diff --git a/client/src/components/run/TerminalView.tsx b/client/src/components/run/TerminalView.tsx
index b485c6f1..dc876ead 100644
--- a/client/src/components/run/TerminalView.tsx
+++ b/client/src/components/run/TerminalView.tsx
@@ -26,7 +26,21 @@ export function TerminalView({ runId, wsBaseUrl }: TerminalViewProps) {
   const containerRef = useRef<HTMLDivElement>(null);
 
   useEffect(() => {
-    const term = new Terminal({ convertEol: true, fontSize: 13, cursorBlink: true });
+    // An explicit fontFamily pins every glyph to one bundled webfont. Left
+    // unset, xterm falls back to the browser/OS default (`courier-new`),
+    // which on Windows lacks Vietnamese precomposed diacritics — the OS then
+    // substitutes those specific glyphs from a different face with a
+    // mismatched advance width, corrupting the cell it's drawn into even
+    // under the canvas renderer (which only stops a glyph from shoving its
+    // *neighbours*, not from overflowing its own cell). JetBrains Mono is
+    // already bundled for the rest of the UI; main.tsx additionally imports
+    // its `vietnamese` subset so this covers the diacritics too.
+    const term = new Terminal({
+      convertEol: true,
+      fontSize: 13,
+      cursorBlink: true,
+      fontFamily: '"JetBrains Mono", monospace',
+    });
     const fit = new FitAddon();
     term.loadAddon(fit);
     if (containerRef.current) term.open(containerRef.current);
diff --git a/client/src/main.tsx b/client/src/main.tsx
index 38765da0..eedfbf12 100644
--- a/client/src/main.tsx
+++ b/client/src/main.tsx
@@ -57,6 +57,12 @@ import "@fontsource/inter/latin-600.css";
 import "@fontsource/inter/latin-700.css";
 import "@fontsource/jetbrains-mono/latin-400.css";
 import "@fontsource/jetbrains-mono/latin-500.css";
+// The lane terminal renders live Vietnamese output in this font (TerminalView),
+// which the latin subset above doesn't cover — without it, missing glyphs fall
+// back to another installed face with a different advance width and get
+// visually mangled inside xterm's fixed-width cells.
+import "@fontsource/jetbrains-mono/vietnamese-400.css";
+import "@fontsource/jetbrains-mono/vietnamese-500.css";
 import App from "./App";
 import "./i18n";
 import "./index.css";