diff --git a/src/index.ts b/src/index.ts
index 4ac489a..4638174 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1182,18 +1182,35 @@ async function attachAgentList(
 /**
  * The WS close reason for a failed terminal attach. A missing configured
  * shell gets a SHORT machine-readable marker (`shell-not-found:<name>`,
- * capped — a WS close reason allows at most 123 bytes) that the client maps
- * to a localized, actionable banner; every other failure keeps the raw
- * message (the model-side tool errors read it verbatim).
+ * capped by BYTES — a WS close reason allows at most 123 bytes, which `ws`
+ * validates with `Buffer.byteLength`) that the client maps to a localized,
+ * actionable banner; every other failure keeps the raw message (the
+ * model-side tool errors read it verbatim).
  */
 export function wsCloseReasonOf(error: unknown): string {
   if (error instanceof SidebarError && error.code === 'shell-not-found') {
-    const name = shellDisplayName(String(error.meta?.shell ?? '')).slice(0, 100)
+    const name = truncateUtf8Bytes(shellDisplayName(String(error.meta?.shell ?? '')), 100)
     return `shell-not-found:${name}`
   }
   return error instanceof Error ? error.message : String(error)
 }
 
+/**
+ * Truncate to at most `maxBytes` UTF-8 bytes without splitting a code point.
+ * A character-count `slice` does not bound the WS close reason: `ws` measures
+ * `Buffer.byteLength` against its 123-byte cap, and the resulting throw would
+ * replace the very error the reason describes.
+ */
+function truncateUtf8Bytes(value: string, maxBytes: number): string {
+  if (Buffer.byteLength(value) <= maxBytes) return value
+  let truncated = ''
+  for (const character of value) {
+    if (Buffer.byteLength(truncated + character) > maxBytes) break
+    truncated += character
+  }
+  return truncated
+}
+
 /**
  * Wire one terminal socket to its pty: replay transcript, pump both ways.
  * Two attach modes share the wire protocol:
diff --git a/tests/smoke.spec.ts b/tests/smoke.spec.ts
index 54a3770..3ec800c 100644
--- a/tests/smoke.spec.ts
+++ b/tests/smoke.spec.ts
@@ -963,6 +963,19 @@ describe('side card settings routes', () => {
     expect(wsCloseReasonOf('plain')).toBe('plain')
   })
 
+  it('caps the close reason by UTF-8 bytes so the ws 123-byte limit holds', () => {
+    // 200 CJK characters are ~600 bytes: a character-count slice would still
+    // overflow the cap `ws` enforces with Buffer.byteLength.
+    const reason = wsCloseReasonOf(new SidebarError(
+      'shell-not-found',
+      'shell executable not found',
+      400,
+      { shell: `/bin/${'终'.repeat(200)}` },
+    ))
+    expect(reason.startsWith('shell-not-found:')).toBe(true)
+    expect(Buffer.byteLength(reason)).toBeLessThanOrEqual(123)
+  })
+
   it('reads the resolved prefs and writes a patch through the seam', async () => {
     const route = mountWithSettings(createFakeSettings())
     const read = await invoke(route, 'settings.get', {})