# FastPath Bitcoin API Glossary

## x402

An HTTP 402 payment protocol. FastPath uses x402 so bots can pay per API call with USDC and no API key.

## EVM

Ethereum Virtual Machine. FastPath lets EVM-style clients ask Bitcoin questions through familiar JSON-RPC patterns.

## JSON-RPC

A request/response protocol used by Ethereum clients. FastPath accepts Ethereum-style JSON-RPC and translates selected methods into Bitcoin-backed answers.

## sat/vB

Satoshis per virtual byte. This is the common Bitcoin fee-rate unit used to estimate whether a transaction will confirm soon.

## UTXO

Unspent transaction output. Bitcoin wallets spend UTXOs rather than account balances.

## Mempool

The set of valid unconfirmed Bitcoin transactions known to a node.

## RBF

Replace-By-Fee. A transaction policy that allows a wallet to replace an unconfirmed transaction with a higher-fee version.

## CPFP

Child-Pays-For-Parent. A fee bump strategy where a child transaction pays enough fee to help confirm its unconfirmed parent.

## Core v31

Bitcoin Core v31-style node data used by FastPath for mempool package and template intelligence.

## Template Check

A check that compares a transaction against FastPath's live cached optimizer and baseline template signals.

## Optimizer Cache

FastPath's cached result from the current optimizer cycle. It is useful for decision support but does not guarantee mining by any external miner.

## Non-Custodial

FastPath can analyze, plan, and relay signed transactions, but it never holds private keys or signs transactions for users.