diff --git a/src/cortex-agents/src/mention.rs b/src/cortex-agents/src/mention.rs
index 81d9d71c..59bb9550 100644
--- a/src/cortex-agents/src/mention.rs
+++ b/src/cortex-agents/src/mention.rs
@@ -17,6 +17,46 @@
 use regex::Regex;
 use std::sync::LazyLock;
 
+/// Safely get the string slice up to the given byte position.
+///
+/// Returns the slice `&text[..pos]` if `pos` is at a valid UTF-8 character boundary.
+/// If `pos` is inside a multi-byte character, finds the nearest valid boundary
+/// by searching backwards.
+fn safe_slice_up_to(text: &str, pos: usize) -> &str {
+    if pos >= text.len() {
+        return text;
+    }
+    if text.is_char_boundary(pos) {
+        return &text[..pos];
+    }
+    // Find the nearest valid boundary by searching backwards
+    let mut valid_pos = pos;
+    while valid_pos > 0 && !text.is_char_boundary(valid_pos) {
+        valid_pos -= 1;
+    }
+    &text[..valid_pos]
+}
+
+/// Safely get the string slice from the given byte position to the end.
+///
+/// Returns the slice `&text[pos..]` if `pos` is at a valid UTF-8 character boundary.
+/// If `pos` is inside a multi-byte character, finds the nearest valid boundary
+/// by searching forwards.
+fn safe_slice_from(text: &str, pos: usize) -> &str {
+    if pos >= text.len() {
+        return "";
+    }
+    if text.is_char_boundary(pos) {
+        return &text[pos..];
+    }
+    // Find the nearest valid boundary by searching forwards
+    let mut valid_pos = pos;
+    while valid_pos < text.len() && !text.is_char_boundary(valid_pos) {
+        valid_pos += 1;
+    }
+    &text[valid_pos..]
+}
+
 /// A parsed agent mention from user input.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct AgentMention {
@@ -108,10 +148,10 @@ pub fn extract_mention_and_text(
 ) -> Option<(AgentMention, String)> {
     let mention = find_first_valid_mention(text, valid_agents)?;
 
-    // Remove the mention from text
+    // Remove the mention from text, using safe slicing for UTF-8 boundaries
     let mut remaining = String::with_capacity(text.len());
-    remaining.push_str(&text[..mention.start]);
-    remaining.push_str(&text[mention.end..]);
+    remaining.push_str(safe_slice_up_to(text, mention.start));
+    remaining.push_str(safe_slice_from(text, mention.end));
 
     // Trim and normalize whitespace
     let remaining = remaining.trim().to_string();
@@ -123,7 +163,8 @@ pub fn extract_mention_and_text(
 pub fn starts_with_mention(text: &str, valid_agents: &[&str]) -> bool {
     let text = text.trim();
     if let Some(mention) = find_first_valid_mention(text, valid_agents) {
-        mention.start == 0 || text[..mention.start].trim().is_empty()
+        // Use safe slicing to handle UTF-8 boundaries
+        mention.start == 0 || safe_slice_up_to(text, mention.start).trim().is_empty()
     } else {
         false
     }
@@ -196,8 +237,8 @@ pub fn parse_message_for_agent(text: &str, valid_agents: &[&str]) -> ParsedAgent
 
