fix(ui_terminal): truncate by display width, not byte index

crates/ui_terminal/src/lib.rs

@@ -10,6 +10,7 @@ pub mod slash_popup;
 pub mod state;
 pub mod streaming;
 pub mod terminal_color;
+pub mod text_util;
 pub mod textarea;
 pub mod tool_renderers;
 pub mod tool_widget;

crates/ui_terminal/src/message.rs

@@ -3,6 +3,7 @@ use ratatui::prelude::*;
 use ratatui::widgets::{Paragraph, Wrap};
 use tui_markdown as md;
 
+use super::text_util::truncate_with_ellipsis;
 use super::tool_renderers::ToolRendererRegistry;
 use super::tool_widget::{is_full_width_parameter, should_hide_parameter, ToolWidget};
 use code_assistant_core::ui::ToolStatus;
@@ -355,10 +356,6 @@ impl ParameterValue {
 
     pub fn get_display_value(&self) -> String {
         // Truncate long values for regular parameters
-        if self.value.len() > 100 {
-            format!("{}...", &self.value[..97])
-        } else {
-            self.value.clone()
-        }
+        truncate_with_ellipsis(&self.value, 100).into_owned()
     }
 }

crates/ui_terminal/src/text_util.rs

@@ -0,0 +1,79 @@
+//! Text helpers shared by the widget renderers.
+//!
+//! Terminal rendering constantly needs "cut this to N columns". Doing that
+//! with byte slices panics on multi-byte input (`&s[..n]` inside a `ü`), so
+//! all truncation goes through these helpers.
+
+use std::borrow::Cow;
+use unicode_width::{UnicodeWidthChar, UnicodeWidthStr};
+
+/// Longest prefix of `text` that fits into `max_width` terminal columns.
+///
+/// Cuts on character boundaries and never splits a wide glyph.
+pub fn truncate_to_width(text: &str, max_width: usize) -> &str {
+    let mut width = 0usize;
+    for (idx, ch) in text.char_indices() {
+        let ch_width = ch.width().unwrap_or(0);
+        if width + ch_width > max_width {
+            return &text[..idx];
+        }
+        width += ch_width;
+    }
+    text
+}
+
+/// Like [`truncate_to_width`], but marks a cut with a trailing `…`.
+///
+/// The ellipsis is part of the budget, so the result never exceeds
+/// `max_width` columns.
+pub fn truncate_with_ellipsis(text: &str, max_width: usize) -> Cow<'_, str> {
+    if text.width() <= max_width {
+        return Cow::Borrowed(text);
+    }
+    if max_width == 0 {
+        return Cow::Borrowed("");
+    }
+    Cow::Owned(format!("{}…", truncate_to_width(text, max_width - 1)))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn keeps_short_text_untouched() {
+        assert_eq!(truncate_to_width("hello", 10), "hello");
+        assert_eq!(truncate_with_ellipsis("hello", 10), "hello");
+    }
+
+    #[test]
+    fn cuts_on_char_boundaries() {
+        // "Übertragung" — the cut lands inside the multi-byte 'Ü' if done
+        // with byte indices.
+        assert_eq!(truncate_to_width("Übertragung", 1), "Ü");
+        assert_eq!(truncate_to_width("für", 2), "fü");
+        assert_eq!(truncate_with_ellipsis("für alle", 3), "fü…");
+    }
+
+    #[test]
+    fn never_splits_wide_glyphs() {
+        // CJK characters occupy two columns each.
+        assert_eq!(truncate_to_width("日本語", 3), "日");
+        assert_eq!(truncate_to_width("日本語", 4), "日本");
+    }
+
+    #[test]
+    fn degenerate_widths() {
+        assert_eq!(truncate_to_width("abc", 0), "");
+        assert_eq!(truncate_with_ellipsis("abc", 0), "");
+        assert_eq!(truncate_with_ellipsis("abc", 1), "…");
+    }
+
+    #[test]
+    fn result_fits_the_budget() {
+        for max in 0..12 {
+            assert!(truncate_to_width("Fragebogen für Ü", max).width() <= max);
+            assert!(truncate_with_ellipsis("Fragebogen für Ü", max).width() <= max);
+        }
+    }
+}

crates/ui_terminal/src/tool_renderers/command_renderer.rs

@@ -11,6 +11,7 @@ use super::{
 };
 use crate::message::ToolUseBlock;
 use crate::terminal_color;
+use crate::text_util::truncate_to_width;
 use code_assistant_core::ui::ToolStatus;
 
