From abc01942197d70163be212592dc4631ddf83d6d9 Mon Sep 17 00:00:00 2001
From: Xyra Sinclair <xyrasinclair@gmail.com>
Date: Sat, 18 Jul 2026 20:29:02 -0700
Subject: [PATCH] fix(proxy): never route an sk-ant bearer to the OpenAI
 upstream (#49)

Prevent Anthropic bearer tokens from being forwarded to OpenAI when routing /v1/models. Add tests for Anthropic, OpenAI, and x-api-key routing.
---
 src/core/proxy.ts                 |  9 ++++-
 tests/models-auth-routing.test.ts | 67 +++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+), 1 deletion(-)
 create mode 100644 tests/models-auth-routing.test.ts

diff --git a/src/core/proxy.ts b/src/core/proxy.ts
index 8733e0f..f6e67f1 100644
--- a/src/core/proxy.ts
+++ b/src/core/proxy.ts
@@ -564,7 +564,14 @@ function isOpenAIResponsesPath(pathname: string): boolean {
 
 function isCanonicalOpenAIPath(pathname: string, headers: Headers, hasOpenAIKey: boolean): boolean {
   const isModelsPath = pathname === '/v1/models' || pathname.startsWith('/v1/models/');
-  const looksOpenAIAuth = hasOpenAIKey || (headers.has('authorization') && !headers.has('x-api-key'));
+  // `/v1/models` exists on BOTH APIs, so it is routed by auth style — but an
+  // `sk-ant-…` bearer is Anthropic by construction (Claude Code subscription
+  // auth sends `authorization: Bearer sk-ant-oat01-…` with no x-api-key).
+  // Without this check that OAuth token would be forwarded to the OpenAI
+  // upstream: a credential leak, and a guaranteed 401.
+  const bearerIsAnthropic = /^Bearer\s+sk-ant-/i.test(headers.get('authorization') ?? '');
+  const looksOpenAIAuth =
+    hasOpenAIKey || (headers.has('authorization') && !headers.has('x-api-key') && !bearerIsAnthropic);
   return pathname === '/v1/chat/completions'
     || pathname === '/v1/responses'
     || pathname.startsWith('/v1/responses/')
diff --git a/tests/models-auth-routing.test.ts b/tests/models-auth-routing.test.ts
new file mode 100644
index 0000000..5a86843
--- /dev/null
+++ b/tests/models-auth-routing.test.ts
@@ -0,0 +1,67 @@
+/**
+ * /v1/models routing by auth style. The path exists on BOTH APIs, so the
+ * proxy sniffs the auth header — but an `sk-ant-…` bearer is Anthropic by
+ * construction (Claude Code subscription auth sends
+ * `authorization: Bearer sk-ant-oat01-…` with no x-api-key). It must never
+ * be forwarded to the OpenAI upstream. All tokens here are fake; the suite
+ * never touches the network (global fetch is stubbed).
+ */
+import { afterEach, describe, expect, it } from 'vitest';
+import { createProxy } from '../src/core/proxy.js';
+
+const realFetch = globalThis.fetch;
+afterEach(() => {
+  globalThis.fetch = realFetch;
+});
+
+function stubFetch(capture: { url?: string; headers?: Headers }) {
+  globalThis.fetch = (async (input: RequestInfo | URL, init?: RequestInit) => {
+    capture.url = String(input);
+    capture.headers = new Headers(init?.headers);
+    return new Response(JSON.stringify({ data: [] }), {
+      headers: { 'content-type': 'application/json' },
+    });
+  }) as typeof fetch;
+}
+
+describe('/v1/models auth-style routing', () => {
+  const proxy = () => createProxy({});
+
+  it('routes an sk-ant bearer (Claude Code OAuth) to the Anthropic upstream', async () => {
+    const cap: { url?: string; headers?: Headers } = {};
+    stubFetch(cap);
+    await proxy()(
+      new Request('http://localhost/v1/models', {
+        method: 'GET',
+        headers: { authorization: 'Bearer sk-ant-oat01-fake-oauth-token' },
+      }),
+    );
+    expect(cap.url).toBe('https://api.anthropic.com/v1/models');
+    // The token stays on the Anthropic leg, never crosses to api.openai.com.
+    expect(cap.headers?.get('authorization')).toBe('Bearer sk-ant-oat01-fake-oauth-token');
+  });
+
+  it('still routes a non-Anthropic bearer to the OpenAI upstream', async () => {
+    const cap: { url?: string; headers?: Headers } = {};
+    stubFetch(cap);
+    await proxy()(
+      new Request('http://localhost/v1/models', {
+        method: 'GET',
+        headers: { authorization: 'Bearer fake-openai-key' },
+      }),
+    );
+    expect(cap.url).toBe('https://api.openai.com/v1/models');
+  });
+
+  it('still routes x-api-key requests to the Anthropic upstream', async () => {
+    const cap: { url?: string; headers?: Headers } = {};
+    stubFetch(cap);
+    await proxy()(
+      new Request('http://localhost/v1/models', {
+        method: 'GET',
+        headers: { 'x-api-key': 'fake-anthropic-key' },
+      }),
+    );
+    expect(cap.url).toBe('https://api.anthropic.com/v1/models');
+  });
+});