     // Check if message starts with @agent
     if let Some((mention, remaining)) = extract_mention_and_text(text, valid_agents) {
-        // Only trigger if mention is at the start
-        if mention.start == 0 || text[..mention.start].trim().is_empty() {
+        // Only trigger if mention is at the start, using safe slicing for UTF-8 boundaries
+        if mention.start == 0 || safe_slice_up_to(text, mention.start).trim().is_empty() {
             return ParsedAgentMessage::for_agent(mention.agent_name, remaining, text.to_string());
         }
     }
@@ -318,4 +359,99 @@ mod tests {
         assert_eq!(mentions[0].agent_name, "my-agent");
         assert_eq!(mentions[1].agent_name, "my_agent");
     }
+
+    // UTF-8 boundary safety tests
+    #[test]
+    fn test_safe_slice_up_to_ascii() {
+        let text = "hello world";
+        assert_eq!(safe_slice_up_to(text, 5), "hello");
+        assert_eq!(safe_slice_up_to(text, 0), "");
+        assert_eq!(safe_slice_up_to(text, 100), "hello world");
+    }
+
+    #[test]
+    fn test_safe_slice_up_to_multibyte() {
+        // "こんにちは" - each character is 3 bytes
+        let text = "こんにちは";
+        assert_eq!(safe_slice_up_to(text, 3), "こ"); // Valid boundary
+        assert_eq!(safe_slice_up_to(text, 6), "こん"); // Valid boundary
+                                                       // Position 4 is inside the second character, should return "こ"
+        assert_eq!(safe_slice_up_to(text, 4), "こ");
+        assert_eq!(safe_slice_up_to(text, 5), "こ");
+    }
+
+    #[test]
+    fn test_safe_slice_from_multibyte() {
+        let text = "こんにちは";
+        assert_eq!(safe_slice_from(text, 3), "んにちは"); // Valid boundary
+                                                          // Position 4 is inside second character, should skip to position 6
+        assert_eq!(safe_slice_from(text, 4), "にちは");
+        assert_eq!(safe_slice_from(text, 5), "にちは");
+    }
+
+    #[test]
+    fn test_extract_mention_with_multibyte_prefix() {
+        let valid = vec!["general"];
+
+        // Multi-byte characters before mention
+        let result = extract_mention_and_text("日本語 @general search files", &valid);
+        assert!(result.is_some());
+        let (mention, remaining) = result.unwrap();
+        assert_eq!(mention.agent_name, "general");
+        // The prefix should be preserved without panicking
+        assert!(remaining.contains("search files"));
+    }
+
+    #[test]
+    fn test_starts_with_mention_multibyte() {
+        let valid = vec!["general"];
+
+        // Whitespace with multi-byte characters should not cause panic
+        assert!(starts_with_mention("  @general task", &valid));
+
+        // Multi-byte characters before mention - should return false, not panic
+        assert!(!starts_with_mention("日本語 @general task", &valid));
+    }
+
+    #[test]
+    fn test_parse_message_for_agent_multibyte() {
+        let valid = vec!["general"];
+
+        // Multi-byte prefix - should not panic
+        let parsed = parse_message_for_agent("日本語 @general find files", &valid);
+        // Since mention is not at the start, should not invoke task
+        assert!(!parsed.should_invoke_task);
+
+        // Multi-byte in the prompt (after mention)
+        let parsed = parse_message_for_agent("@general 日本語を検索", &valid);
+        assert!(parsed.should_invoke_task);
+        assert_eq!(parsed.agent, Some("general".to_string()));
+        assert_eq!(parsed.prompt, "日本語を検索");
+    }
+
+    #[test]
+    fn test_extract_mention_with_emoji() {
+        let valid = vec!["general"];
+
+        // Emojis are 4 bytes each
+        let result = extract_mention_and_text("🎉 @general celebrate", &valid);
+        assert!(result.is_some());
+        let (mention, remaining) = result.unwrap();
+        assert_eq!(mention.agent_name, "general");
+        assert!(remaining.contains("celebrate"));
+    }
+
+    #[test]
+    fn test_mixed_multibyte_and_ascii() {
+        let valid = vec!["general"];
+
+        // Mix of ASCII, CJK, and emoji
+        let text = "Hello 世界 🌍 @general search for 日本語";
+        let result = extract_mention_and_text(text, &valid);
+        assert!(result.is_some());
+        let (mention, remaining) = result.unwrap();
+        assert_eq!(mention.agent_name, "general");
+        // Should not panic and produce valid output
+        assert!(!remaining.is_empty());
+    }
 }
diff --git a/src/cortex-cli/src/import_cmd.rs b/src/cortex-cli/src/import_cmd.rs
index 696d93ae..38b25f86 100644
--- a/src/cortex-cli/src/import_cmd.rs
+++ b/src/cortex-cli/src/import_cmd.rs
@@ -357,31 +357,47 @@ fn validate_export_messages(messages: &[ExportMessage]) -> Result<()> {
     for (idx, message) in messages.iter().enumerate() {
         // Check for base64-encoded image data in content
         // Common pattern: "data:image/png;base64,..." or "data:image/jpeg;base64,..."
