Unified LLM API Gateway for Multiple Providers

A unified LLM API gateway is a single HTTP endpoint your application talks to, which then routes requests to one or more underlying model providers — Claude, GPT, Gemini, open-weight models — behind a consistent request/response shape. Instead of writing separate client code for each provider's SDK, auth scheme, and streaming format, your app calls one API, and the gateway handles translation, routing, retries, and usage tracking.

If you're searching for this, you're probably past the "call one model's SDK directly" stage. You have multiple teams or products using different models, you want fallback when a provider has an outage, or you're tired of every service in your stack hardcoding a specific vendor's SDK. The rest of this article covers what a gateway actually needs to do, a minimal architecture you can build yourself, and where a managed layer like SubToAPI fits into the picture for the Claude side of that stack.

What "Unified" Actually Means in Practice

A gateway that's genuinely unified does more than proxy requests. At minimum it needs to normalize:

• Request schema — one message format (role/content arrays, system prompts, tool definitions) that gets translated into each provider's native shape before the call
• Response schema — consistent fields for text, tool calls, stop reason, and token usage regardless of which model answered
• Streaming — server-sent events or chunked responses in one format, even if the underlying provider streams differently
• Auth — a single API key scheme for your own clients, with provider credentials stored once, server-side
• Errors — mapped to a consistent set of HTTP status codes instead of each provider's own error taxonomy

Without that normalization layer, you don't have a gateway — you have several SDKs wired into the same codebase, which is what you were trying to avoid.

Why Teams Build One

The usual triggers for building a multi-provider gateway:

• Vendor risk — a single provider outage shouldn't take your product down if a fallback model can serve degraded answers
• Cost routing — cheaper or faster models for low-stakes tasks, stronger models for complex ones, decided per-request
• Team sprawl — five engineering teams each hardcoding provider SDKs means five places to update when an API changes
• Governance — centralized logging, rate limiting, and spend visibility across every model call in the company, not scattered across services

None of these require a third-party product — they're solvable with a reasonably sized internal service. The question is whether building and maintaining that service is worth the engineering time versus the alternative.

A Minimal Gateway Architecture

A working gateway is smaller than people expect. The core is a router function that picks a provider based on request metadata, and a set of adapters that translate to/from each provider's native format.

// gateway/route.js
const PROVIDERS = {
  claude: { base: "https://api.subtoapi.app/v1/messages", keyEnv: "SUBTOAPI_KEY" },
  openai: { base: "https://api.openai.com/v1/chat/completions", keyEnv: "OPENAI_KEY" },
};

async function callProvider(provider, payload) {
  const cfg = PROVIDERS[provider];
  const res = await fetch(cfg.base, {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env[cfg.keyEnv]}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify(payload),
  });
  if (!res.ok) throw new Error(`${provider} error: ${res.status}`);
  return res.json();
}

async function route(request) {
  const provider = request.preferredModel ?? pickProvider(request);
  try {
    return await callProvider(provider, toProviderFormat(provider, request));
  } catch (err) {
    const fallback = provider === "claude" ? "openai" : "claude";
    return await callProvider(fallback, toProviderFormat(fallback, request));
  }
}

This is deliberately simplified — toProviderFormat and pickProvider carry most of the real complexity (message translation, tool schema mapping, token budgeting), but the shape of the problem is this: one entrypoint, pluggable adapters, fallback logic at the routing layer, not scattered through application code.

Where a Provider-Specific Layer Fits

Building the router is the easy part. The harder, more tedious part is getting a clean, production-grade HTTPS interface out of each individual provider — API keys you can issue per application, streaming that doesn't drop chunks, tool use handled correctly, and usage metadata you can actually bill against.

This is the gap SubToAPI fills for the Claude side of a multi-provider setup. It turns your existing Claude access into a proper API: scoped sub_live_... keys per application instead of one shared credential, streaming and tool use that match the normal Messages format, and usage metadata attached to every response so you can track cost per key or per team without parsing logs yourself. You point your gateway's Claude adapter at SubToAPI instead of managing raw provider credentials directly, and you get key rotation, seat-based access, and a dashboard for free.

curl https://api.subtoapi.app/v1/messages \
  -H "Authorization: Bearer $SUBTOAPI_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4-5",
    "max_tokens": 1024,
    "messages": [{"role": "user", "content": "Summarize this ticket."}]
  }'

This doesn't replace the routing layer that spans multiple vendors — SubToAPI is specifically the Claude leg of that stack — but it removes a chunk of the operational work (key management, per-app scoping, usage tracking) that you'd otherwise have to build for every provider node in your gateway. See the quickstart and Messages API reference for the full request shape, or check pricing if you're evaluating it against the cost of building that layer yourself.

Operational Concerns Beyond Routing

A gateway that only routes requests is half-finished. Things worth planning for before you ship one internally:

• Per-key rate limits so one noisy application can't starve others sharing the gateway
• Timeout and retry budgets that differ by provider, since latency profiles vary significantly
• Usage attribution down to the application or team level, not just total spend
• Schema versioning for your unified format, since provider APIs change independently of each other

Most teams underestimate the second and third items until a cost spike or an outage forces the issue.

Questions

Does a unified gateway add noticeable latency? A well-built one adds low single-digit milliseconds for routing logic — negligible next to model inference time. The risk is a poorly designed adapter layer that does unnecessary serialization or extra network hops.

Should I build this myself or use a managed product? If you only need one provider, a managed API layer like SubToAPI for Claude is faster to ship than a custom gateway. If you genuinely need multiple providers behind one interface, you'll likely build a thin router and use provider-specific managed layers underneath it for the pieces you don't want to maintain.

Can I switch providers without changing application code? Only if your normalized request/response schema is strict and your adapters are kept in sync with provider API changes. This is the main ongoing maintenance cost of running a gateway, regardless of how many providers it supports.