Claude Sonnet 5 appeared on OpenRouter the day it launched, listed as anthropic/claude-sonnet-5. Using it through OpenRouter gives you one API key for many models, easy provider fallback, and simple model routing. This guide gets you set up.

New to the platform? See our OpenRouter complete guide first.

Why use OpenRouter for Sonnet 5

• One key, many models. Call Sonnet 5, Opus 4.8, and non-Claude models through a single endpoint.
• Fallback. If one provider has an outage or rate limit, route to another automatically.
• Easy comparison. Swap models by changing a string, which is handy for testing Sonnet 5 against alternatives.

Step 1: Get an OpenRouter API key

1. Create an account at openrouter.ai.
2. Add credits or connect billing.
3. Generate an API key and store it in an environment variable:

export OPENROUTER_API_KEY="your-key-here"

Keep the key out of your code. See secure AI API keys.

Step 2: Call Sonnet 5

OpenRouter uses an OpenAI-compatible API, so most SDKs work with a base URL change. In Python:

from openai import OpenAI

client = OpenAI(
    base_url="https://openrouter.ai/api/v1",
    api_key="your-openrouter-key",
)

completion = client.chat.completions.create(
    model="anthropic/claude-sonnet-5",
    messages=[{"role": "user", "content": "Write a Python function to debounce calls."}],
)

print(completion.choices[0].message.content)

The model string is anthropic/claude-sonnet-5.

Step 3: Add provider fallback

OpenRouter lets you specify fallback models so a request still succeeds if your first choice is unavailable. A sensible pattern is to fall back from Sonnet 5 to another capable model, or to route the hardest tasks to Opus 4.8. This is the core value of a router for production reliability.

Step 4: Route by task

Because switching models is just a string change, you can route routine work to Sonnet 5 and escalate hard tasks to anthropic/claude-opus-4-8. This mirrors the two-model strategy in our Sonnet 5 vs Opus 4.8 comparison without managing two providers.

Pricing note

OpenRouter passes through model pricing plus a small fee. Sonnet 5’s introductory rate is $2 input and $10 output per million tokens. Remember the new tokenizer can raise effective token counts by up to 1.35 times; see pricing explained. For native access without the router fee, use the Sonnet 5 API directly.

Setting up fallback in practice

OpenRouter lets you pass a list of models so a request still succeeds if your first choice is rate-limited or unavailable. Conceptually you specify a primary and one or more fallbacks. A sensible production setup makes Sonnet 5 the primary and a comparable model the backup, so an Anthropic capacity issue does not take your feature down. This reliability is the main reason teams put a router in front of a single provider, and it is hard to replicate with the native API alone.

Routing by task difficulty

Because changing models is just a string change, you can build simple routing logic in your application:

• Route routine requests to anthropic/claude-sonnet-5 at low or medium effort.
• Route the hardest requests to anthropic/claude-opus-4-8.
• Optionally route bulk, cost-sensitive work to a cheaper third model.

This mirrors the two-model strategy from our Sonnet 5 vs Opus 4.8 comparison, but managed in one place through a single key and bill.

Watching your costs

OpenRouter shows per-request cost in its dashboard, which makes it easy to spot expensive workflows. Keep two things in mind. First, OpenRouter adds a small fee on top of model pricing, so for steady single-provider use the native Sonnet 5 API is marginally cheaper. Second, Sonnet 5’s new tokenizer can raise effective token counts by up to 1.35 times, so the same prompt costs a little more than the old Sonnet. Our token efficiency guide covers how to keep that in check with caching and context trimming.

When to use OpenRouter versus the native API

Use OpenRouter when you value provider fallback, want to compare several models quickly, or prefer one key and one bill across vendors. Use the native API when you run steady, single-provider traffic and want the lowest per-token cost and the most direct access to Anthropic features.

Frequently asked questions

What is the OpenRouter model string for Sonnet 5? anthropic/claude-sonnet-5.

Why use OpenRouter instead of the native API? For one key across many models, automatic provider fallback, and easy model routing. The native API avoids the router fee.

Can I set up fallback to Opus 4.8? Yes. OpenRouter supports fallback and routing, so you can escalate hard tasks to Opus 4.8.

Is Sonnet 5 cheaper on OpenRouter? OpenRouter passes through model pricing plus a small fee, so native access is marginally cheaper.

Does OpenRouter support the full 1M context window for Sonnet 5? Yes, OpenRouter passes through the model’s capabilities, including the one million token context window. As always, large context costs input tokens on every call, so trim what you send.

Can I set effort levels through OpenRouter? Effort control is exposed through the model’s parameters. If your routing layer supports passing model-specific settings, you can control reasoning depth; otherwise you may need the native API for the finest control. See the effort levels guide.

Is OpenRouter reliable enough for production? Its main production value is exactly reliability: fallback across providers means a single vendor outage does not take your feature down. For steady single-provider traffic without fallback needs, the native API is marginally cheaper.

Can I track spend per model in OpenRouter? Yes. The dashboard breaks down usage and cost by model and request, which makes it easy to see how much of your spend goes to Sonnet 5 versus a fallback or Opus 4.8.

Does using OpenRouter add latency? There is a small routing overhead, but for most applications it is negligible compared to model generation time. The reliability benefit of fallback usually outweighs it for production traffic.

The bottom line

OpenRouter is the easiest way to use Sonnet 5 alongside other models with built-in fallback and routing. Set the model to anthropic/claude-sonnet-5, add a fallback, and route hard tasks to Opus 4.8. For native setup, see the API guide, and for the full model picture, the complete guide.