diff --git a/containers/api-proxy/server.js b/containers/api-proxy/server.js
index 010ba5f1..829b632d 100644
--- a/containers/api-proxy/server.js
+++ b/containers/api-proxy/server.js
@@ -149,24 +149,44 @@ function normalizeBasePath(rawPath) {
 
 /**
  * Build the full upstream path by joining basePath, reqUrl's pathname, and query string.
+ * Applies provider-safe defaults and avoids duplicate prefixing when the incoming
+ * path already includes the configured base path.
  *
  * Examples:
- *   buildUpstreamPath('/v1/chat/completions', 'api.openai.com', '')
- *     → '/v1/chat/completions'
+ *   buildUpstreamPath('/responses', 'api.openai.com', '')
+ *     → '/v1/responses'
  *   buildUpstreamPath('/v1/chat/completions', 'host.databricks.com', '/serving-endpoints')
  *     → '/serving-endpoints/v1/chat/completions'
  *   buildUpstreamPath('/v1/messages?stream=true', 'host.com', '/anthropic')
  *     → '/anthropic/v1/messages?stream=true'
  *
- * @param {string} reqUrl - The incoming request URL (must start with '/')
+ * @param {string} reqUrl - The incoming request URL (must start with '/' and not '//')
  * @param {string} targetHost - The upstream hostname (used only to parse the URL)
  * @param {string} basePath - Normalized base path prefix (e.g. '/serving-endpoints' or '')
  * @returns {string} Full upstream path including query string
  */
 function buildUpstreamPath(reqUrl, targetHost, basePath) {
+  if (typeof reqUrl !== 'string' || !reqUrl.startsWith('/') || reqUrl.startsWith('//')) {
+    throw new Error('URL must be a relative origin-form path');
+  }
+
   const targetUrl = new URL(reqUrl, `https://${targetHost}`);
-  const prefix = basePath === '/' ? '' : basePath;
-  return prefix + targetUrl.pathname + targetUrl.search;
+  const pathname = targetUrl.pathname;
+  let prefix = basePath === '/' ? '' : basePath;
+
+  // OpenAI's canonical API paths are versioned under /v1, while some newer
+  // clients (for example Codex CLI with OPENAI_BASE_URL pointing at the sidecar)
+  // send unversioned paths like /responses. Add /v1 only for the default
+  // OpenAI host when no explicit base path is configured.
+  if (!prefix && targetUrl.hostname === 'api.openai.com') {
+    prefix = '/v1';
+  }
+
+  if (prefix && (pathname === prefix || pathname.startsWith(`${prefix}/`))) {
+    return pathname + targetUrl.search;
+  }
+
+  return prefix + pathname + targetUrl.search;
 }
 
 /**
@@ -418,7 +438,7 @@ function proxyRequest(req, res, targetHost, injectHeaders, provider, basePath =
   });
 
   // Validate that req.url is a relative path (prevent open-redirect / SSRF)
-  if (!req.url || !req.url.startsWith('/')) {
+  if (!req.url || !req.url.startsWith('/') || req.url.startsWith('//')) {
     const duration = Date.now() - startTime;
     metrics.gaugeDec('active_requests', { provider });
     metrics.increment('requests_total', { provider, method: req.method, status_class: '4xx' });
@@ -687,7 +707,7 @@ function proxyWebSocket(req, socket, head, targetHost, injectHeaders, provider,
   }
 
   // ── Validate: relative path only (prevent SSRF) ────────────────────────
-  if (!req.url || !req.url.startsWith('/')) {
+  if (!req.url || !req.url.startsWith('/') || req.url.startsWith('//')) {
     logRequest('warn', 'websocket_upgrade_rejected', {
       request_id: requestId,
       provider,
diff --git a/containers/api-proxy/server.test.js b/containers/api-proxy/server.test.js
index a83617a4..60b08865 100644
--- a/containers/api-proxy/server.test.js
+++ b/containers/api-proxy/server.test.js
@@ -347,6 +347,11 @@ describe('buildUpstreamPath', () => {
     it('should handle root path with no base path', () => {
       expect(buildUpstreamPath('/', HOST, '')).toBe('/');
     });
+
+    it('should reject protocol-relative URLs to prevent host override', () => {
+      expect(() => buildUpstreamPath('//evil.com/v1/chat/completions', HOST, ''))
+        .toThrow('URL must be a relative origin-form path');
+    });
   });
 
   describe('Databricks serving-endpoints (single-segment base path)', () => {
@@ -406,6 +411,21 @@ describe('buildUpstreamPath', () => {
         .toBe('/v1/chat/completions');
     });
 
+    it('should map unversioned /responses to /v1/responses for api.openai.com', () => {
+      expect(buildUpstreamPath('/responses', 'api.openai.com', ''))
+        .toBe('/v1/responses');
+    });
+
+    it('should preserve already-versioned OpenAI responses path', () => {
+      expect(buildUpstreamPath('/v1/responses', 'api.openai.com', ''))
+        .toBe('/v1/responses');
+    });
+
+    it('should map unversioned /responses to /v1/responses when OpenAI host includes port', () => {
+      expect(buildUpstreamPath('/responses', 'api.openai.com:443', ''))
+        .toBe('/v1/responses');
+    });
+
     it('should preserve /v1/messages exactly (Anthropic standard path)', () => {
       expect(buildUpstreamPath('/v1/messages', 'api.anthropic.com', ''))
         .toBe('/v1/messages');
@@ -438,6 +458,12 @@ describe('buildUpstreamPath', () => {
         .toBe('/v1/messages');
     });
 
+    it('should not force /v1 for non-OpenAI custom targets', () => {
+      const target = 'my-gateway.example.com';
+      expect(buildUpstreamPath('/responses', target, ''))
+        .toBe('/responses');
+    });
+
     it('should produce wrong hostname if scheme is NOT stripped (demonstrating the bug)', () => {
       // Without normalizeApiTarget, the scheme-prefixed value causes
       // new URL() to parse 'https' as the hostname instead of the real host
@@ -575,6 +601,13 @@ describe('proxyWebSocket', () => {
       expect(socket.destroy).toHaveBeenCalled();
     });
 
+    it('rejects a protocol-relative URL with 400 (SSRF prevention)', () => {
+      const socket = makeMockSocket();
+      proxyWebSocket(makeUpgradeReq({ url: '//evil.com/v1/responses' }), socket, Buffer.alloc(0), 'api.openai.com', {}, 'openai');
+      expect(socket.write).toHaveBeenCalledWith(expect.stringContaining('HTTP/1.1 400 Bad Request'));
+      expect(socket.destroy).toHaveBeenCalled();
+    });
+
     it('rejects a null URL with 400', () => {
       const socket = makeMockSocket();
       proxyWebSocket(makeUpgradeReq({ url: null }), socket, Buffer.alloc(0), 'api.openai.com', {}, 'openai');
@@ -946,4 +979,3 @@ describe('resolveOpenCodeRoute', () => {
     expect(route.headers['x-api-key']).toBeUndefined();
   });
 });
-