tx402

Resilient x402 buyer SDK for Python. Deterministic multi-chain payment routing, local spend guardrails, and a drop-in httpx wrapper for autonomous agents.

⚠️ This is a placeholder release (0.0.0). The package name is reserved; the SDK is under active development and nothing is implemented yet. Do not depend on this version. The first functional release will be 0.1.0.

What it will do

An AI agent running a 50-step workflow cannot afford to have step 45 die because one payment facilitator rate-limited it or the merchant wanted USDC on a chain the agent wasn't configured for. tx402 wraps a normal HTTP client and handles the 402 Payment Required handshake:

from tx402 import Policy, Tx402Client

client = Tx402Client(
    evm_signer=evm_signer,
    solana_signer=solana_signer,
    policy=Policy(
        max_per_request="0.50 USDC",
        max_per_hour="10.00 USDC",
        allowed_networks=["eip155:8453", "solana:mainnet"],
    ),
)

response = client.post(url, json={"prompt": "Hello"})

Under the hood, on a 402, it will: decode the challenge, enforce your spend policy before any key is touched, deterministically pick a route across the networks the merchant actually offered (scored by balance, fee, and local endpoint health), reserve the spend, sign one authorization, and retry exactly once with it.

Both Tx402Client and AsyncTx402Client are provided.

Design commitments

• Non-custodial. The core API accepts signer abstractions, never raw private key strings. Private keys never leave your process, and are never logged or transmitted.
• Policy before signature. Budget caps, domain allowlists, and network allowlists are evaluated and the spend is reserved before a signer is ever invoked.
• Integer money. All amounts are integer atomic units. float money inputs are rejected, not coerced — a cap that rounds is not a cap.
• No backend. No tx402-operated service, no telemetry, no phone-home. The only network calls are to the merchant you asked for and the RPC endpoints you configured.
• Same-chain only. It pays on a network the merchant offered and you can sign for. It will not bridge or swap behind your back; if you can't pay, you get a typed InsufficientLiquidityError.

Install

pip install tx402            # core: protocol codec + HTTP transport
pip install "tx402[evm]"     # + Base / EVM signing
pip install "tx402[svm]"     # + Solana signing
pip install "tx402[all]"     # everything

Requires Python 3.10+.

Status

Pre-alpha. Targeting Base and Solana for the first release, with a TypeScript SDK (tx402 on npm) at behavioral parity — both are validated against the same language-neutral conformance fixtures.

License

Apache-2.0