API Gateway for Anthropic and OpenAI Models

What an API gateway for Anthropic and OpenAI models actually does

If you're searching for this, you likely already have access to Claude, GPT models, or both, and you need a single layer that sits between your application code and those providers. An API gateway for Anthropic and OpenAI models handles authentication, request routing, rate limiting, logging, and often format translation, so your app doesn't need separate SDKs, separate key management, and separate error-handling logic for each provider.

The short answer: you either build this yourself with a thin proxy service, or you use a hosted gateway that already normalizes the differences between Claude's Messages API and OpenAI's Chat Completions API. Both are valid depending on your team size and how much time you want to spend maintaining infrastructure that isn't your product.

Why teams end up needing a gateway

Most teams don't start here. They integrate directly with one provider's SDK, ship a feature, and move on. The gateway need shows up later, usually from one of these triggers:

• Multiple provider keys scattered across services. Each microservice or environment has its own ANTHROPIC_API_KEY or OPENAI_API_KEY, with no central place to rotate, revoke, or audit them.
• Cost and usage visibility problems. Finance asks "how much are we spending on LLM calls per team or per feature" and nobody has a clean answer because usage is buried in provider dashboards.
• Need for per-application keys. You want to issue a scoped key to a specific app or customer integration without handing out your root provider credentials.
• Reliability concerns. A single provider outage or rate limit shouldn't take down your whole product if you have access to more than one model provider.

A gateway addresses all four by putting one authenticated HTTP endpoint between your code and the providers, with its own key format, logging, and limits.

Build vs. buy

Building your own proxy

A basic version is a small HTTP service that:

1. Accepts requests with your own internal API key
2. Maps the request to the correct provider's request shape
3. Forwards it with the real provider credentials attached server-side
4. Streams or returns the response, optionally translating the format back

This is reasonable for a weekend project and gives you full control. The maintenance cost shows up later: provider APIs change their streaming formats, tool-calling schemas, and error codes independently, and you're now responsible for tracking both. If you're a two-person team, that's a meaningful ongoing cost against a problem that isn't your core product.

Using a hosted gateway

A hosted gateway trades that ongoing maintenance for a subscription. For Claude specifically, SubToAPI turns your existing Claude access into a standard HTTPS API: you get application-scoped keys (sub_live_...), streaming support, tool use, usage metadata per key, and team seats in one dashboard. Instead of writing a proxy, you call one endpoint with a bearer token.

Example request against the Messages-compatible endpoint:

curl https://api.subtoapi.app/v1/messages \
  -H "Authorization: Bearer $SUBTOAPI_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "Summarize this ticket in two sentences."}
    ]
  }'

The same pattern applies to streaming and tool calls — see the streaming docs and tools docs for request shapes. If you're running a product that calls both Claude and OpenAI models, you'd pair this kind of gateway on the Anthropic side with whatever you're using for OpenAI, or route both through a gateway layer you write that calls out to each provider's dedicated endpoint.

What to look for in a gateway, regardless of provider

Whether you build or buy, these are the properties that actually matter in production:

• Scoped, revocable keys. You should be able to issue a key per application or per environment and kill it without touching other integrations.
• Streaming support that matches the underlying provider. Token-by-token streaming shouldn't be an afterthought bolted on later; it changes how you structure your client code.
• Usage metadata per request. Token counts and latency per call, attributable to a specific key, are what let you actually answer "what is this costing us" without guessing.
• Team-level access control. If more than one person deploys against the gateway, you need seats and permissions, not a shared .env file passed around in Slack.
• Predictable pricing. Per-seat or flat subscription pricing is easier to budget against than trying to forecast token volume months ahead.

A practical migration path

If you're moving from direct provider SDK calls to a gateway, do it incrementally:

1. Pick one service or endpoint that calls Claude or OpenAI directly.
2. Swap the base URL and auth header for the gateway, keeping the request body shape the same where possible.
3. Confirm streaming and error handling behave the same way under load.
4. Roll the change out to the rest of your services once you trust the integration.

For Claude specifically, the quickstart and Messages API reference walk through this exact swap — most teams get a working request within a few minutes of generating a key from signup.

Pricing shape

Hosted gateways typically charge per seat rather than per token, since the token cost still flows through to the underlying provider. SubToAPI's plans run Solo at €9 for individual use, Team at €19/seat for small teams needing shared dashboards and key management, and Scale at €49/seat for larger usage with more seats and oversight. Full details are on the pricing page, and every plan starts with a free trial so you can test the integration against your real workload before committing.

Questions

Does an API gateway for Anthropic and OpenAI models change the response format? A good gateway keeps the response shape close to the underlying provider's native format so existing client code needs minimal changes. Full translation between formats (e.g., Claude's Messages shape to OpenAI's Chat Completions shape) is a separate, more invasive feature — check whether you actually need that or just need unified auth and logging.

Can I use one gateway key across multiple applications? You can, but it defeats one of the main benefits: scoped revocation. Issue separate keys per application or environment so you can rotate or kill access to one integration without affecting others.

Is a gateway necessary if I only use one provider? Even with a single provider, a gateway is useful once you have more than one application or team member making calls — it centralizes key management, usage visibility, and rate limiting that otherwise gets duplicated across services.