> ## Documentation Index
> Fetch the complete documentation index at: https://docs.swarms.world/llms.txt
> Use this file to discover all available pages before exploring further.

# Building Agents with OpenRouter

> One API key, hundreds of models — use OpenRouter to access models from every major provider through a single interface.

[OpenRouter](https://openrouter.ai) is a unified API gateway to hundreds of models from every major provider (Anthropic, OpenAI, Google, Mistral, Meta, DeepSeek, and more). One API key, one billing relationship, hundreds of models.

This is the right pick when you want to A/B test models freely, avoid lock-in, or simplify procurement.

## Installation

```bash theme={null}
pip install -U swarms
```

## Environment Setup

```bash theme={null}
export OPENROUTER_API_KEY="sk-or-..."
```

Get an API key at [openrouter.ai/keys](https://openrouter.ai/keys).

## Quick Start

OpenRouter model names follow the pattern `openrouter/<provider>/<model>`:

```python theme={null}
from swarms import Agent

agent = Agent(
    agent_name="OpenRouter-Agent",
    model_name="openrouter/anthropic/claude-3.5-sonnet",
    max_loops=1,
)

print(agent.run("Summarize the case for API gateways in three paragraphs."))
```

## Popular Model Names

| Model             | `model_name`                                     |
| ----------------- | ------------------------------------------------ |
| Claude 3.5 Sonnet | `"openrouter/anthropic/claude-3.5-sonnet"`       |
| Claude Opus 4.8   | `"openrouter/anthropic/claude-opus-4-8"`         |
| GPT-4.1           | `"openrouter/openai/gpt-4.1"`                    |
| GPT-5.4           | `"openrouter/openai/gpt-5.4"`                    |
| Gemini 2.5 Pro    | `"openrouter/google/gemini-2.5-pro"`             |
| Llama 3.3 70B     | `"openrouter/meta-llama/llama-3.3-70b-instruct"` |
| Qwen 2.5 72B      | `"openrouter/qwen/qwen-2.5-72b-instruct"`        |
| DeepSeek R1       | `"openrouter/deepseek/deepseek-r1"`              |
| Mistral Large     | `"openrouter/mistralai/mistral-large"`           |

Browse the [full catalog](https://openrouter.ai/models) for hundreds more.

## Tool Use

Tools work on OpenRouter for any underlying model that supports function calling:

```python theme={null}
from swarms import Agent

def get_weather(city: str) -> str:
    """Return the current weather for a city."""
    return f"{city}: 21°C, partly cloudy"

agent = Agent(
    agent_name="OpenRouter-Assistant",
    model_name="openrouter/anthropic/claude-3.5-sonnet",
    tools=[get_weather],
    max_loops=3,
)

print(agent.run("What's the weather in Tokyo right now?"))
```

## Streaming

```python theme={null}
from swarms import Agent

agent = Agent(
    agent_name="Streaming-OpenRouter",
    model_name="openrouter/openai/gpt-4.1",
    streaming_on=True,
    max_loops=1,
)

agent.run("Walk me through how a B-tree index works in a relational database.")
```

## A/B Testing Models

Because every model is available through the same key, OpenRouter is ideal for running side-by-side comparisons:

```python theme={null}
from swarms import Agent, ConcurrentWorkflow

candidates = [
    "openrouter/anthropic/claude-3.5-sonnet",
    "openrouter/openai/gpt-4.1",
    "openrouter/google/gemini-2.5-pro",
    "openrouter/meta-llama/llama-3.3-70b-instruct",
]

agents = [
    Agent(
        agent_name=name.split("/")[-1],
        model_name=name,
        system_prompt="Answer in under 150 words.",
        max_loops=1,
    )
    for name in candidates
]

workflow = ConcurrentWorkflow(agents=agents)
results = workflow.run("What are the most underrated trade-offs in monolithic vs. microservice architectures?")

for name, response in results.items():
    print(f"\n=== {name} ===\n{response}")
```

## Provider Routing

OpenRouter automatically routes between provider hosts for the same model. You can pin to a specific provider or let OpenRouter pick the cheapest/fastest:

```python theme={null}
from swarms import Agent

# Let OpenRouter route automatically
agent = Agent(
    agent_name="Auto-Routed",
    model_name="openrouter/meta-llama/llama-3.3-70b-instruct",
    max_loops=1,
)

# See https://openrouter.ai/docs#provider-routing for advanced provider preferences
```

## Production Defaults

```python theme={null}
from swarms import Agent

agent = Agent(
    agent_name="Production-OpenRouter",
    model_name="openrouter/anthropic/claude-3.5-sonnet",
    max_loops=1,
    persistent_memory=True,
    context_compression=True,
    context_length=128_000,
    autosave=True,
    retry_attempts=3,
    print_on=False,
)
```

## Next Steps

* [Building Agents with Anthropic](/examples/model-providers/anthropic) — direct Anthropic integration
* [Building Agents with OpenAI](/examples/model-providers/openai) — direct OpenAI integration
* [Model Providers Overview](/integrations/model-providers)