---
title: "tx402"
description: "A resilient, non-custodial buyer-side SDK for the x402 HTTP payment protocol, in TypeScript and Python."
source: https://docs.tx402.io/
---

# tx402

## What it does

`tx402` wraps a normal HTTP client. When a server answers **`402 Payment Required`**, tx402
interprets the challenge, checks it against **your** spend policy, picks a payment route across
the chains the merchant offered, signs one authorization, and retries the request. You get the
response body. Roughly a hundred lines of fragile glue code become three.

It is **non-custodial** and **buyer-side only**. tx402 never holds funds, never calls a
facilitator's `/verify` or `/settle` — settlement is the merchant's job — and never accepts a
raw private key in its main configuration.

    Policy evaluation and an atomic budget reservation both complete **before** a signer is
    ever invoked. A refused request costs zero signatures, and a dry run costs zero of both.
    Offered several chains? The same challenge and the same health state select the same
    route every time — viability, then your preference, then fee, then health.
    Every amount is an integer in atomic units, end to end. No float, no `Number`, no
    rounding surprise between the quote and the signature.
    If a signature reached the merchant and the outcome is unknown, you get a distinct error
    and the budget stays reserved — so the same money is never spent twice by accident.

## Install

```bash
# TypeScript / Node 20+
npm install tx402                    # core + CLI, no chain
npm install tx402 @x402/evm viem     # + Base / EVM

# Python 3.10+
pip install tx402                    # core + CLI, no chain
pip install "tx402[evm]"             # + Base / EVM
```

Both packages are unscoped and named `tx402`. Chain support is opt-in: the core import path
loads **no chain library** in either language. Add the chain row for anything that reaches a
chain — the [quickstart](/start/quickstart/) lists Solana's too.

## Three lines

```ts title="TypeScript"
import { createTx402Client } from "tx402";

const tx402 = createTx402Client({
  signers: { evm },
  policy: { maxPerRequest: "0.10 USDC", maxPerHour: "5.00 USDC" },
});

const response = await tx402.fetch("https://api.example.com/paid-resource");
```

```python title="Python"
from tx402 import Policy, Tx402Client

with Tx402Client(
    evm_signer=evm,
    policy=Policy(max_per_request="0.10 USDC", max_per_hour="5.00 USDC"),
) as tx402:
    response = tx402.get("https://api.example.com/paid-resource")
```

Or without writing any code at all:

```bash
npx tx402 call https://api.example.com/paid-resource --max-spend "0.10 USDC" --dry-run
```

`--dry-run` does everything a real call does **except** reserve budget and sign. It is the
fastest way to find out what a merchant is asking for and what tx402 would do about it.

It is not, however, free of setup: planning ranks the offered routes by reading your balance on
each, so `--dry-run` needs a **chain row installed and a key configured**, and says so with exit
`5` if either is missing. The question that needs neither is `client.inspect()`, which returns
the merchant's terms without touching a chain.

## Where to go next

- **[Quickstart](/start/quickstart/)** — a real paid call on a testnet, in under five minutes.
- **[The request lifecycle](/guides/lifecycle/)** — what happens between your call and the response.
- **[Spend policy](/guides/policy/)** — the guardrails, and the order they run in.
- **[Error reference](/reference/errors/)** — all seventeen errors and the nine exit codes.
- **[Security model](/security/)** — the threat model, and what tx402 does and does not defend.
- **[Hosted tools](/tools/)** — free browser tools for inspecting, verifying and debugging x402
  endpoints, built on this SDK.

## More documentation

- Documentation index (Markdown): https://docs.tx402.io/sitemap.md
- Machine index: https://docs.tx402.io/llms.txt · full text: https://docs.tx402.io/llms-full.txt
- This page: https://docs.tx402.io/