-        if let Some(data_uri_start) = message.content.find("data:image/")
-            && let Some(base64_marker) = message.content[data_uri_start..].find(";base64,")
-        {
-            let base64_start = data_uri_start + base64_marker + 8; // 8 = len(";base64,")
-            let remaining = &message.content[base64_start..];
-
-            // Find end of base64 data (could end with quote, whitespace, or end of string)
-            let base64_end = remaining
-                .find(['"', '\'', ' ', '\n', ')'])
-                .unwrap_or(remaining.len());
-            let base64_data = &remaining[..base64_end];
-
-            // Validate the base64 data
-            if !base64_data.is_empty() {
-                let engine = base64::engine::general_purpose::STANDARD;
-                if let Err(e) = engine.decode(base64_data) {
-                    bail!(
-                        "Invalid base64 encoding in message {} (role: '{}'): {}\n\
-                            The image data starting at position {} has invalid base64 encoding.\n\
-                            Please ensure all embedded images use valid base64 encoding.",
-                        idx + 1,
-                        message.role,
-                        e,
-                        data_uri_start
-                    );
+        if let Some(data_uri_start) = message.content.find("data:image/") {
+            // Use safe slicing with .get() to avoid panics on multi-byte UTF-8 boundaries
+            let content_after_start = match message.content.get(data_uri_start..) {
+                Some(s) => s,
+                None => continue, // Invalid byte offset, skip this message
+            };
+
+            if let Some(base64_marker) = content_after_start.find(";base64,") {
+                let base64_start = data_uri_start + base64_marker + 8; // 8 = len(";base64,")
+
+                // Safe slicing for the remaining content after base64 marker
+                let remaining = match message.content.get(base64_start..) {
+                    Some(s) => s,
+                    None => continue, // Invalid byte offset, skip this message
+                };
+
+                // Find end of base64 data (could end with quote, whitespace, or end of string)
+                let base64_end = remaining
+                    .find(['"', '\'', ' ', '\n', ')'])
+                    .unwrap_or(remaining.len());
+
+                // Safe slicing for the base64 data
+                let base64_data = match remaining.get(..base64_end) {
+                    Some(s) => s,
+                    None => continue, // Invalid byte offset, skip this message
+                };
+
+                // Validate the base64 data
+                if !base64_data.is_empty() {
+                    let engine = base64::engine::general_purpose::STANDARD;
+                    if let Err(e) = engine.decode(base64_data) {
+                        bail!(
+                            "Invalid base64 encoding in message {} (role: '{}'): {}\n\
+                                The image data starting at position {} has invalid base64 encoding.\n\
+                                Please ensure all embedded images use valid base64 encoding.",
+                            idx + 1,
+                            message.role,
+                            e,
+                            data_uri_start
+                        );
+                    }
                 }
             }
         }
@@ -395,13 +411,24 @@ fn validate_export_messages(messages: &[ExportMessage]) -> Result<()> {
                     // Try to find and validate any base64 in the arguments
                     for (pos, _) in args_str.match_indices(";base64,") {
                         let base64_start = pos + 8;
-                        let remaining = &args_str[base64_start..];
+
+                        // Safe slicing for the remaining content after base64 marker
+                        let remaining = match args_str.get(base64_start..) {
+                            Some(s) => s,
+                            None => continue, // Invalid byte offset, skip this occurrence
+                        };
+
                         let base64_end = remaining
                             .find(|c: char| {
                                 c == '"' || c == '\'' || c == ' ' || c == '\n' || c == ')'
                             })
                             .unwrap_or(remaining.len());
-                        let base64_data = &remaining[..base64_end];
+
+                        // Safe slicing for the base64 data
+                        let base64_data = match remaining.get(..base64_end) {
+                            Some(s) => s,
+                            None => continue, // Invalid byte offset, skip this occurrence
+                        };
 
