OpenRouter Fundamentals Basics

8 examples to get you started with OpenRouter - 5 basic and 3 intermediate.

You will authenticate against the OpenAI-compatible gateway, call chat completions, swap model slugs, inspect routing metadata, and handle the errors agents hit first.

Prerequisites

• Python 3.11+ recommended
• An OpenRouter account and API key
• Credits for paid models, or willingness to use :free variants with tight rate limits

python -m venv .venv && source .venv/bin/activate
pip install openai httpx
export OPENROUTER_API_KEY="sk-or-..."

Basic Examples

1. Point the OpenAI Client at OpenRouter

Use the standard OpenAI Python SDK with OpenRouter's base URL.

import os
from openai import OpenAI
 
client = OpenAI(
    base_url="https://openrouter.ai/api/v1",
    api_key=os.environ["OPENROUTER_API_KEY"],
)
 
resp = client.chat.completions.create(
    model="openai/gpt-4o-mini",  # verify slug at build
    messages=[{"role": "user", "content": "Reply with the word pong."}],
)
print(resp.choices[0].message.content)

• Same chat.completions surface most agent frameworks already use.
• Base URL is the entire integration switch for many apps.
• Model must be an OpenRouter slug, not only a vendor marketing name.

Related: What OpenRouter Actually Is: One API for 300+ Models

2. Add Optional App Attribution Headers

Optional headers help rank your app on OpenRouter and do not change model behavior.

resp = client.chat.completions.create(
    model="openai/gpt-4o-mini",
    messages=[{"role": "user", "content": "Hello"}],
    extra_headers={
        "HTTP-Referer": "https://example.com",  # your site
        "X-OpenRouter-Title": "My Agent Demo",
    },
)

• Headers are optional for correctness.
• Set them once in client default_headers for cleaner call sites.
• Use a stable app name so usage analytics stay coherent.

3. Swap Models by Changing Only the Slug

Keep messages and tools fixed; change the model string.

MODELS = [
    "openai/gpt-4o-mini",
    "anthropic/claude-3.5-sonnet",  # verify at build
    "google/gemini-2.5-flash",
]
 
prompt = [{"role": "user", "content": "In one sentence, what is an agent loop?"}]
 
for model in MODELS:
    r = client.chat.completions.create(model=model, messages=prompt)
    print(model, "=>", r.choices[0].message.content)

• This is the core OpenRouter value for agents and evals.
• Put slugs in config or env, not hardcoded in ten modules.
• Re-run golden tasks after every slug change.

Related: Model Slugs and Naming Conventions on OpenRouter

4. Try a Free Variant (Mind the Caps)

Free models use a :free suffix and platform rate limits.

resp = client.chat.completions.create(
    model="meta-llama/llama-3.3-70b-instruct:free",  # example; verify at build
    messages=[{"role": "user", "content": "Say hi in five words."}],
)
print(resp.choices[0].message.content)

• Free tier is for prototyping, not production agent load.
• Platform caps (verify at build): often 20 req/min; 50 req/day if lifetime credit purchases are under 10, else 1000 req/day.
• A 402 can still appear if account balance is negative, even for free models.

Related: Free Models on OpenRouter: What's Available and Their Limits

5. Read Usage and Provider Hints from the Response

Agents should log cost drivers every turn.

resp = client.chat.completions.create(
    model="openai/gpt-4o-mini",
    messages=[{"role": "user", "content": "Count to three."}],
)
print("usage:", resp.usage)
# Provider may appear on the object or in raw JSON depending on SDK version
print(resp.model_dump().get("provider"))

• Token usage drives credit spend.
• Provider identity matters for debugging latency and policy.
• Store model slug + usage on every agent turn trace.

Intermediate Examples

6. Stream Tokens for Interactive Agents

stream = client.chat.completions.create(
    model="openai/gpt-4o-mini",
    messages=[{"role": "user", "content": "List three agent stop conditions."}],
    stream=True,
)
for chunk in stream:
    delta = chunk.choices[0].delta.content or ""
    print(delta, end="", flush=True)
print()

• Streaming uses SSE under the hood.
• Mid-stream errors may arrive as stream events after HTTP 200 - handle both paths.
• Buffer partial tool-call JSON carefully if you stream tool arguments.

Related: Streaming Responses Through OpenRouter

7. List Models Programmatically

Do not hardcode the entire catalog in source control.

import os
import httpx
 
r = httpx.get(
    "https://openrouter.ai/api/v1/models",
    headers={"Authorization": f"Bearer {os.environ['OPENROUTER_API_KEY']}"},
    timeout=30.0,
)
r.raise_for_status()
models = r.json()["data"]
print("count:", len(models))
print("sample:", models[0]["id"])

• Catalog size and prices change; refresh at deploy or build time.
• Filter client-side by modality, price, or context length for agent routers.
• Cache with a TTL so cold starts do not hammer the models endpoint.

8. Handle 402 and 429 the Way Production Agents Should

from openai import APIStatusError
 
def complete(model: str, messages: list) -> str:
    try:
        resp = client.chat.completions.create(model=model, messages=messages)
        return resp.choices[0].message.content or ""
    except APIStatusError as e:
        code = e.status_code
        if code == 402:
            raise RuntimeError("Insufficient OpenRouter credits or key limit") from e
        if code == 429:
            raise RuntimeError("Rate limited; back off or switch model") from e
        raise
 
print(complete("openai/gpt-4o-mini", [{"role": "user", "content": "ok"}]))

• 402: add credits, raise per-key limits, or fix negative balance.
• 429: exponential backoff; on free variants, buy credits threshold or switch to paid slug.
• Agent loops should map these to stop reasons, not infinite retries.

Related: How OpenRouter's Credit and Billing Model Works

Related

• What OpenRouter Actually Is: One API for 300+ Models - gateway definition
• How OpenRouter's Unified API Normalizes Different Providers - why one client works
• OpenRouter Setup & Routing Basics - keys and first completion depth
• Calling OpenRouter from Python and TypeScript SDKs - multi-language recipes
• OpenRouter Fundamentals Best Practices - fit checklist for projects

Stack versions: Pins from the category manifest (verify at build): OpenRouter (~315+ models, July 2026 pricing/fees); LangGraph 1.0+; CrewAI 1.14+; Microsoft Agent Framework 1.0; Vercel AI SDK 6; Pydantic AI (latest); LlamaIndex (latest); OpenAI Agents SDK (latest + MCP); MCP (Linux Foundation governance); A2A (HTTP+SSE+JSON-RPC 2.0); Solana @solana/web3.js + @solana/spl-token.