fix: prevent Discord message fragmentation during streaming (fixes #81)

src/discord.rs

@@ -217,31 +217,25 @@ async fn stream_prompt(
                 text_buf.push_str("⚠️ _Session expired, starting fresh..._\n\n");
             }
 
-            // Spawn edit-streaming task
+            // Spawn edit-streaming task — only edits the single message, never sends new ones.
+            // Long content is truncated during streaming; final multi-message split happens after.
             let edit_handle = {
                 let ctx = ctx.clone();
                 let mut buf_rx = buf_rx.clone();
                 tokio::spawn(async move {
                     let mut last_content = String::new();
-                    let mut current_edit_msg = msg_id;
                     loop {
                         tokio::time::sleep(std::time::Duration::from_millis(1500)).await;
                         if buf_rx.has_changed().unwrap_or(false) {
                             let content = buf_rx.borrow_and_update().clone();
                             if content != last_content {
-                                if content.len() > 1900 {
-                                    let chunks = format::split_message(&content, 1900);
-                                    if let Some(first) = chunks.first() {
-                                        let _ = edit(&ctx, channel, current_edit_msg, first).await;
-                                    }
-                                    for chunk in chunks.iter().skip(1) {
-                                        if let Ok(new_msg) = channel.say(&ctx.http, chunk).await {
-                                            current_edit_msg = new_msg.id;
-                                        }
-                                    }
+                                let display = if content.chars().count() > 1900 {
+                                    let truncated = format::truncate_chars(&content, 1900);
+                                    format!("{truncated}…")
                                 } else {
-                                    let _ = edit(&ctx, channel, current_edit_msg, &content).await;
-                                }
+                                    content.clone()
+                                };
+                                let _ = edit(&ctx, channel, msg_id, &display).await;
                                 last_content = content;
                             }
                         }

src/format.rs

@@ -1,35 +1,53 @@
-/// Split text into chunks at line boundaries, each <= limit chars.
+/// Split text into chunks at line boundaries, each <= limit Unicode characters (UTF-8 safe).
+/// Discord's message limit counts Unicode characters, not bytes.
 pub fn split_message(text: &str, limit: usize) -> Vec<String> {
-    if text.len() <= limit {
+    if text.chars().count() <= limit {
         return vec![text.to_string()];
     }
 
     let mut chunks = Vec::new();
     let mut current = String::new();
+    let mut current_len: usize = 0;
 
     for line in text.split('\n') {
+        let line_chars = line.chars().count();
         // +1 for the newline
-        if !current.is_empty() && current.len() + line.len() + 1 > limit {
+        if !current.is_empty() && current_len + line_chars + 1 > limit {
             chunks.push(current);
             current = String::new();
+            current_len = 0;
         }
         if !current.is_empty() {
             current.push('\n');
+            current_len += 1;
         }
-        // If a single line exceeds limit, hard-split it
-        if line.len() > limit {
-            for chunk in line.as_bytes().chunks(limit) {
-                if !current.is_empty() {
+        // If a single line exceeds limit, hard-split on char boundaries
+        if line_chars > limit {
+            for ch in line.chars() {
+                if current_len + 1 > limit {
                     chunks.push(current);
+                    current = String::new();
+                    current_len = 0;
                 }
-                current = String::from_utf8_lossy(chunk).to_string();
+                current.push(ch);
+                current_len += 1;
             }
         } else {
             current.push_str(line);
+            current_len += line_chars;
         }
     }
     if !current.is_empty() {
         chunks.push(current);
     }
     chunks
 }
+
+/// Truncate a string to at most `limit` Unicode characters.
+/// Discord's message limit counts Unicode characters, not bytes.
+pub fn truncate_chars(s: &str, limit: usize) -> &str {
+    match s.char_indices().nth(limit) {
+        Some((idx, _)) => &s[..idx],
+        None => s,
+    }
+}