LLM Gateway for Multiple Providers: What You Need to Know

An LLM gateway for multiple providers is a single API layer that sits between your application and the various model providers you use — Claude, GPT, Gemini, open-source models — so your code talks to one consistent interface instead of juggling different SDKs, auth schemes, and response formats.

The short answer to "how do I build or choose one": you need a service (self-hosted or managed) that normalizes requests and responses across providers, handles API keys and routing, and gives you unified logging and usage data. Most teams end up needing this the moment they use more than one model provider in production, because each provider ships its own client library, its own streaming format, and its own way of reporting token usage.

Why teams reach for a gateway

If you start with a single provider, a direct SDK integration is fine. The problem shows up once you add a second model for redundancy, cost, or capability reasons. At that point you're maintaining:

• Separate authentication flows per provider
• Different request/response schemas (OpenAI's messages array isn't identical to Anthropic's)
• Different streaming protocols (SSE formats vary)
• Separate billing dashboards with no combined view of spend
• Provider-specific rate limit handling scattered across your codebase

A gateway collapses this into one contract. Your application code calls one endpoint, sends a reasonably standard payload, and the gateway translates it to whatever the underlying provider expects.

What a good multi-provider gateway actually needs

Not every "gateway" product does the same thing. When evaluating one, check for these specifics rather than trusting the marketing copy:

1. Consistent request/response shape. The gateway should accept a normalized message format and return normalized output, regardless of which provider handled the call. If you still have to write provider-specific parsing code downstream, the gateway isn't doing its job.

2. Streaming support. Token-by-token streaming behaves differently across providers. A real gateway exposes one streaming interface (typically server-sent events) that works the same way no matter which model is behind it.

3. Per-key scoping. You want application-level API keys, not shared provider credentials. If a key leaks or a project gets decommissioned, you revoke one key without touching the underlying provider account.

4. Usage and cost visibility. Token counts, request counts, and cost estimates per key, per project, per team member. Without this, "multi-provider" just means multiple places to check your bill.

5. Tool/function calling passthrough. If your app uses tool use, the gateway needs to pass tool definitions and tool results through correctly for each provider's format, not just plain text completions.

6. Team and seat management. Once more than one person or service touches the gateway, you need role-based keys and seat-level controls rather than one shared secret in an environment variable.

Build vs. buy

You can build a minimal gateway yourself: a thin proxy service that accepts your normalized schema, maps it to each provider's SDK, and forwards the response. This is reasonable if you only need two providers and don't need usage analytics or team features. The cost shows up in maintenance — every time a provider changes their API (new model versions, deprecated parameters, different streaming chunk formats), you're the one updating the translation layer.

A managed gateway trades that maintenance burden for a subscription, and typically adds the operational pieces (dashboards, key rotation, usage export) that are tedious to build well.

Where SubToAPI fits

SubToAPI isn't a multi-provider router — it does one thing specifically: it turns your existing Claude access into a clean HTTPS API with application keys (sub_live_...), streaming, tool use, and usage metadata. If Claude is one of the providers behind your gateway setup, SubToAPI is the piece that handles the Claude side cleanly, so you're not hand-rolling authentication and key management for that provider while your gateway layer routes traffic between Claude and whatever else you're using.

In practice, that looks like: your internal gateway or router decides which provider handles a given request, and for the Claude branch, it calls SubToAPI's endpoint the same way it would call any other provider's API.

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."}
    ]
  }'

Because it issues scoped application keys, you can hand one key to your gateway's Claude adapter and a different key to a staging environment, then track usage separately for each without managing raw account credentials in multiple places. See the quickstart for setup, messages for the request format, streaming for SSE details, and tools for function-calling support.

A minimal routing example

If you're wiring up your own lightweight gateway logic in front of two providers, the pattern usually looks like this:

async function callModel(provider, payload) {
  if (provider === "claude") {
    return fetch("https://api.subtoapi.app/v1/messages", {
      method: "POST",
      headers: {
        "Authorization": `Bearer ${process.env.SUBTOAPI_KEY}`,
        "Content-Type": "application/json"
      },
      body: JSON.stringify(payload)
    });
  }
  // other provider branches go here
}

The important part isn't the code — it's that each provider branch is isolated, so a change to one provider's API doesn't ripple through your whole application.

Getting started

If you're standardizing on Claude as one of your providers and want the API layer handled for you, start with a free trial and check pricing — plans start at €9/month for solo use, with team and scale tiers for shared keys and higher usage.

Questions

Does a multi-provider gateway slow down requests compared to calling providers directly? There's a small added hop, but for most applications the latency difference is negligible next to model inference time itself. The tradeoff is usually worth it for the consistency and visibility you gain.

Can I use a gateway with just one provider today and add more later? Yes. Starting with a normalized interface for a single provider means adding a second one later doesn't require rewriting your application code, only adding a new routing branch.

Does SubToAPI route between different LLM providers? No — SubToAPI specifically turns your Claude access into an API with keys, streaming, and tool use. If you need multi-provider routing, you'd pair it with your own router or gateway layer and use SubToAPI as the Claude endpoint.