Restrict Vercel Node stream path to OpenAI chat to avoid Gemini empty responses

internal/js/chat-stream/index.js

@@ -59,8 +59,9 @@ async function handler(req, res) {
     return;
   }
 
-  // Keep all non-stream behavior on Go side to avoid compatibility regressions.
-  if (!toBool(payload.stream)) {
+  // Keep all non-stream behavior and non-OpenAI-chat paths on Go side to avoid
+  // protocol-shape regressions (e.g. Gemini/Claude clients expecting their own formats).
+  if (!toBool(payload.stream) || !isNodeStreamSupportedPath(req.url || '')) {
     await proxyToGo(req, res, rawBody);
     return;
   }
@@ -76,6 +77,23 @@ function isVercelRuntime() {
   return asString(process.env.VERCEL) !== '' || asString(process.env.NOW_REGION) !== '';
 }
 
+function isNodeStreamSupportedPath(rawURL) {
+  const path = extractPathname(rawURL);
+  return path === '/v1/chat/completions';
+}
+
+function extractPathname(rawURL) {
+  const text = asString(rawURL);
+  if (!text) {
+    return '';
+  }
+  const q = text.indexOf('?');
+  if (q >= 0) {
+    return text.slice(0, q);
+  }
+  return text;
+}
+
 module.exports = handler;
 
 module.exports.__test = {
@@ -89,4 +107,6 @@ module.exports.__test = {
   boolDefaultTrue,
   filterIncrementalToolCallDeltasByAllowed,
   estimateTokens,
+  isNodeStreamSupportedPath,
+  extractPathname,
 };

tests/node/chat-stream.test.js

@@ -18,6 +18,8 @@ const {
   boolDefaultTrue,
   filterIncrementalToolCallDeltasByAllowed,
   shouldSkipPath,
+  isNodeStreamSupportedPath,
+  extractPathname,
 } = handler.__test;
 
 test('chat-stream exposes parser test hooks', () => {
@@ -225,3 +227,15 @@ test('shouldSkipPath skips dynamic response/fragments/*/status paths only', () =
   assert.equal(shouldSkipPath('response/fragments/8/status'), true);
   assert.equal(shouldSkipPath('response/status'), false);
 });
+
+test('node stream path guard only allows /v1/chat/completions', () => {
+  assert.equal(isNodeStreamSupportedPath('/v1/chat/completions'), true);
+  assert.equal(isNodeStreamSupportedPath('/v1/chat/completions?x=1'), true);
+  assert.equal(isNodeStreamSupportedPath('/v1beta/models/gemini-2.5-flash:streamGenerateContent'), false);
+  assert.equal(isNodeStreamSupportedPath('/anthropic/v1/messages'), false);
+});
+
+test('extractPathname strips query only', () => {
+  assert.equal(extractPathname('/v1/chat/completions?stream=true'), '/v1/chat/completions');
+  assert.equal(extractPathname('/v1beta/models/gemini-2.5-flash:streamGenerateContent?key=1'), '/v1beta/models/gemini-2.5-flash:streamGenerateContent');
+});