# Satoshi API - Bitcoin Fee Window Watch

> Satoshi API - Bitcoin Fee Window Watch is a paid API for AI agents from bitcoinsapi.com, paid per call via x402, $0.1/call, status unknown (last checked 2026-09-14).

Starts a fee-window watch that monitors Bitcoin mempool fees and alerts when fees drop below (or meet) a specified sat/vB threshold within a defined time window

## Facts

- Endpoint: POST https://bitcoinsapi.com/api/v1/watch/fee-window
- Price: $0.1/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/satoshi-api-bitcoin-fee-window-watch-b734d135
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_DjGOGALnIP5DY45uIUI2K

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 satoshi-api-bitcoin-fee-window-watch-b734d135 -d '<json body>'
```

Example prompt: Watch Bitcoin fees for the next 6 hours and let me know when they drop below 5 sat/vB — label it 'send-savings-tx' so I know which watch it is.

## When to prefer this

Use this endpoint when an agent or app needs to defer a Bitcoin transaction until fees are favorable, rather than polling repeatedly on its own. It is ideal when you want a managed fee-watch with a briefing and poll-back pattern rather than fetching instantaneous fee data. Prefer this over a simple fee-lookup endpoint when the goal is conditional triggering — wait until fees are below X — rather than just knowing the current fee level.

## Known failure modes

- Missing or invalid threshold_sat_vb — returns validation error
- Invalid condition value — watch cannot be created
- window_hours out of acceptable range — rejected
- Payment not included or insufficient — x402 payment required response
- Network or mempool data unavailable — service may return error or degraded briefing

## How this service works

Know when to send Bitcoin and when to wait. Satoshi API gives apps and AI agents live fee recommendations, mempool context, and cost estimates. Free, open source.

## Output

Returns a JSON object with a human-readable briefing string summarizing the watch, a watch object confirming the condition and threshold, poll instructions for the agent to check status (free after the paid watch start), an agent_action object with next steps, and status information. The response also echoes back the x402 price paid ($0.10).

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "label": {
   "type": "string"
  },
  "condition": {
   "enum": [
    "below",
    "above"
   ],
   "type": "string"
  },
  "window_hours": {
   "type": "integer"
  },
  "threshold_sat_vb": {
   "type": "number",
   "description": "Fee threshold in sat/vB."
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "schema": {
  "type": "object",
  "required": [
   "briefing",
   "watch",
   "poll"
  ],
  "properties": {
   "poll": {
    "type": "object"
   },
   "watch": {
    "type": "object"
   },
   "status": {
    "type": "object"
   },
   "briefing": {
    "type": "string"
   },
   "agent_action": {
    "type": "object"
   }
  }
 },
 "example": {
  "poll": {
   "method": "GET",
   "payment": "free_poll_after_paid_start"
  },
  "watch": {
   "condition": "below",
   "threshold_sat_vb": 5
  },
  "briefing": "Bitcoin fee-window watch",
  "x402_price_usd": "$0.10"
 },
 "mimeType": "application/json"
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/satoshi-api-bitcoin-fee-window-watch-b734d135/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from bitcoinsapi.com](https://www.zero.xyz/host/bitcoinsapi.com/llms.txt)