# FastPath Bitcoin Mempool Transaction Lookup

> FastPath Bitcoin Mempool Transaction 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).

Fetches Bitcoin mempool details for a specific transaction by txid, returning virtual size, fee rate, and RBF replaceability status.

## Facts

- Endpoint: GET https://api.nativebtc.org/v1/mempool/tx/%7Btxid%7D
- 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-bitcoin-mempool-transaction-lookup-b308ab64
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_wwFmqON8AgS21f_9i1A8d

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-bitcoin-mempool-transaction-lookup-b308ab64
```

Example prompt: Can you check the mempool status of Bitcoin transaction 81c97a6c5b98a1019095de132727cda868694fb5d92534ebb07e475619e38325 — I want to know its fee rate in sat/vB, virtual size, and whether it can be replaced with RBF?

## When to prefer this

Use this endpoint when you need real-time mempool data for a specific Bitcoin transaction — especially fee rate in sat/vB, virtual size, and RBF status. It is ideal for agents that need to decide whether to bump a transaction's fee, verify a payment is in-flight, or programmatically monitor unconfirmed transactions. Prefer this over general block explorer APIs if you need structured JSON output compatible with Ethereum-style tooling or are already using the FastPath/NativeBTC stack.

## Known failure modes

- Transaction not found in mempool (already confirmed or never broadcast) — success: false
- Invalid or malformed txid format — likely 400 or error response
- Transaction already confirmed and no longer in mempool
- Payment failure via x402 — 402 response if USDC payment not included or insufficient
- Network timeout if mempool is under heavy load

## 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 with the transaction ID, virtual size in vbytes (vsize), fee rate in satoshis per virtual byte (feeRateSatVb), whether the transaction signals BIP-125 RBF replaceability (bip125Replaceable), and a success boolean indicating whether the transaction was found in the mempool.

## Request schema (JSON Schema)

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

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "txid": "81c97a6c5b98a1019095de132727cda868694fb5d92534ebb07e475619e38325",
  "vsize": 110,
  "success": true,
  "feeRateSatVb": 2,
  "bip125Replaceable": true
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fastpath-bitcoin-mempool-transaction-lookup-b308ab64/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)