API Gateway for Anthropic and OpenAI: What You Need

If you're searching for an API gateway for Anthropic and OpenAI, you're probably trying to solve one of two problems: you want a single integration point for multiple LLM providers, or you have access to a model (often through a Claude Pro/Max subscription or an OpenAI consumer plan) and need a proper HTTPS API with keys, streaming, and usage tracking in front of it. Both problems have the same shape — you need a gateway layer that normalizes authentication, request/response formats, and observability across providers.

This article covers what an API gateway for Anthropic and OpenAI actually needs to do, the trade-offs between building one yourself and using a hosted service, and how to get a working setup in minutes rather than days.

What "API Gateway for Anthropic and OpenAI" Actually Means

Anthropic and OpenAI have different request shapes, different auth headers, different streaming formats (SSE event names differ), and different tool-calling schemas. If you're building a product that might switch providers, support both, or just want provider-agnostic observability, you need a layer that:

• Issues your own application-scoped API keys instead of handing out raw provider credentials
• Translates your request into the correct provider format
• Normalizes streaming so your frontend doesn't need provider-specific parsing logic
• Logs usage (tokens, cost, latency) per key, per user, or per team
• Enforces rate limits and quotas independent of the provider's own limits

Without this layer, every client application that needs LLM access ends up with a raw provider key baked in somewhere, no visibility into who used what, and brittle code that breaks when a provider changes their SDK.

Build vs. Hosted Gateway

There are two realistic paths.

Build your own gateway. A thin proxy service (often built on a reverse proxy or a small Node/Express or FastAPI app) that holds the real provider credentials and exposes your own endpoints. This gives you full control but means you own:

• Key issuance and revocation
• Rate limiting and quota enforcement
• Streaming proxying (SSE passthrough with reconnect handling)
• Usage metering and billing data
• Multi-tenant access control if you have teams or customers

This is a reasonable amount of work for a side project but turns into a maintenance burden once you have paying customers who expect uptime, audit logs, and seat management.

Use a hosted gateway. This is where a service like SubToAPI fits. It turns your existing Claude access into a clean HTTPS API: you get application API keys (sub_live_...), streaming, tool use, usage metadata, and team seats, all from one dashboard, without running your own proxy infrastructure. If your core need is Anthropic access specifically — not a multi-provider abstraction — a hosted single-provider gateway like this removes most of the operational work in one step.

What to Check Before Choosing a Gateway

Whether you build or buy, verify these before committing:

1. Streaming support. Does it proxy Server-Sent Events correctly, including partial token deltas? Test this with a long response and confirm you get incremental chunks, not a buffered dump at the end.
2. Tool/function calling passthrough. If your app uses tool use, confirm the gateway doesn't strip or reshape tool schemas.
3. Per-key usage visibility. You want to see token counts and cost per API key, not just an aggregate bill.
4. Team and seat management. If more than one person or service will use the gateway, you need scoped keys per team member, not one shared secret.
5. Latency overhead. A gateway adds a network hop. Measure it — it should be low double-digit milliseconds, not seconds.

Example: Calling Through a Gateway

Here's what a typical request through a gateway-style API looks like once you have an application key. This example uses SubToAPI's Messages endpoint:

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." }
    ]
  }'

And streaming, which is where a well-built gateway earns its keep:

const res = await fetch("https://api.subtoapi.app/v1/messages", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.SUBTOAPI_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    model: "claude-sonnet-4-5",
    max_tokens: 1024,
    stream: true,
    messages: [{ role: "user", content: "Write a haiku about deploys." }],
  }),
});

const reader = res.body.getReader();
const decoder = new TextDecoder();
while (true) {
  const { done, value } = await reader.read();
  if (done) break;
  console.log(decoder.decode(value));
}

The request and response shape is the same whether you're calling it from a serverless function, a backend service, or a client-side proxy — which is the whole point of putting a gateway in front of the raw provider API. See the quickstart, messages docs, and streaming docs for the full reference.

Multi-Provider Abstraction vs. Single-Provider Depth

If your actual goal is "route to whichever model is cheapest or fastest," you want a true multi-provider router with fallback logic — that's a different tool than what's described here, and it usually means managing both an Anthropic key and an OpenAI key behind your own routing rules.

If your actual goal is "get a reliable, metered HTTPS API in front of Claude without paying per-token API rates on top of my subscription," a dedicated Claude gateway covers that case more directly, with less surface area to maintain. Pricing is flat per seat (Solo €9, Team €19/seat, Scale €49/seat) rather than token-metered, which matters if your usage is bursty or hard to forecast.

Getting Started

1. Sign up and create a workspace.
2. Generate an application key.
3. Point your app at /v1/messages with that key instead of a raw provider credential.
4. Add streaming and tool use once the base integration works.

A free trial is available at /signup if you want to test the gateway against real traffic before committing to a plan.

questions

Do I need separate gateways for Anthropic and OpenAI, or can one tool handle both? A multi-provider gateway can route to both, but it adds complexity (two credential sets, two rate-limit models). If you only need Claude, a dedicated Claude API gateway is simpler to operate and reason about.

Does adding a gateway slow down streaming responses? A well-built proxy adds low single-digit to low double-digit milliseconds of overhead and passes through SSE chunks as they arrive, so perceived latency to the first token stays close to native.

Can I use a gateway if I only have a Claude subscription, not a direct API account? Yes — that's the specific use case SubToAPI solves: it converts existing Claude access into a standard HTTPS API with keys, streaming, and usage metadata without requiring a separate pay-per-token API account.