How to Turn Claude Into a REST API Endpoint

If you want to call Claude from a mobile app, a frontend, a webhook, or an internal tool, you need something that looks like a normal REST API: a URL, an Authorization header, a JSON body, and predictable JSON (or streamed) responses. Claude itself is accessible through Anthropic's API, but that's built for direct developer access with a single account-level key — it's not designed to be handed out to multiple apps, teams, or client-side services with per-app rate limits and usage tracking.

Turning Claude into a REST API endpoint means putting a layer in front of it that does three things: issues its own API keys (one per app, per environment, per team member), forwards requests to Claude in the same request/response shape developers already expect, and gives you visibility into who used what. You can build this layer yourself with a small proxy server, or use a hosted service like SubToAPI that already does it. Below is how both approaches work.

What "REST API endpoint" actually requires

A proper REST endpoint for Claude needs more than just forwarding a POST request. At minimum:

• Its own auth scheme — API keys you can issue and revoke per application, not your raw Anthropic credentials embedded in a client.
• A stable request/response contract — JSON in, JSON out, with documented fields, independent of upstream changes.
• Streaming support — server-sent events or chunked responses for token-by-token output in chat UIs.
• Tool/function calling passthrough — if your app uses Claude's tool use, the endpoint needs to relay tool call requests and results correctly.
• Usage metadata — token counts and costs per request, ideally per API key, so you can attribute spend.
• Rate limiting and error handling — so one misbehaving client doesn't exhaust your whole quota.

Skip any of these and you end up rebuilding them later once you have more than one app or team member calling Claude.

Option 1: Build your own proxy

A minimal version is a small Express (or FastAPI, Go, etc.) server that accepts requests, validates your own API key, and forwards to Anthropic's API:

app.post("/v1/messages", async (req, res) => {
  const key = req.headers.authorization?.replace("Bearer ", "");
  if (!isValidAppKey(key)) return res.status(401).json({ error: "invalid key" });

  const upstream = await fetch("https://api.anthropic.com/v1/messages", {
    method: "POST",
    headers: {
      "x-api-key": process.env.ANTHROPIC_API_KEY,
      "anthropic-version": "2023-06-01",
      "content-type": "application/json",
    },
    body: JSON.stringify(req.body),
  });

  const data = await upstream.json();
  logUsage(key, data.usage);
  res.json(data);
});

This works for a prototype. The problems show up once you need it in production:

• Streaming requires you to pipe SSE chunks through correctly and handle client disconnects.
• Per-key rate limiting and usage tracking mean building a database, a dashboard, and revocation logic.
• Team seats mean auth roles, invites, and per-seat billing.
• You're now on-call for a piece of infrastructure that sits between every one of your apps and Claude.

If you only need this for a single internal script, build it. If you need it across multiple apps or a team, it's usually cheaper to not build it.

Option 2: Use a hosted layer

SubToAPI turns your existing Claude access into an HTTPS endpoint without you running any infrastructure. You get application API keys (sub_live_...) instead of sharing raw credentials, and every request goes through the same REST contract:

curl https://api.subtoapi.app/v1/messages \
  -H "Authorization: Bearer $SUBTOAPI_KEY" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-sonnet-4-5",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "Summarize this changelog in 3 bullets"}
    ]
  }'

The response is standard JSON with the message content and usage data attached, so you can log token counts per API key without building that yourself. Streaming works the same way — set "stream": true and read the SSE chunks as they arrive, covered in the streaming docs. Tool use requests pass through the same endpoint and are documented in the tools guide.

Because keys are scoped per application, you can issue one for your production app, one for staging, and one for each internal script, then revoke any of them individually if a key leaks — without touching the others. Team plans add seats so each developer gets their own key under one dashboard, which matters once more than one person is calling Claude from your codebase.

Choosing between the two

Build your own proxy if:

• You only need one client calling Claude.
• You already run infrastructure for API gateways and don't mind maintaining another one.
• You need custom logic (e.g., request rewriting) that a generic layer wouldn't support.

Use a hosted endpoint if:

• You want multiple apps or environments each with their own key today.
• You want usage and cost visibility without building a logging pipeline.
• You'd rather ship your product than maintain a proxy.

Getting started with SubToAPI takes about the same time as writing the minimal proxy above: sign up, grab a key, and send your first request. The quickstart walks through creating a key and making your first call, and the pricing page covers the Solo, Team, and Scale plans if you need multiple seats later. There's a free trial at signup if you want to test it against your existing integration first.

Practical checklist

Before you wire Claude into your app as a REST endpoint, confirm you have:

1. A key scheme that lets you issue and revoke keys per app.
2. Streaming support if your UI shows tokens as they arrive.
3. Tool use passthrough if your app relies on function calling.
4. Usage metadata per key so you can track cost by app or team member.
5. A documented request/response format, per the Messages API docs, so you're not reverse-engineering behavior later.

Whether you build this yourself or use SubToAPI, the goal is the same: a stable, authenticated, well-documented HTTP endpoint that any client — mobile, web, or backend script — can call without knowing anything about how Claude itself works underneath.

FAQ

Is there an official Claude REST API I can just call directly? Yes — Anthropic provides a direct API, but it's meant for single-account developer access with one shared key. It doesn't give you per-app keys, seat management, or a dashboard for tracking usage across a team, which is why people put a layer like SubToAPI in front of it.

Do I need to change my code if I switch from a custom proxy to a hosted one? Minimal changes. If your proxy already mirrors the standard Messages API request/response shape, switching to SubToAPI mostly means updating the base URL and using an sub_live_... key instead of your own.

Can I still use streaming and tool calls through a REST wrapper? Yes. A properly built wrapper passes through SSE streaming and tool use requests unchanged — see the streaming and tools docs for the exact request format.