 /// Expand tab characters to spaces (4-space tab stops).
@@ -71,11 +72,7 @@ impl ToolRenderer for CommandToolRenderer {
                         .bg(bg),
                 );
                 let max_cmd_len = row_width.saturating_sub(2);
-                let display = if cmd.value.len() > max_cmd_len {
-                    &cmd.value[..max_cmd_len]
-                } else {
-                    cmd.value.as_str()
-                };
+                let display = truncate_to_width(&cmd.value, max_cmd_len);
                 buf.set_string(
                     area.x + 4,
                     y,
@@ -103,11 +100,7 @@ impl ToolRenderer for CommandToolRenderer {
                         Style::default().bg(bg),
                     );
                     let expanded = expand_tabs(line);
-                    let display = if expanded.len() > row_width {
-                        &expanded[..row_width]
-                    } else {
-                        expanded.as_str()
-                    };
+                    let display = truncate_to_width(&expanded, row_width);
                     buf.set_string(
                         area.x + 2,
                         y,

crates/ui_terminal/src/tool_renderers/compact_renderer.rs

@@ -5,11 +5,13 @@
 
 use ratatui::prelude::*;
 use ratatui::style::{Color, Modifier, Style};
+use unicode_width::UnicodeWidthStr;
 
 use super::{
     push_error_history_line, render_error_line, render_tool_header, tool_header_line, ToolRenderer,
 };
 use crate::message::ToolUseBlock;
+use crate::text_util::truncate_to_width;
 use code_assistant_core::ui::ToolStatus;
 
 /// Renderer for read/explore tools: read_files, list_files, list_projects,
@@ -46,15 +48,11 @@ impl ToolRenderer for CompactToolRenderer {
                 CompactLine::Item(text) => {
                     buf.set_string(area.x + 2, y, "- ", Style::default().fg(Color::DarkGray));
                     let max_len = area.width.saturating_sub(4) as usize;
-                    let display = if text.len() > max_len {
-                        &text[..max_len]
-                    } else {
-                        text.as_str()
-                    };
+                    let display = truncate_to_width(&text, max_len);
                     buf.set_string(area.x + 4, y, display, Style::default().fg(Color::Gray));
                 }
                 CompactLine::KeyValue(key, value) => {
-                    let key_len = key.len() as u16;
+                    let key_len = key.width() as u16;
                     buf.set_string(area.x + 2, y, &key, Style::default().fg(Color::Cyan));
                     buf.set_string(
                         area.x + 2 + key_len,
@@ -63,11 +61,7 @@ impl ToolRenderer for CompactToolRenderer {
                         Style::default().fg(Color::White),
                     );
                     let max_len = area.width.saturating_sub(4 + key_len) as usize;
-                    let display = if value.len() > max_len {
-                        &value[..max_len]
-                    } else {
-                        value.as_str()
-                    };
+                    let display = truncate_to_width(&value, max_len);
                     buf.set_string(
                         area.x + 4 + key_len,
                         y,

crates/ui_terminal/src/tool_renderers/mod.rs

@@ -15,6 +15,7 @@ use ratatui::prelude::*;
 use ratatui::style::{Color, Modifier, Style};
 
 use super::message::ToolUseBlock;
+use super::text_util::truncate_to_width;
 use code_assistant_core::ui::ToolStatus;
 
 /// Trait for custom tool block renderers.
@@ -154,11 +155,7 @@ pub fn render_error_line(tool_block: &ToolUseBlock, area: Rect, buf: &mut Buffer
         if let Some(ref message) = tool_block.status_message {
             if y < area.y + area.height {
                 let max_len = area.width.saturating_sub(2) as usize;
-                let display = if message.len() > max_len {
-                    &message[..max_len]
-                } else {
-                    message.as_str()
-                };
+                let display = truncate_to_width(message, max_len);
                 buf.set_string(area.x + 2, y, display, Style::default().fg(Color::LightRed));
                 return y + 1;
             }

crates/ui_terminal/src/tool_renderers/sub_agent_renderer.rs

@@ -15,6 +15,7 @@ use ratatui::widgets::Paragraph;
 
 use super::{status_color, ToolRenderer};
 use crate::message::ToolUseBlock;
+use crate::text_util::truncate_with_ellipsis;
 use code_assistant_core::agent::sub_agent::{SubAgentOutput, SubAgentToolStatus};
 use code_assistant_core::ui::ToolStatus;
 
