fix(runtime): avoid encoding latin1 directory headers

packages/ui/src/lib/runtime-fetch.test.ts

@@ -394,9 +394,15 @@ describe('runtimeFetch header sanitization', () => {
     expect(result).toBeFalsy();
   });
 
-  test('sanitizeHeadersForBrowser always encodes directory hints with marker', () => {
+  test('sanitizeHeadersForBrowser leaves Latin-1 directory hints unchanged', () => {
     const path = 'C:\\work\\foo%20bar';
     const result = sanitizeHeadersForBrowser({ 'x-opencode-directory': path });
+    expect(result).toBeFalsy();
+  });
+
+  test('sanitizeHeadersForBrowser encodes non-Latin-1 directory hints with marker', () => {
+    const path = 'D:\\文件夹';
+    const result = sanitizeHeadersForBrowser({ 'x-opencode-directory': path });
     expect(result).toBeTruthy();
     const encoded = Object.fromEntries(result!);
     expect(encoded['x-opencode-directory']).toBe(encodeURIComponent(path));

packages/ui/src/lib/runtime-fetch.ts

@@ -99,9 +99,9 @@ const shouldAttachRuntimeAuth = (input: string | URL | Request): boolean => {
 // Headers API only accepts ISO-8859-1 (Latin-1) characters. Any value containing
 // characters outside \u0000-\u00FF causes "Failed to construct/set 'Headers':
 // String contains non ISO-8859-1 code point." Encode those values so they round-trip
-// safely through the browser's Headers API. Directory hints are always encoded
-// with an explicit marker header so the server decodes only values produced by
-// this transport and preserves literal percent sequences from direct clients.
+// safely through the browser's Headers API. Directory hints get an explicit marker
+// only when encoded, so plain ASCII paths remain compatible with routes that read
+// the header directly.
 export const isLatin1Safe = (value: string): boolean => {
   for (let i = 0; i < value.length; i += 1) {
     if (value.charCodeAt(i) > 0xFF) return false;
@@ -109,9 +109,7 @@ export const isLatin1Safe = (value: string): boolean => {
   return true;
 };
 
-const shouldEncodeHeaderValue = (key: string, value: string): boolean => (
-  key.toLowerCase() === 'x-opencode-directory' || !isLatin1Safe(value)
-);
+const shouldEncodeHeaderValue = (_key: string, value: string): boolean => !isLatin1Safe(value);
 
 export const sanitizeHeadersForBrowser = (init?: HeadersInit): [string, string][] | undefined => {
   if (!init) return undefined;