                         if !base64_data.is_empty() {
                             let engine = base64::engine::general_purpose::STANDARD;
diff --git a/src/cortex-cli/src/lock_cmd.rs b/src/cortex-cli/src/lock_cmd.rs
index dc652cad..1caa3d3c 100644
--- a/src/cortex-cli/src/lock_cmd.rs
+++ b/src/cortex-cli/src/lock_cmd.rs
@@ -114,6 +114,15 @@ fn validate_session_id(session_id: &str) -> Result<()> {
     )
 }
 
+/// Safely get a string prefix by character count, not byte count.
+/// This avoids panics on multi-byte UTF-8 characters.
+fn safe_char_prefix(s: &str, max_chars: usize) -> &str {
+    match s.char_indices().nth(max_chars) {
+        Some((byte_idx, _)) => &s[..byte_idx],
+        None => s, // String has fewer than max_chars characters
+    }
+}
+
 /// Get the lock file path.
 fn get_lock_file_path() -> PathBuf {
     dirs::home_dir()
@@ -156,7 +165,7 @@ pub fn is_session_locked(session_id: &str) -> bool {
     match load_lock_file() {
         Ok(lock_file) => lock_file.locked_sessions.iter().any(|entry| {
             entry.session_id == session_id
-                || session_id.starts_with(&entry.session_id[..8.min(entry.session_id.len())])
+                || session_id.starts_with(safe_char_prefix(&entry.session_id, 8))
         }),
         Err(_) => false,
     }
@@ -308,7 +317,7 @@ async fn run_list(args: LockListArgs) -> Result<()> {
         println!("{}", "-".repeat(60));
 
         for entry in &lock_file.locked_sessions {
-            let short_id = &entry.session_id[..8.min(entry.session_id.len())];
+            let short_id = safe_char_prefix(&entry.session_id, 8);
             println!("  {} - locked at {}", short_id, entry.locked_at);
             if let Some(ref reason) = entry.reason {
                 println!("    Reason: {}", reason);
@@ -332,7 +341,7 @@ async fn run_check(args: LockCheckArgs) -> Result<()> {
         e.session_id == args.session_id
             || args
                 .session_id
-                .starts_with(&e.session_id[..8.min(e.session_id.len())])
+                .starts_with(safe_char_prefix(&e.session_id, 8))
     });
 
     if is_locked {
@@ -342,7 +351,7 @@ async fn run_check(args: LockCheckArgs) -> Result<()> {
             e.session_id == args.session_id
                 || args
                     .session_id
-                    .starts_with(&e.session_id[..8.min(e.session_id.len())])
+                    .starts_with(safe_char_prefix(&e.session_id, 8))
         }) && let Some(ref reason) = entry.reason
         {
             println!("Reason: {}", reason);
@@ -508,4 +517,39 @@ mod tests {
         let path_str = path.to_string_lossy();
         assert!(path_str.contains(".cortex"));
     }
+
+    #[test]
+    fn test_safe_char_prefix_ascii() {
+        // ASCII strings should work correctly
+        assert_eq!(safe_char_prefix("abcdefghij", 8), "abcdefgh");
+        assert_eq!(safe_char_prefix("abc", 8), "abc");
+        assert_eq!(safe_char_prefix("", 8), "");
+        assert_eq!(safe_char_prefix("12345678", 8), "12345678");
+    }
+
+    #[test]
+    fn test_safe_char_prefix_utf8_multibyte() {
+        // Multi-byte UTF-8 characters should not panic
+        // Each emoji is 4 bytes, so 8 chars = 32 bytes
+        let emoji_id = "🔥🎉🚀💡🌟✨🎯🔮extra";
+        assert_eq!(safe_char_prefix(emoji_id, 8), "🔥🎉🚀💡🌟✨🎯🔮");
+
+        // Mixed ASCII and multi-byte
+        let mixed = "ab🔥cd🎉ef";
+        assert_eq!(safe_char_prefix(mixed, 4), "ab🔥c");
+        assert_eq!(safe_char_prefix(mixed, 8), "ab🔥cd🎉ef");
+
+        // Chinese characters (3 bytes each)
+        let chinese = "中文测试会话标识符";
+        assert_eq!(safe_char_prefix(chinese, 4), "中文测试");
+    }
+
+    #[test]
+    fn test_safe_char_prefix_boundary() {
+        // Edge cases
+        assert_eq!(safe_char_prefix("a", 0), "");
+        assert_eq!(safe_char_prefix("a", 1), "a");
+        assert_eq!(safe_char_prefix("🔥", 1), "🔥");
+        assert_eq!(safe_char_prefix("🔥", 0), "");
+    }
 }
diff --git a/src/cortex-cli/src/utils/notification.rs b/src/cortex-cli/src/utils/notification.rs
index 4656e223..8edd2c93 100644
--- a/src/cortex-cli/src/utils/notification.rs
+++ b/src/cortex-cli/src/utils/notification.rs
@@ -63,7 +63,14 @@ pub fn send_task_notification(session_id: &str, success: bool) -> Result<()> {
         "Cortex Task Failed"
     };
 
