Best LLM Gateway in 2025: How to Choose One

If you're searching for the "best LLM gateway," you're probably trying to solve one of two problems: you need a single API layer in front of multiple model providers, or you already have access to a model (through a subscription, a team plan, or a specific provider) and you want to turn it into a proper HTTPS API your app can call. There isn't one universal "best" gateway — the right answer depends on which problem you actually have. This article breaks down the criteria that matter and where different types of gateways fit.

What an LLM gateway actually needs to do

Strip away the marketing and an LLM gateway has a short list of real jobs:

• Authentication — issue scoped API keys instead of sharing raw provider credentials
• A stable request/response format — so your code doesn't break when a provider changes something upstream
• Streaming support — token-by-token output over SSE or chunked responses
• Tool/function calling — passing tool definitions and getting structured calls back
• Usage visibility — knowing which key, team, or feature consumed how many tokens
• Team access control — seats, roles, and separate keys per environment or developer

A gateway that does all six well is more useful than one that adds a dozen extra features but is shaky on the basics. When you're evaluating options, test each of these directly rather than trusting a feature list.

The two kinds of "LLM gateway"

This is where a lot of confusion comes from, and it's worth being explicit about it.

Multi-provider routers. These sit in front of several LLM providers (OpenAI, Anthropic, open models, etc.) and let you switch or load-balance between them through one interface. They're useful if you genuinely need provider redundancy or want to A/B test models. The tradeoff is that you're managing separate billing and rate limits with each underlying provider, and the gateway itself is another piece of infrastructure to run or pay for.

Access-to-API converters. These take an existing plan or subscription you already have — for example, Claude access — and expose it as a clean application API: a sub_live_... key, JSON in and out, streaming, tool calling. You're not adding a new provider relationship; you're making the one you already have programmable. This is a narrower job than a full multi-provider router, but for a lot of teams it's the actual problem they have: "I have Claude, I want to call it from my backend with proper keys and usage tracking, without wiring OAuth or session tokens into my app."

If your goal is provider redundancy across many models, look at multi-provider routers. If your goal is making one subscription usable as an API for a team or a product, a converter like SubToAPI is a more direct fit, and it's simpler to reason about since there's only one upstream to think about.

Evaluation checklist

Whatever you're comparing, run through this list before committing:

1. Does it support streaming out of the box? Test it with a long response and check time-to-first-token, not just total latency.
2. Can it handle tool use / function calling in the same shape your existing code expects, or will you need a translation layer?
3. How are API keys scoped? Per-project keys, per-environment keys, and easy revocation matter more once you have more than one developer touching the system.
4. What does usage reporting actually show you? Token counts per key are the minimum; per-team or per-feature breakdowns save you from guessing where cost is going.
5. What happens when a request fails or times out? Look at retry behavior and error payloads, not just the happy path.
6. Pricing model — per-seat, per-token, or flat fee, and whether it matches how your team is actually structured.

A concrete example: calling a gateway

Regardless of which gateway you pick, the integration pattern usually looks similar. Here's what it looks like against SubToAPI, which converts an existing Claude subscription into an application 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": 500,
    "messages": [
      {"role": "user", "content": "Summarize this changelog in three bullets."}
    ]
  }'

And streaming from Node:

const res = await fetch("https://api.subtoapi.app/v1/messages", {
  method: "POST",
  headers: {
    Authorization: `Bearer ${process.env.SUBTOAPI_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    model: "claude-sonnet-4",
    max_tokens: 500,
    stream: true,
    messages: [{ role: "user", content: "Draft a release note." }],
  }),
});

for await (const chunk of res.body) {
  process.stdout.write(chunk.toString());
}

Notice there's no OAuth dance, no session cookies, no browser automation — just a bearer key and JSON, which is the whole point of putting a gateway in front of your model access in the first place.

Where SubToAPI fits

SubToAPI is built specifically for the second category above: turning your existing Claude access into a proper API rather than routing across multiple providers. It gives you application keys (sub_live_...), streaming, tool use, usage metadata per key, and team seats in one dashboard. Plans start at €9 for Solo, €19/seat for Team, and €49/seat for Scale, with a free trial at signup — see /pricing for details. If you want to see the exact request/response shapes, /docs/messages and /docs/streaming cover the core endpoints, and /docs/tools covers function calling. The /docs/quickstart page gets you from signup to a working key in a few minutes.

If what you need is a multi-provider router, that's a different tool and a different evaluation. But if the actual problem is "I have Claude access and I want to call it like an API," that's a narrower, more solvable problem — and worth not overcomplicating with infrastructure meant for a different use case.

Questions

Is a multi-provider router always better than a single-provider gateway? No. Routers add value when you genuinely need to switch between models or providers. If you only use one provider, a router adds complexity — separate rate limits, inconsistent tool-calling behavior across models — without a corresponding benefit.

Do I need a gateway if I already have direct API access from my provider? If your provider already gives you application API keys, usage dashboards, and team seats, you may not need one. Gateways add value when your existing access is subscription- or session-based and you need to turn it into something your backend can call programmatically.

What's the minimum feature set to look for? Stable request/response formatting, real streaming support, and per-key usage visibility. Everything else — tool calling, team seats, retries — matters, but these three determine whether the gateway is usable at all in production.