# FastPath API - Bitcoin Transaction Replacement Lookup

> FastPath API - Bitcoin Transaction Replacement Lookup is a paid API for AI agents from api.nativebtc.org, paid per call via x402, $0.001/call, status unknown (last checked 2026-09-12).

Returns RBF (Replace-By-Fee) replacement history and status for a specific Bitcoin transaction by txid

## Facts

- Endpoint: GET https://api.nativebtc.org/v1/mempool/tx/%7Btxid%7D/replacements
- Price: $0.001/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-12
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/fastpath-api-bitcoin-transaction-replacement-lookup-b0e7a673
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_irbXHL0kha5-RZ0FAeYYB

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability fastpath-api-bitcoin-transaction-replacement-lookup-b0e7a673
```

Example prompt: Can you check whether Bitcoin transaction 81c97a6c5b98a1019095de132727cda868694fb5d92534ebb07e475619e38325 has been replaced in the mempool — I want to know if it was bumped via RBF and what replaced it.

## When to prefer this

Use this endpoint when you need to determine whether a specific Bitcoin transaction was replaced via Replace-By-Fee (RBF) and want the full replacement chain. Prefer it over generic blockchain explorers when you need programmatic access to RBF replacement data, especially when building agents or automation that track payment finality on Bitcoin. It is purpose-built for mempool intelligence and uses Ethereum-style JSON-RPC conventions familiar to EVM developers.

## Known failure modes

- Invalid or malformed txid returns an error or empty result
- Transaction not found in mempool history returns success:false or empty replacements
- Network timeout or upstream Bitcoin node unavailability
- Payment required (402) if x402 payment header is missing or invalid
- Transaction too old may not be indexed in mempool history

## How this service works

FastPath translates Ethereum-style JSON-RPC requests into Bitcoin data, fee intelligence, mempool insight, and Core v31 package signals. Paid by x402.

## Output

Returns a JSON object containing the original txid, a success boolean, a replaced boolean indicating whether the transaction was superseded via RBF, and an array of replacement transactions (empty if not replaced).

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "properties": {
   "type": "string"
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "txid": "81c97a6c5b98a1019095de132727cda868694fb5d92534ebb07e475619e38325",
  "success": true,
  "replaced": false,
  "replacements": []
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fastpath-api-bitcoin-transaction-replacement-lookup-b0e7a673/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.nativebtc.org](https://www.zero.xyz/host/api.nativebtc.org/llms.txt)