-    let short_id = &session_id[..8.min(session_id.len())];
+    // Use safe UTF-8 slicing - find the last valid char boundary at or before position 8
+    let short_id = session_id
+        .char_indices()
+        .take_while(|(idx, _)| *idx < 8)
+        .map(|(idx, ch)| idx + ch.len_utf8())
+        .last()
+        .and_then(|end| session_id.get(..end))
+        .unwrap_or(session_id);
     let body = format!("Session: {}", short_id);
 
     let urgency = if success {
diff --git a/src/cortex-resume/src/resume_picker.rs b/src/cortex-resume/src/resume_picker.rs
index 9bf8832a..7b0ee9a6 100644
--- a/src/cortex-resume/src/resume_picker.rs
+++ b/src/cortex-resume/src/resume_picker.rs
@@ -153,12 +153,15 @@ fn format_relative_time(time: &chrono::DateTime<chrono::Utc>) -> String {
     }
 }
 
-/// Truncate string to fit width.
+/// Truncate string to fit width, handling multi-byte UTF-8 safely.
 fn truncate_string(s: &str, width: usize) -> String {
-    if s.len() <= width {
+    // Count actual character width, not byte length
+    let char_count = s.chars().count();
+    if char_count <= width {
         s.to_string()
     } else if width > 3 {
-        format!("{}...", &s[..width - 3])
+        let truncated: String = s.chars().take(width - 3).collect();
+        format!("{}...", truncated)
     } else {
         s.chars().take(width).collect()
     }
@@ -176,4 +179,40 @@ mod tests {
         let hour_ago = now - chrono::Duration::hours(2);
         assert_eq!(format_relative_time(&hour_ago), "2h ago");
     }
+
+    #[test]
+    fn test_truncate_string_ascii() {
+        // Short string, no truncation needed
+        assert_eq!(truncate_string("hello", 10), "hello");
+
+        // Exact fit
+        assert_eq!(truncate_string("hello", 5), "hello");
+
+        // Needs truncation
+        assert_eq!(truncate_string("hello world", 8), "hello...");
+
+        // Very short width
+        assert_eq!(truncate_string("hello", 3), "hel");
+        assert_eq!(truncate_string("hello", 2), "he");
+    }
+
+    #[test]
+    fn test_truncate_string_utf8() {
+        // UTF-8 multi-byte characters (Japanese)
+        let japanese = "こんにちは世界"; // 7 chars
+        assert_eq!(truncate_string(japanese, 10), japanese); // No truncation
+        assert_eq!(truncate_string(japanese, 7), japanese); // Exact fit
+        assert_eq!(truncate_string(japanese, 6), "こんに..."); // Truncated (3 chars + ...)
+
+        // UTF-8 with emoji
+        let emoji = "Hello 🌍🌎🌏"; // 9 chars: H,e,l,l,o, ,🌍,🌎,🌏
+        assert_eq!(truncate_string(emoji, 20), emoji); // No truncation
+        assert_eq!(truncate_string(emoji, 9), emoji); // Exact fit (9 chars)
+        assert_eq!(truncate_string(emoji, 8), "Hello..."); // Truncated (5 chars + ...)
+
+        // Mixed UTF-8 and ASCII
+        let mixed = "路径/path/文件"; // 11 chars
+        assert_eq!(truncate_string(mixed, 20), mixed); // No truncation
+        assert_eq!(truncate_string(mixed, 8), "路径/pa..."); // Truncated
+    }
 }