# LangChain

> Use Inferit from LangChain in Python and JavaScript: an API key, the OpenAI-compatible base URL, model auto or a named model, and pay per request with x402 from LangChain JS. Testnet: test ITC, no monetary value.

LangChain's ChatOpenAI takes a base URL, so Inferit works as its model in Python and in JavaScript. Chains, agents and tools built on it run unchanged.

## 1. Get an API key

Create a key in the [Buy workspace](https://inferit.app/buy) ([Quickstart](https://inferit.cc/docs/quickstart.md), steps 1 to 6) and export it. The key spends your prepaid balance: test ITC on the testnet, a test token with no monetary value.

```bash
export INFERIT_API_KEY='ik_...'
```

The base URL is `https://api-production-c74b9.up.railway.app/v1`. Any OpenAI-compatible client works with it (chat completions, streaming included).

## 2. Python

`langchain-openai`'s `ChatOpenAI` with Inferit's base URL. `model_name` in the reply's metadata is the model that answered.

_chat.py_

```python
# pip install langchain-openai
import os
from langchain_openai import ChatOpenAI

llm = ChatOpenAI(
    base_url="https://api-production-c74b9.up.railway.app/v1",
    api_key=os.environ["INFERIT_API_KEY"],
    model="qwen/qwen-2.5-14b-instruct",  # "auto": Inferit picks the model
)

reply = llm.invoke("Say hello in five words.")
print(reply.content)
print(reply.response_metadata.get("model_name"))  # the model that answered
```

## 3. JavaScript

`@langchain/openai` takes the base URL under `configuration`, which it passes to the OpenAI client.

_chat.mjs_

```javascript
// npm i @langchain/openai @langchain/core
import { ChatOpenAI } from '@langchain/openai';

const llm = new ChatOpenAI({
  model: 'qwen/qwen-2.5-14b-instruct', // "auto": Inferit picks the model
  apiKey: process.env.INFERIT_API_KEY,
  configuration: { baseURL: 'https://api-production-c74b9.up.railway.app/v1' },
});

const reply = await llm.invoke('Say hello in five words.');
console.log(reply.content);
console.log(reply.response_metadata?.model_name); // the model that answered
```

## Model auto, or a named model

With `"auto"` as the model, Inferit picks the model: the best value on a published quality ranking for this request, at that model's cheapest healthy seller ([Automatic routing](https://inferit.cc/docs/routing.md)). The response names the model that answered, in its `model` field and the `x-inferit-model` header.

Auto needs a ranked model with a live seller. `GET /v1/routing` lists the models it can choose from now in `candidates_now`; while that list is empty, a request with auto answers `503 no_eligible_model`. Then name a catalog model with a live seller instead, such as `qwen/qwen-2.5-14b-instruct`, and Inferit picks the seller. The samples on this page send auto while it has a candidate, and that model otherwise.

## Pay per request with x402 (JavaScript)

With no API key, LangChain JS can pay for each request with x402: give the OpenAI client a `fetch` that answers the API's `402` with the standard x402 v2 client. The OpenAI client always sends an `Authorization` header, so the wrapper removes it first; with it, the API would read the request as an API-key call. Each payment is at least 0.01 ITC, is credited to the paying address's balance, and each request is charged its exact cost ([agents.md](https://inferit.cc/agents.md), [API reference](https://inferit.cc/docs/api-reference.md#x402)). The client caps one payment at 1 ITC; that is a setting in your client, not a rule of the contract.

_chat-x402.mts_

```typescript
// npm i @langchain/openai @langchain/core @x402/fetch @x402/evm viem
import { ChatOpenAI } from '@langchain/openai';
import { wrapFetchWithPayment, x402Client } from '@x402/fetch';
import { ExactEvmScheme } from '@x402/evm/exact/client';
import { generatePrivateKey, privateKeyToAccount } from 'viem/accounts';

const account = privateKeyToAccount((process.env.AGENT_KEY ?? generatePrivateKey()) as `0x${string}`);

// Testnet only: Inferit's faucet gives this address 1,000 free test ITC (the claim costs nothing).
await fetch('https://api-production-c74b9.up.railway.app/v1/faucet', {
  method: 'POST',
  headers: { 'content-type': 'application/json' },
  body: JSON.stringify({ address: account.address }),
});

// The standard x402 v2 client pays only in tokens it was told about: allow ITC,
// and cap one payment at 1 ITC (1000000 base units, 6 decimals).
const ITC = { network: 'eip155:1874', asset: '0x2E672dFE33EA977FD064E01aDe7d8c73B3Be7fBB' } as const;
const client = new x402Client()
  .register('eip155:*', new ExactEvmScheme(account))
  .setSpendControls({ allowedAssets: [{ ...ITC, maxAmountPerPayment: '1000000' }] });
const paidFetch = wrapFetchWithPayment(fetch, client);

// The OpenAI client always sends an Authorization header. Drop it, so the API
// answers with a 402 quote, which paidFetch pays and retries with PAYMENT-SIGNATURE.
const x402Fetch: typeof fetch = (input, init = {}) => {
  const headers = new Headers(init.headers);
  headers.delete('authorization');
  return paidFetch(input, { ...init, headers });
};

const llm = new ChatOpenAI({
  model: 'qwen/qwen-2.5-14b-instruct',
  apiKey: 'x402', // never sent: x402Fetch removes it
  maxTokens: 64, // the quote is the worst case for this body: a small limit keeps it small
  configuration: { baseURL: 'https://api-production-c74b9.up.railway.app/v1', fetch: x402Fetch },
});
console.log((await llm.invoke('Say hello in five words.')).content);
```

The x402 sample is JavaScript only. From Python, the exchange can be signed by hand, as in the Python sample on [Agents](https://inferit.cc/agents); or use an API key.

## Checked

The Python and JavaScript samples (with an API key) ran against the hosted testnet API on 2026-10-13, with a named model (auto had no candidate that day).

The x402 variant was run up to the API's `402` quote: the request reached the API with no `Authorization` header and the x402 client read the quote. The paid retry is the same exchange as on [Agents](https://inferit.cc/agents).

---

Canonical page: https://inferit.cc/docs/guides/langchain
All pages as markdown: https://inferit.cc/llms.txt
Testnet demo: test tokens, no monetary value.