anthropic auth proxy for multi-user claude code

Local HTTP proxy on 127.0.0.1:5051 intercepts Claude Code API requests
from sandboxed member users, injects the real OAuth token server-side,
and forwards to Anthropic. Users only see a proxy secret, never the
real credentials.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/servers/api/anthropic-proxy.ts

@@ -0,0 +1,92 @@
+import { join } from 'node:path';
+import { homedir } from 'node:os';
+
+const PROXY_PORT = Number(process.env.ANTHROPIC_PROXY_PORT ?? '5051');
+const ANTHROPIC_API_BASE = 'https://api.anthropic.com';
+const CREDENTIALS_PATH = join(homedir(), '.claude', '.credentials.json');
+
+// Generate a random proxy secret at startup — shared with Claude Code spawns
+// Prefix with sk-ant- so Claude Code accepts it as a valid API key format
+export const proxySecret = `sk-ant-proxy01-${crypto.randomUUID()}`;
+
+type CredentialsFile = {
+  claudeAiOauth?: {
+    accessToken?: string;
+  };
+};
+
+async function readOAuthToken(): Promise<string | null> {
+  try {
+    const file = Bun.file(CREDENTIALS_PATH);
+    if (!(await file.exists())) return null;
+    const data = (await file.json()) as CredentialsFile;
+    return data.claudeAiOauth?.accessToken?.trim() || null;
+  } catch {
+    return null;
+  }
+}
+
+export function startAnthropicProxy() {
+  Bun.serve({
+    port: PROXY_PORT,
+    hostname: '127.0.0.1',
+    idleTimeout: 0,
+
+    async fetch(req) {
+      const url = new URL(req.url);
+
+      // Validate proxy secret
+      const incomingKey = req.headers.get('x-api-key');
+      if (incomingKey !== proxySecret) {
+        return new Response(JSON.stringify({ error: 'Unauthorized' }), {
+          status: 401,
+          headers: { 'Content-Type': 'application/json' },
+        });
+      }
+
+      // Read fresh OAuth token
+      const token = await readOAuthToken();
+      if (!token) {
+        return new Response(JSON.stringify({ error: 'No OAuth token available' }), {
+          status: 502,
+          headers: { 'Content-Type': 'application/json' },
+        });
+      }
+
+      // Build upstream URL
+      const upstream = `${ANTHROPIC_API_BASE}${url.pathname}${url.search}`;
+
+      // Clone headers, replace proxy secret with real OAuth token
+      const headers = new Headers(req.headers);
+      headers.set('x-api-key', token);
+      headers.delete('host');
+
+      // Read request body fully before forwarding — avoids stream-in-stream issues
+      const body = req.body ? await req.arrayBuffer() : null;
+
+      // Forward request
+      const upstreamRes = await fetch(upstream, {
+        method: req.method,
+        headers,
+        body,
+      });
+
+      // Build clean response headers
+      const resHeaders = new Headers();
+      for (const [key, value] of upstreamRes.headers) {
+        const lower = key.toLowerCase();
+        // Skip hop-by-hop and encoding headers that may have been consumed by fetch
+        if (lower === 'content-encoding' || lower === 'content-length' || lower === 'transfer-encoding') continue;
+        resHeaders.set(key, value);
+      }
+
+      return new Response(upstreamRes.body, {
+        status: upstreamRes.status,
+        statusText: upstreamRes.statusText,
+        headers: resHeaders,
+      });
+    },
+  });
+
+  console.log(`[anthropic-proxy] listening on 127.0.0.1:${PROXY_PORT}`);
+}