fix: stabilize vercel api proxy routing

api/_proxy.js

@@ -68,21 +68,43 @@ function stripProxyPrefix(pathname, prefixes) {
   return pathname.slice(matchedPrefix.length) || '/';
 }
 
+function buildTargetPathFromQuery(incomingUrl, pathQueryParam) {
+  if (!pathQueryParam) {
+    return null;
+  }
+
+  const pathFromQuery = incomingUrl.searchParams.get(pathQueryParam);
+
+  if (!pathFromQuery) {
+    return '/';
+  }
+
+  incomingUrl.searchParams.delete(pathQueryParam);
+
+  return pathFromQuery.startsWith('/') ? pathFromQuery : `/${pathFromQuery}`;
+}
+
 function buildTargetUrl(requestUrl, options, targetBase) {
   const incomingUrl = new URL(requestUrl, 'https://vercel.local');
-  const pathname = stripProxyPrefix(incomingUrl.pathname, normalizeProxyPrefixes(options));
+  const pathname =
+    buildTargetPathFromQuery(incomingUrl, options.pathQueryParam) ??
+    stripProxyPrefix(incomingUrl.pathname, normalizeProxyPrefixes(options));
 
   const targetUrl = new URL(pathname || '/', targetBase);
-  targetUrl.search = incomingUrl.search;
+  targetUrl.search = incomingUrl.searchParams.toString();
 
   return targetUrl;
 }
 
-async function proxyRequest(request, response, { envName, prefix, prefixes }) {
+async function proxyRequest(request, response, { envName, pathQueryParam, prefix, prefixes }) {
   let targetUrl;
 
   try {
-    targetUrl = buildTargetUrl(request.url, { prefix, prefixes }, requireProxyTarget(envName));
+    targetUrl = buildTargetUrl(
+      request.url,
+      { pathQueryParam, prefix, prefixes },
+      requireProxyTarget(envName),
+    );
   } catch (error) {
     response.statusCode = 500;
     response.setHeader('content-type', 'application/json; charset=utf-8');

api/_proxy.test.js

@@ -43,4 +43,18 @@ describe('buildTargetUrl', () => {
 
     expect(targetUrl.toString()).toBe('https://backend.example.com/be10/api/v1/scan');
   });
+
+  it('builds the target path from a Vercel rewrite query parameter', () => {
+    const targetUrl = buildTargetUrl(
+      '/api/be3?path=api/v1/scan/subscribe&guest_uuid=guest-1',
+      {
+        pathQueryParam: 'path',
+      },
+      targetBase,
+    );
+
+    expect(targetUrl.toString()).toBe(
+      'https://backend.example.com/api/v1/scan/subscribe?guest_uuid=guest-1',
+    );
+  });
 });

api/be1.js

@@ -0,0 +1,8 @@
+const { proxyRequest } = require('./_proxy');
+
+module.exports = function handler(request, response) {
+  return proxyRequest(request, response, {
+    envName: 'BE1_PROXY_TARGET_URL',
+    pathQueryParam: 'path',
+  });
+};

api/be3.js

@@ -0,0 +1,8 @@
+const { proxyRequest } = require('./_proxy');
+
+module.exports = function handler(request, response) {
+  return proxyRequest(request, response, {
+    envName: 'BE3_PROXY_TARGET_URL',
+    pathQueryParam: 'path',
+  });
+};

vercel.json

@@ -2,11 +2,11 @@
   "rewrites": [
     {
       "source": "/be1/:path*",
-      "destination": "/api/be1/:path*"
+      "destination": "/api/be1?path=:path*"
     },
     {
       "source": "/be3/:path*",
-      "destination": "/api/be3/:path*"
+      "destination": "/api/be3?path=:path*"
     }
   ]
 }