How to Secure a Claude API Proxy Endpoint

If you're running a proxy in front of the Claude API — for a frontend app, a mobile client, or internal tooling — the core security question is simple: how do you let clients call the model without exposing your Anthropic API key or letting anyone run up your bill? The answer combines four things: never ship the real key to the client, put a scoped credential in front of every request, rate-limit and authenticate callers, and log enough to detect abuse before it drains your budget.

This matters because a Claude API proxy is, by definition, a privileged relay. Your server holds a key that can spend real money and access real data. Any gap between "my proxy is reachable" and "my proxy is protected" becomes an open invoice for whoever finds it. Below is a practical checklist for closing that gap, whether you're hand-rolling the proxy or evaluating a managed option.

Never let the raw Anthropic key touch the client

The most common mistake is putting the Anthropic API key in frontend JavaScript, a mobile app bundle, or a public repo's .env.example that gets committed by accident. Once a key is in client-side code, it's public — full stop, regardless of minification or obfuscation.

The fix is structural: your Claude key lives only on a server you control, and every client — web, mobile, CLI — talks to your endpoint using a different, scoped credential. That credential should be:

• Revocable independently of your main Anthropic key
• Attributable to a specific user, app, or environment
• Rate-limited on its own, so one compromised client doesn't affect others

This is exactly the pattern SubToAPI implements: it converts your Claude access into application API keys (sub_live_...) that your apps call instead of the raw Anthropic credential. If a sub_live_ key leaks, you revoke it from the dashboard without touching the underlying Claude access. See the quickstart for the request shape.

Authenticate every request, not just the first one

A proxy endpoint needs auth on every call, not a session cookie set once at login. Minimum viable setup:

Authorization: Bearer sub_live_xxxxxxxxxxxxxxxx

Check this header server-side before forwarding anything to Claude. If you're building your own proxy layer (rather than using a managed one), a basic Express middleware looks like this:

function requireApiKey(req, res, next) {
  const auth = req.headers.authorization || "";
  const key = auth.startsWith("Bearer ") ? auth.slice(7) : null;

  if (!key || !isValidKey(key)) {
    return res.status(401).json({ error: "invalid_api_key" });
  }

  req.clientId = lookupClientId(key);
  next();
}

Don't rely on IP allowlisting alone — client IPs change, mobile apps rotate networks, and CDN edge IPs are shared. Use it as a secondary signal, not the primary control.

Rate-limit per key, not just globally

A global rate limit protects your infrastructure but not your budget. If one API key gets compromised or one bug causes a retry loop, you want that isolated to the offending key, not throttling your whole service.

Implement limits at two levels:

• Per-key requests/minute — catches runaway loops and scraping
• Per-key token/spend ceiling — catches expensive prompts or large context windows being abused

const limits = { requestsPerMinute: 60, maxTokensPerDay: 500000 };

async function checkLimit(clientId) {
  const usage = await getUsage(clientId);
  if (usage.requestsThisMinute >= limits.requestsPerMinute) {
    throw new RateLimitError("too_many_requests");
  }
  if (usage.tokensToday >= limits.maxTokensPerDay) {
    throw new RateLimitError("daily_token_limit_exceeded");
  }
}

If you don't want to build and maintain this yourself, this is one of the reasons a managed layer is worth considering — SubToAPI applies per-key limits and exposes usage metadata per request so you can see exactly which key is driving cost, without writing your own accounting system.

Validate and constrain what the proxy will forward

A proxy that blindly forwards whatever JSON body a client sends is a liability. At minimum:

• Whitelist the model names you allow (don't let clients specify arbitrary or future models you haven't budgeted for)
• Cap max_tokens server-side, don't trust client-supplied values above your ceiling
• Strip or validate system prompts if clients shouldn't be able to override your instructions
• Reject tool definitions you didn't author, if you're using tool use and don't want clients injecting arbitrary function schemas

const ALLOWED_MODELS = new Set(["claude-sonnet-4", "claude-haiku-4"]);
const MAX_TOKENS_CEILING = 4096;

function sanitizeRequest(body) {
  if (!ALLOWED_MODELS.has(body.model)) {
    throw new Error("model_not_allowed");
  }
  body.max_tokens = Math.min(body.max_tokens || 1024, MAX_TOKENS_CEILING);
  return body;
}

Keep secrets out of logs and error responses

It's easy to accidentally log full request bodies (which may contain user data) or leak stack traces that reveal internal endpoints. Log request metadata — client ID, model, token counts, latency, status code — and avoid logging full prompt/response content unless you have a specific retention policy and consent story for it. Return generic error messages to clients; keep detailed errors server-side only.

Use HTTPS and short-lived infrastructure credentials

This sounds obvious but is worth stating: the proxy itself must be served over HTTPS, and if it runs on cloud infrastructure, use short-lived IAM credentials or secret managers (not .env files checked into version control) to store the Anthropic key. Rotate the underlying key periodically and immediately if you suspect exposure.

Consider whether you need to build this at all

Building a secure Claude API proxy — auth, per-key rate limiting, request validation, usage tracking, logging — is a legitimate amount of infrastructure for something that isn't your core product. SubToAPI packages this: application keys, streaming, tool use, and usage metadata in one dashboard, with team seats if multiple people need scoped access. Plans start at €9/month for solo use, with a free trial at signup; see pricing for team and scale tiers. If your proxy is a means to an end rather than the product itself, it's worth comparing the time cost of maintaining your own security layer against a managed one.

FAQs

Do I need a proxy if I'm only calling Claude from my backend? No — if all calls originate server-side and no credential is ever sent to a browser or mobile client, you don't need a separate proxy layer for security purposes, though you may still want one for rate limiting or multi-provider routing.

Is API key rotation enough to secure a proxy? No. Rotation limits damage after a leak but doesn't prevent abuse from a valid, unrevoked key. You still need per-key rate limits, request validation, and monitoring to catch misuse in real time.

Can I rate-limit by IP instead of by API key? IP-based limiting is a useful secondary control but insufficient alone — shared networks, mobile carriers, and CDNs make IP a poor proxy for identity. Always key your limits to the authenticated credential first.