
+                msg.lines()
+                    .map(|line| format!("> {}", line))
+                    .collect::<Vec<_>>()
+                    .join("\n")
+            },
+            None => {
+                execute!(
+                    session.stderr,
+                    style::SetForegroundColor(Color::Yellow),
+                    style::Print("\nNo assistant message found to reply to.\n\n"),
+                    style::SetForegroundColor(Color::Reset)
+                )?;
+
+                return Ok(ChatState::PromptUser {
+                    skip_printing_tools: true,
+                });
+            },
+        };
+
+        let content = match open_editor(Some(initial_text.clone())) {
+            Ok(content) => content,
+            Err(err) => {
+                execute!(
+                    session.stderr,
+                    style::SetForegroundColor(Color::Red),
+                    style::Print(format!("\nError opening editor: {}\n\n", err)),
+                    style::SetForegroundColor(Color::Reset)
+                )?;
+
+                return Ok(ChatState::PromptUser {
+                    skip_printing_tools: true,
+                });
+            },
+        };
+
+        Ok(
+            match content.trim().is_empty() || content.trim() == initial_text.trim() {
+                true => {
+                    execute!(
+                        session.stderr,
+                        style::SetForegroundColor(Color::Yellow),
+                        style::Print("\nNo changes made in editor, not submitting.\n\n"),
+                        style::SetForegroundColor(Color::Reset)
+                    )?;
+
+                    ChatState::PromptUser {
+                        skip_printing_tools: true,
+                    }
+                },
+                false => {
+                    execute!(
+                        session.stderr,
+                        style::SetForegroundColor(Color::Green),
+                        style::Print("\nContent loaded from editor. Submitting prompt...\n\n"),
+                        style::SetForegroundColor(Color::Reset)
+                    )?;
+
+                    // Display the content as if the user typed it
+                    execute!(
+                        session.stderr,
+                        style::SetAttribute(style::Attribute::Reset),
+                        style::SetForegroundColor(Color::Magenta),
+                        style::Print("> "),
+                        style::SetAttribute(style::Attribute::Reset),
+                        style::Print(&content),
+                        style::Print("\n")
+                    )?;
+
+                    // Process the content as user input
+                    ChatState::HandleInput { input: content }
+                },
+            },
+        )
+    }
+}

crates/chat-cli/src/cli/chat/prompt.rs

@@ -54,6 +54,7 @@ pub const COMMANDS: &[&str] = &[
     "/clear",
     "/help",
     "/editor",
+    "/reply",
     "/issue",
     "/quit",
     "/tools",