Claude API Wrapper for Multiple LLM Providers: A Guide

What "multi-provider wrapper" actually means

A Claude API wrapper for multiple LLM providers is a thin abstraction layer that sits between your application code and the underlying model APIs (Claude, GPT-4, Gemini, Llama, etc.), normalizing requests and responses so your app doesn't have to know which provider it's talking to. You write one function signature — sendMessage(prompt, options) — and the wrapper translates it into the correct payload format for whichever provider is configured, then normalizes the response back into a shape your app already understands.

The reason people search for this is almost always one of three things: they want to avoid vendor lock-in, they want to A/B different models for cost or quality, or they're building a product that lets end users pick their own model. All three are solvable with the same basic pattern, and you don't need a heavyweight framework to do it — a well-designed wrapper is usually under 200 lines of code.

The core design pattern

Every multi-provider wrapper boils down to three pieces:

1. A normalized request shape — role-based messages, a system prompt, max tokens, temperature, and a tools array if you need function calling.
2. Provider adapters — one small module per provider that maps the normalized request into that provider's specific API format and maps the response back.
3. A router — picks which adapter to call, based on config, cost rules, or fallback logic.

Here's a minimal version of that pattern:

// adapters/claude.js
async function callClaude(messages, opts) {
  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: opts.model || "claude-3-5-sonnet",
      max_tokens: opts.maxTokens || 1024,
      messages
    })
  });
  const data = await res.json();
  return {
    text: data.content[0].text,
    usage: data.usage,
    provider: "claude"
  };
}

module.exports = { callClaude };

// adapters/openai.js
async function callOpenAI(messages, opts) {
  const res = await fetch("https://api.openai.com/v1/chat/completions", {
    method: "POST",
    headers: {
      "Authorization": `Bearer ${process.env.OPENAI_KEY}`,
      "Content-Type": "application/json"
    },
    body: JSON.stringify({
      model: opts.model || "gpt-4o",
      messages
    })
  });
  const data = await res.json();
  return {
    text: data.choices[0].message.content,
    usage: data.usage,
    provider: "openai"
  };
}

module.exports = { callOpenAI };

// router.js
const { callClaude } = require("./adapters/claude");
const { callOpenAI } = require("./adapters/openai");

async function complete(messages, opts = {}) {
  const provider = opts.provider || "claude";
  if (provider === "claude") return callClaude(messages, opts);
  if (provider === "openai") return callOpenAI(messages, opts);
  throw new Error(`Unknown provider: ${provider}`);
}

module.exports = { complete };

That's genuinely most of what multi-provider abstraction requires. The hard parts aren't the happy path — they're the edge cases below.

Where multi-provider wrappers actually get hard

Streaming formats differ. Claude sends server-sent events with content_block_delta chunks; other providers use different event names and delta shapes. If you normalize streaming, do it at the adapter level so your app only ever consumes one chunk format.

Tool/function calling isn't standardized. Claude's tool_use blocks, OpenAI's function calling, and other providers' approaches all encode arguments and results differently. Your normalized shape needs to pick a lowest-common-denominator format and have each adapter translate into and out of it.

Token counting and pricing aren't comparable. A "token" means something slightly different per tokenizer, and prices vary by an order of magnitude across providers. If cost-based routing is your goal, pull usage metadata from each response and track it centrally rather than trying to estimate upfront.

Rate limits and retry behavior differ per provider. Build retry/backoff logic once in the router, not duplicated in every adapter.

Auth and billing are separate per provider, which means separate API keys, separate dashboards, separate invoices — unless the Claude side of your stack is already consolidated.

Where SubToAPI fits

If your multi-provider wrapper includes Claude as one of its backends, you still need a clean, authenticated HTTPS endpoint to call. SubToAPI turns your existing Claude access into that endpoint: an application API key (sub_live_...), standard request/response shapes over HTTPS, streaming, tool use, and usage metadata per call — all the things your callClaude adapter needs without you having to manage raw provider credentials or billing separately.

This matters more as your wrapper grows: once you have five engineers calling Claude through the same router, you want team seats and a single dashboard for usage, not five people juggling separate keys. SubToAPI handles that part — team seats are built in on the Team (€19/seat) and Scale (€49/seat) plans, with a Solo plan at €9 for individual use and a free trial to test the integration before committing.

SubToAPI doesn't wrap other LLM providers — it's specifically the clean Claude layer. But that's exactly the piece you want underneath a multi-provider abstraction: a stable, well-documented adapter target so the rest of your routing logic doesn't have to think about Anthropic's raw API surface at all. Start with the quickstart and the Messages API reference to see the exact request/response shape your Claude adapter should target.

Build vs. buy

If you only need two or three providers and your routing logic is simple (manual provider selection, basic fallback), writing the ~150-line wrapper above is faster than adopting a framework. If you need dynamic routing based on cost, latency, or model capability across many providers, a dedicated LLM gateway product is worth evaluating — but even then, you'll still want each individual provider connection to be as clean as possible, which is the problem SubToAPI solves specifically for Claude.

Questions

Do I need a wrapper if I only use Claude? No — if Claude is your only provider, skip the abstraction layer and call the API directly. Wrappers earn their complexity only when you need to switch or compare providers.

Can SubToAPI route requests to non-Claude providers? No. SubToAPI is a Claude-specific API layer — it gives you clean HTTPS access, keys, and usage tracking for Claude. You'd use it as the Claude adapter inside your own multi-provider router.

What's the biggest gotcha when normalizing multiple providers? Tool/function calling. Each provider encodes tool definitions and results differently, and get this wrong and your fallback logic will silently break when switching models mid-conversation.