Claude API Gateway for Multiple Projects: Setup Guide

If you're running more than one app, client, or internal tool on top of Claude, sharing a single Anthropic API key across all of them quickly becomes a liability. One project can burn through your rate limit, you can't tell which codebase generated which bill, and revoking access for one team means breaking everything else. A Claude API gateway for multiple projects solves this by sitting between your applications and Claude, issuing separate credentials per project while routing everything through one account.

The short answer: you need a layer that (1) issues distinct, revocable API keys per project or team, (2) tracks usage and cost per key, and (3) forwards requests to Claude without each app needing direct access to your master Anthropic credentials. You can build this yourself with a small proxy service, or use a managed gateway like SubToAPI that does it out of the box. The rest of this post covers both paths.

Why one shared Claude API key doesn't scale

A single key works fine for a side project. It breaks down once you have:

• Multiple products (a chatbot, an internal support tool, a client-facing integration) sharing the same Anthropic bill with no cost attribution.
• Multiple teams who each need their own quota, so one runaway script doesn't starve another team's traffic.
• Contractors or external partners who need temporary, scoped access you can kill instantly without affecting anyone else.
• Compliance or audit requirements where you need to know exactly which service made which call.

Mixing all of this under one ANTHROPIC_API_KEY means every project effectively has root access to your entire Claude spend.

What a Claude API gateway actually does

A gateway is a thin routing and policy layer. Instead of each project talking directly to Anthropic's endpoint, it talks to your gateway's endpoint with a project-scoped key. The gateway then:

1. Authenticates the incoming key and maps it to a project.
2. Applies that project's rate limits, model restrictions, or budget caps.
3. Forwards the request to Claude using the underlying account credentials.
4. Logs token usage, latency, and cost against that specific key.
5. Streams the response back unchanged.

From the application's point of view, it's calling an API that looks and behaves like Claude's — same request/response shape — but with per-project isolation baked in.

Option 1: build a minimal gateway yourself

If you only have two or three projects and want full control, a lightweight proxy is a reasonable DIY approach:

// gateway.js — minimal per-project Claude proxy
import express from "express";

const app = express();
app.use(express.json());

const PROJECT_KEYS = {
  "proj_marketing": { rpm: 20, model: "claude-3-5-sonnet-latest" },
  "proj_support":   { rpm: 60, model: "claude-3-5-haiku-latest" },
};

app.post("/v1/messages", async (req, res) => {
  const key = req.headers["x-project-key"];
  const project = PROJECT_KEYS[key];
  if (!project) return res.status(401).json({ error: "invalid project key" });

  // (rate limiting logic per project would go here)

  const response = 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, model: project.model }),
  });

  const data = await response.json();
  res.json(data);
});

app.listen(3000);

This works, but you quickly end up re-implementing things that are tedious to get right: per-key rate limiting, streaming support, retry/backoff on Anthropic's side, usage dashboards, and key rotation. It's fine for a prototype; it becomes real engineering work once you have more than a handful of projects or need team-level billing.

Option 2: use a managed API gateway

This is where a service like SubToAPI fits. It converts your existing Claude access into an HTTPS API with per-application keys (sub_live_...), so each project gets its own credential without you building a proxy from scratch.

A typical setup for multiple projects looks like this:

1. Sign up and connect your Claude access (/signup).
2. Create a separate application key for each project — marketing bot, support assistant, internal tooling — each one independently revocable.
3. Point each project at the same gateway endpoint, swapping only the key:

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

1. Check usage per key from the dashboard to see which project is driving cost.
2. Add teammates as seats instead of sharing raw keys — Solo is €9, Team is €19/seat, Scale is €49/seat, and every plan starts with a free trial.

Because requests go through the same gateway, you also get streaming, tool use, and usage metadata consistently across every project rather than reimplementing it per app. See /docs/quickstart, /docs/streaming, and /docs/tools for the specifics.

Structuring projects behind a gateway

Whether you build or buy, a few practices make a multi-project gateway manageable:

• One key per project, not per environment-and-project combo you can't track. Keep staging and production keys separate so a bug in staging can't blow through your production budget.
• Name keys by purpose, not by person. proj_support_prod survives a team reorg better than johns_key.
• Set per-project model defaults. A high-volume classification task should default to a cheaper, faster model; a customer-facing writing tool should default to a stronger one.
• Review usage weekly during rollout, then monthly once patterns stabilize. Spikes usually mean a retry loop or a prompt that's too long, not organic growth.
• Rotate keys when a project ends or a contractor's engagement finishes. Revoking one key should never require touching another project's credentials.

Choosing between DIY and managed

Build your own proxy if you have one or two projects, stable traffic, and in-house infra time to maintain it. Reach for a managed gateway once you're past that: three or more projects, multiple teams needing visibility into their own usage, or clients who need isolated, revocable access without you writing custom infrastructure. The full endpoint reference is at /docs/messages, and pricing details are at /pricing.

Questions

Does a Claude API gateway change how I call the Messages API? No. A well-built gateway mirrors Anthropic's request and response shape, so your existing prompts, streaming logic, and tool-use code work unchanged — you just swap the endpoint and the key.

Can I set different rate limits for different projects on the same gateway? Yes, that's one of the main reasons to use a gateway. Each project key can carry its own rate limit and model defaults so one project's traffic spike doesn't starve another's.

Is a gateway only useful for large teams? No. Even a solo developer running two or three side projects benefits from separate keys and per-project usage visibility — it's what makes it possible to know what each project actually costs before it becomes a problem.