A Unified API for Claude and OpenAI Models

What "unified API" actually means here

If you're searching for a unified API for Claude and OpenAI models, you're probably trying to solve one of two problems: you want to call both providers from a single code path without maintaining two SDKs and two auth schemes, or you want to hand a single API key to internal teams and let them pick whichever model fits their task. Either way, the answer is the same — you need a thin abstraction layer that normalizes requests and responses across providers, so your application code doesn't care which model actually generates the completion.

There are two ways to get this: build a small internal proxy that translates your app's calls into each provider's native format, or use a hosted gateway that already does this and gives you one API key, one billing surface, and one set of logs. Both are valid depending on your team's size and how much time you want to spend on plumbing instead of product.

Why teams end up needing this

A single-provider integration works fine until one of these happens:

• Model-specific strengths. Claude tends to be stronger on long-context reasoning and instruction-following in agentic workflows; some OpenAI models are cheaper or faster for short, high-volume tasks. Teams want to route by task, not by whichever SDK they happened to install first.
• Vendor risk. Rate limits, pricing changes, or outages on one provider shouldn't take your whole product down. A unified layer lets you fail over or A/B test providers without touching business logic.
• Procurement and access. Getting direct API access approved with one vendor can be slower than others, and teams don't want to block a launch on a single approval queue.
• Billing sprawl. Two providers means two invoices, two usage dashboards, and two sets of API keys to rotate. Consolidating that into one dashboard is a real operational win, not just a nicety.

The two architectures

1. You build the proxy

This is a small backend service — often just an Express or FastAPI app — that exposes one endpoint like /v1/chat and internally maps the request to either Anthropic's Messages API or OpenAI's Chat Completions API based on a model or provider field. You own:

• Request/response schema translation (message roles, system prompts, tool schemas differ between providers)
• Streaming normalization (both use server-sent events but with different chunk formats)
• Auth and key storage for both upstream providers
• Usage logging and cost attribution per team or app

This is the right call if you have very specific routing logic (e.g., cost-based failover, custom retry policies) and engineering time to maintain it as both providers evolve their APIs.

2. You use a hosted gateway

A hosted gateway does the translation for you and gives your team a single API key to distribute. SubToAPI takes this approach for Claude specifically: it turns your existing Claude access into a clean HTTPS API with application-scoped keys (sub_live_...), so instead of managing raw provider credentials across your codebase, every service or team member gets its own key, scoped and revocable independently. That solves the "one API surface, many consumers" half of the unified-API problem even before you add a second model provider into the mix.

The tradeoff with any hosted layer, including this one, is that it doesn't erase the underlying differences between models — Claude's tool-use format and OpenAI's function-calling format are not identical — but it does mean you're not building and maintaining the plumbing yourself, and you get streaming, usage metadata, and team seat management out of the box.

A minimal internal router, if you build your own

If you're rolling your own unified layer, keep the interface intentionally small. Here's the shape most teams converge on:

async function generate({ provider, model, messages, stream = false }) {
  if (provider === "anthropic") {
    return fetch("https://api.subtoapi.app/v1/messages", {
      method: "POST",
      headers: {
        "Authorization": `Bearer ${process.env.SUBTOAPI_KEY}`,
        "Content-Type": "application/json"
      },
      body: JSON.stringify({ model, messages, stream, max_tokens: 1024 })
    });
  }

  if (provider === "openai") {
    return fetch("https://api.openai.com/v1/chat/completions", {
      method: "POST",
      headers: {
        "Authorization": `Bearer ${process.env.OPENAI_KEY}`,
        "Content-Type": "application/json"
      },
      body: JSON.stringify({ model, messages, stream })
    });
  }

  throw new Error(`Unknown provider: ${provider}`);
}

This is deliberately naive — it doesn't normalize the response shape yet — but it's the right starting point. Add a thin response mapper on top that extracts content, usage, and stop_reason into a common object your app consumes, and you've got a working unified interface without a heavy framework.

For the Claude side specifically, using an application key through /docs/quickstart instead of a raw Anthropic key means you don't have to build separate key rotation and scoping logic for that provider — you get it as part of the dashboard.

What to normalize (and what not to bother with)

Focus your abstraction effort on:

• Message format — role names, system prompt placement, multi-turn history
• Streaming events — map both providers' chunk formats to one event type your frontend consumes (see /docs/streaming for how this looks on the Claude side)
• Tool/function calling — the schemas differ enough that this is usually the most work; see /docs/tools for the Claude tool-use format
• Usage and cost fields — token counts and pricing units aren't identical across providers, so normalize to a common input_tokens / output_tokens shape early

Don't bother trying to make model behavior identical — prompts that work well on one model often need retuning on the other. A unified API should abstract the transport and format, not the model's actual output.

Getting started

If your immediate need is consolidating Claude access across a team — separate keys per service, usage visibility, streaming, and tool use — that's a narrower problem than a full multi-provider gateway, and it's the one SubToAPI is built for. You can add OpenAI as a second provider in your own routing layer once the Claude side is standardized. Check /pricing for plan details or start with /signup to get an application key in a few minutes.

questions

Do I need a unified API if I only use one model provider today? Not immediately, but if you expect to add a second provider or need per-team key management and usage tracking, building the abstraction early is cheaper than retrofitting it later.

Can I normalize streaming responses across Claude and OpenAI? Yes — both use SSE-based streaming, but the chunk formats differ. You'll need a small mapping layer to convert each into a common event shape your frontend can consume regardless of provider.

Does a unified API layer add latency? A well-built proxy adds low single-digit milliseconds of overhead. The bigger latency factor is always the underlying model's response time, not the routing layer.