Unified Gateway for Claude API, OpenAI and Anthropic

A "unified gateway" for Claude API, OpenAI, and Anthropic is an internal API layer that lets your application call different LLM providers through one consistent interface, instead of hardcoding separate SDKs, auth schemes, and request formats for each vendor. If you're searching for this, you're probably trying to solve one of two problems: you want to avoid vendor lock-in by abstracting provider differences, or you're already running both OpenAI and Claude in production and tired of maintaining duplicate integration code.

The short answer is that there's no single hosted product that merges OpenAI and Anthropic into one official API — each vendor ships its own SDK, auth model, and request schema. What teams actually build is a thin routing layer: a normalized internal schema, one adapter per provider, and a router that picks the right backend based on model name, cost, or availability. This article walks through that pattern and where a service like SubToAPI fits into the Anthropic side of it.

Why Teams Build a Unified Gateway

Running multiple model providers side by side is common for a few practical reasons:

• Fallback and redundancy. If one provider has an outage or rate-limits you, traffic can fail over to the other without changing application code.
• Cost and quality tradeoffs. Some tasks are cheaper or better suited to one model family; a gateway lets you route per-request.
• Avoiding lock-in. If your product logic talks to a stable internal schema instead of a vendor SDK, swapping or adding providers later is a config change, not a rewrite.
• Centralized observability. One place to log tokens, latency, and errors across every model you use.

The tradeoff is that you now own an abstraction layer, and that layer has to deal with real differences between OpenAI's Chat Completions format and Anthropic's Messages format.

The Core Problem: Schema and Auth Differences

OpenAI and Anthropic diverge in several places that matter for a gateway:

• Request shape. OpenAI uses a messages array with roles like system, user, assistant; Anthropic's Messages API separates system as a top-level field and structures content as typed blocks (text, tool_use, tool_result).
• Tool calling. Both support tool use, but the JSON shapes for tool definitions, invocations, and results are not identical.
• Streaming events. Both stream via server-sent events, but event names and payload structure differ.
• Auth and provisioning. OpenAI issues API keys per account; Anthropic's direct API requires its own billing setup, and if you're working from a Claude subscription rather than a pay-as-you-go API account, you don't get an API key at all by default.

That last point is where a lot of unified-gateway projects stall. People already paying for Claude want to reuse that access programmatically, but the subscription product and the API product are separate. This is the specific gap SubToAPI closes: it turns your existing Claude access into a standard HTTPS API with sub_live_... application keys, so the Anthropic side of your gateway has the same clean, key-based auth model as the OpenAI side.

A Practical Gateway Pattern

A minimal unified gateway normalizes requests to one internal shape, then dispatches to a provider-specific adapter. Conceptually:

async function callModel({ provider, model, messages, system, tools }) {
  if (provider === "openai") {
    return callOpenAI({ model, messages, tools });
  }
  if (provider === "anthropic") {
    return callClaude({ model, messages, system, tools });
  }
  throw new Error(`Unknown provider: ${provider}`);
}

The callClaude adapter, backed by SubToAPI, looks like a standard HTTPS call:

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,
    "system": "You are a routing-aware assistant.",
    "messages": [
      {"role": "user", "content": "Summarize this ticket in two sentences."}
    ]
  }'

The callOpenAI adapter hits OpenAI's Chat Completions endpoint with its own schema. Your router doesn't need to know the difference — it just picks a provider and model, and the adapters handle translation to and from your internal format.

For streaming, both providers support SSE, but your gateway should normalize the chunk events into one internal stream format before they reach your frontend. SubToAPI's streaming behaves like the standard Anthropic streaming contract, so the adapter mainly needs to parse content_block_delta events and re-emit them in your gateway's own shape — see /docs/streaming for the exact event sequence.

What a Gateway Needs From Each Provider

Regardless of how many providers you wire in, each one needs to supply a few baseline things for the gateway to be reliable:

• Stable API keys, not shared account credentials, so you can issue separate keys per service or environment.
• Usage metadata per request (input/output tokens) so your gateway can attribute cost per team, project, or customer.
• Tool use support if any of your routed models need function calling — see /docs/tools for how Claude's tool definitions map into a request.
• Team-level key management, since a gateway used by multiple services or multiple engineers needs per-seat keys rather than one shared secret.

SubToAPI covers these for the Claude side: issued application keys, per-request usage metadata, tool use, streaming, and team seats on the Team and Scale plans, all managed from one dashboard. That means your Anthropic adapter can be built exactly like your OpenAI adapter — point it at a base URL, send a Bearer token, parse a JSON or SSE response — without any subscription-specific plumbing.

Getting Started

If you're assembling this kind of gateway and the Anthropic leg is the missing piece, the fastest path is:

1. Sign up and generate a sub_live_... key at /signup.
2. Send your first request following /docs/quickstart.
3. Wire the Messages format into your router using /docs/messages as the schema reference.
4. Add streaming and tool use to your adapter once the base request works, using /docs/streaming and /docs/tools.
5. Compare plans at /pricing if you're provisioning keys for a team rather than a single project.

From there, the OpenAI side of your gateway is whatever adapter you already have, and the router logic — fallback, cost-based routing, model selection — stays entirely in your own code, independent of either vendor.

questions

Does a unified gateway mean one API key works for both OpenAI and Anthropic models? No. Each provider still requires its own key. A unified gateway means your application code talks to one internal interface, while separate adapters handle each provider's auth and schema underneath.

Can I use my Claude subscription instead of Anthropic's pay-as-you-go API for the Claude leg of my gateway? Not directly through Anthropic, but SubToAPI converts your existing Claude access into standard HTTPS API keys with streaming and tool use, so you can plug it into a gateway the same way you would any API-based provider.

What's the hardest part of building a unified gateway across providers? Normalizing request and response schemas, especially around tool calling and streaming events, since OpenAI and Anthropic structure both differently even though the underlying capability is similar.