@@ -47,16 +48,6 @@ impl ToolRenderer for SubAgentToolRenderer {
     }
 }
 
-/// Truncate `text` to at most `max` chars, appending `…` when cut.
-fn truncate(text: &str, max: usize) -> String {
-    if text.chars().count() > max {
-        let cut: String = text.chars().take(max).collect();
-        format!("{cut}…")
-    } else {
-        text.to_string()
-    }
-}
-
 /// Build the styled lines for a sub-agent tool block. Shared by the live
 /// viewport, the height calculation, and the scrollback history.
 fn sub_agent_lines(tool_block: &ToolUseBlock) -> Vec<Line<'static>> {
@@ -74,7 +65,8 @@ fn sub_agent_lines(tool_block: &ToolUseBlock) -> Vec<Line<'static>> {
         ),
     ];
     if let Some(instructions) = tool_block.parameters.get("instructions") {
-        let summary = truncate(instructions.value.trim(), INSTRUCTIONS_SUMMARY_LEN);
+        let summary =
+            truncate_with_ellipsis(instructions.value.trim(), INSTRUCTIONS_SUMMARY_LEN);
         if !summary.is_empty() {
             header.push(Span::styled(
                 format!(": {summary}"),

crates/ui_terminal/src/tool_widget.rs

@@ -2,6 +2,7 @@ use code_assistant_core::ui::ToolStatus;
 use ratatui::prelude::*;
 
 use super::message::ToolUseBlock;
+use super::text_util::{truncate_to_width, truncate_with_ellipsis};
 use super::tool_renderers::ToolRendererRegistry;
 
 /// Custom ratatui widget for rendering tool use blocks.
@@ -149,11 +150,7 @@ impl<'a> ToolWidget<'a> {
         // Error status message
         if let Some(ref message) = self.tool_block.status_message {
             if self.tool_block.status == ToolStatus::Error && current_y < area.y + area.height {
-                let display_text = if message.len() > area.width as usize {
-                    &message[..area.width as usize]
-                } else {
-                    message
-                };
+                let display_text = truncate_to_width(message, area.width as usize);
                 buf.set_string(
                     area.x + 2,
                     current_y,
@@ -173,15 +170,12 @@ impl<'a> ToolWidget<'a> {
                     if current_y >= area.y + area.height {
                         break;
                     }
-                    let truncated = if line.len() > (area.width.saturating_sub(4)) as usize {
-                        format!("{}...", &line[..(area.width.saturating_sub(7)) as usize])
-                    } else {
-                        line.to_string()
-                    };
+                    let truncated =
+                        truncate_with_ellipsis(line, area.width.saturating_sub(4) as usize);
                     buf.set_string(
                         area.x + 2,
                         current_y,
-                        &truncated,
+                        truncated.as_ref(),
                         Style::default().fg(Color::Gray),
                     );
                     current_y += 1;
@@ -212,3 +206,38 @@ pub(super) fn should_hide_parameter(tool_name: &str, param_name: &str, param_val
         _ => false,
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::message::{ParameterValue, ToolUseBlock};
+    use indexmap::IndexMap;
+
+    /// Output lines containing multi-byte characters used to panic here:
+    /// the truncation sliced by byte index and landed inside a `ü`.
+    #[test]
+    fn fallback_renders_multibyte_output_without_panicking() {
+        let mut parameters = IndexMap::new();
+        parameters.insert(
+            "path".to_string(),
+            ParameterValue::new("Übertragungsprotokoll für die steuerliche Erfassung".to_string()),
+        );
+        let tool = ToolUseBlock {
+            // A tool without a registered renderer → generic fallback.
+            name: "delete_files".to_string(),
+            id: "test-id".to_string(),
+            parameters,
+            status: ToolStatus::Error,
+            status_message: Some("Datei konnte nicht gelöscht werden — Zugriff verweigert".into()),
+            output: Some(
+                "/Users/x/Downloads/050_202_00962_Uebertragungsprotokoll_Fragebogen_zur_steuerlichen_Erfassung_für_Einzelunternehmen.pdf\n日本語のファイル名.pdf".to_string(),
+            ),
+        };
+
+        for width in [1u16, 7, 20, 40, 104, 120] {
+            let area = Rect::new(0, 0, width, 10);
+            let mut buf = Buffer::empty(area);
+            ToolWidget::new(&tool).render(area, &mut buf);
+        }
+    }
+}