A Unified API Gateway for Multiple LLM Providers

A unified API gateway for multiple LLM providers is a single HTTP layer that sits between your application and the model backends you actually use — Claude, GPT, Gemini, open-weight models behind your own inference server, and so on. Instead of writing separate client code, separate auth handling, and separate retry logic for each provider, your app talks to one endpoint and one request/response shape, and the gateway deals with the differences underneath.

Most teams reach for this once they're running more than one model in production: one provider for long-context reasoning, another for cheap classification, a third as a failover when rate limits hit. Without a gateway, that usually means duplicated SDK code, scattered API keys, inconsistent streaming formats, and no single place to see what you're spending across providers. A gateway fixes that by normalizing the interface and centralizing the operational stuff — auth, logging, cost tracking, team access — in one place.

What the gateway actually needs to do

A gateway that's worth building (or paying for) handles more than just forwarding requests. The core responsibilities are:

• Auth normalization — one API key scheme for your app, mapped internally to whatever each provider requires
• Request/response translation — a consistent message format (system/user/assistant roles, tool calls, stop reasons) regardless of which provider handled the call
• Streaming — server-sent events or chunked responses in a single format your frontend can consume the same way every time
• Tool/function calling — a shared schema for defining tools and receiving tool_use blocks, even if each provider's native tool format differs
• Usage and cost metadata — token counts and spend attached to every response so you can bill internally or just monitor burn rate
• Rate limit and failover logic — retries, backoff, and optionally routing to a secondary provider when the primary is throttled
• Team and key management — per-application or per-developer keys, scoped and revocable, without sharing one root credential

If you only need two of these, you might not need a full gateway yet — a thin wrapper function might do. But once you have multiple teams, multiple apps, or multiple providers in the mix, the lack of a shared layer starts showing up as inconsistent error handling and nobody being sure which key is burning through quota.

Build vs. buy

Building your own gateway is straightforward in theory: a router that inspects a provider field, calls the right SDK, and reshapes the response. In practice it gets complicated fast — streaming formats differ in annoying ways, tool-calling schemas aren't compatible, and usage metadata is reported differently by every provider. Maintaining that translation layer as providers update their APIs is ongoing work, not a one-time project.

The alternative is to use a managed API layer for the pieces that are slow to build yourself, and keep your own router thin. For the Claude side specifically, SubToAPI does exactly this: it turns your existing Claude access into a clean HTTPS API with application-scoped keys (sub_live_...), streaming, tool use, and usage metadata built in, plus team seats so you're not sharing one credential across a whole engineering org.

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."}
    ]
  }'

If you're assembling a multi-provider gateway, this is the kind of response shape you want every node behind it to return — predictable fields, usage data attached, no provider-specific quirks leaking through. You can then put your own thin router in front of SubToAPI and whatever other provider endpoints you use:

async function callLLM(provider, payload) {
  const endpoints = {
    claude: "https://api.subtoapi.app/v1/messages",
    // other providers go here
  };

  const res = await fetch(endpoints[provider], {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.SUBTOAPI_KEY}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify(payload),
  });

  if (!res.ok) {
    throw new Error(`Provider ${provider} failed: ${res.status}`);
  }
  return res.json();
}

This pattern — one normalized node per provider, a thin router on top — is cheaper to maintain than writing a custom gateway from scratch, because the hard parts (streaming, tool schemas, key scoping, usage tracking) are already solved for the Claude node. You still own the routing and fallback logic, but that's a small, stable piece of code compared to reimplementing provider clients.

Practical tips for running one

A few things that matter once the gateway is in production:

• Log at the gateway, not in each app. Every request/response pair, latency, and token count should land in one place so you can debug and audit without hunting through service logs.
• Scope keys per application, not per developer. Rotating or revoking a compromised key shouldn't mean rotating credentials for your whole team.
• Keep streaming consistent. If one provider streams differently than another, normalize it at the gateway — don't push that complexity into frontend code. See streaming for how this looks in practice.
• Treat tool calling as a shared contract. Define your tool schemas once and translate them per provider at the gateway layer, not in application code. Details are in the tools docs.
• Track cost per app, not just per account. Usage metadata attached to every response makes it possible to attribute spend without a separate billing integration.

Getting started with a single provider through a gateway-style API is the easiest way to validate the pattern before adding more providers. You can get a working integration up in a few minutes — the quickstart covers the full request/response cycle, and pricing lays out the plans if you're evaluating for a team.

Questions

Does a unified gateway add latency compared to calling providers directly? It adds one network hop, which is typically a few milliseconds — negligible compared to LLM inference time. The operational benefits (centralized keys, consistent logging, easier failover) almost always outweigh this.

Can I use a unified gateway with just one provider for now and add more later? Yes, and it's a reasonable way to start. Normalize your request/response handling around one provider's gateway API first, then add a router and a second provider node once you actually need multi-provider routing.

Does SubToAPI support multiple LLM providers? No — SubToAPI is focused on turning your Claude access into a clean, team-ready HTTPS API. If you need a true multi-provider gateway, you'd use SubToAPI as the normalized Claude node behind your own routing layer. Sign up at /signup to try it.