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diff --git a/packages/meshbay-hub/src/meshbay_hub/static/portable-name.js b/packages/meshbay-hub/src/meshbay_hub/static/portable-name.js
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+/**
+ * A name that can be written on every platform a member saves to
+ * (docs/MESHBAY_DESIGN.md §10).
+ *
+ * A node serves a file under the name its own disk gave it, and a name that is
+ * fine on ext4 can be impossible on Windows or on an exFAT drive: reserved
+ * characters, a trailing dot or space, `CON` or `aux.txt`. The node never
+ * rewrites a name — it is the string that opens the file — so the client does,
+ * at the moment it saves, and says so.
+ *
+ * The same rules as `meshbay_common.paths.sanitize_for_download`, and held to
+ * them by `test_portable_name_parity.py`: two copies of a rule that differ
+ * decide differently which files get renamed.
+ */
+
+const WINDOWS_RESERVED = new Set([
+  'CON', 'PRN', 'AUX', 'NUL',
+  ...[1, 2, 3, 4, 5, 6, 7, 8, 9].map((i) => `COM${i}`),
+  ...[1, 2, 3, 4, 5, 6, 7, 8, 9].map((i) => `LPT${i}`),
+]);
+
+const RESERVED_CHARS = new Set('<>:"/\\|?*');
+// The bidirectional controls: "invoice\u202efdp.exe" displays as
+// "invoiceexe.pdf", and a saved name must say what the file is.
+const BIDI_CONTROLS = new Set('\u061c\u200e\u200f\u202a\u202b\u202c\u202d\u202e'
+                              + '\u2066\u2067\u2068\u2069');
+
+const reserved = (c) => RESERVED_CHARS.has(c) || BIDI_CONTROLS.has(c)
+  || c.charCodeAt(0) < 32;
+
+function isPortable(name) {
+  if (!name || name === '.' || name === '..') return false;
+  for (const c of name) if (reserved(c)) return false;
+  if (name.endsWith(' ') || name.endsWith('.')) return false;
+  return !WINDOWS_RESERVED.has(name.split('.', 1)[0].toUpperCase());
+}
+
+/** `name` if it can be written everywhere, otherwise the nearest name that can. */
+export function portableName(name, replacement = '_') {
+  const text = String(name ?? '');
+  if (isPortable(text)) return text;
+  let out = Array.from(text).map((c) => (reserved(c) ? replacement : c)).join('');
+  out = out.replace(/[ .]+$/, '');
+  const dot = out.indexOf('.');
+  let stem = dot === -1 ? out : out.slice(0, dot);
+  const rest = dot === -1 ? '' : out.slice(dot);
+  if (WINDOWS_RESERVED.has(stem.toUpperCase())) stem += replacement;
+  out = stem + rest;
+  return out || 'unnamed';
+}
+
+/** Every segment of a relative path made portable, keeping `/` between them. */
+export function portablePath(path) {
+  return String(path ?? '').split('/')
+    .map((segment) => (segment ? portableName(segment) : segment)).join('/');
+}