Keyword extraction

Top keywords and two-word phrases by frequency (stopwords removed). Cheap, deterministic signal for routing, tagging, and dedup.

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Input

Field	Type	Description
text *	string	Text to analyze (max 500KB)
limit	number	Max keywords (default 15)

Example output

{
  "keywords": [
    {
      "term": "payment",
      "count": 9
    }
  ],
  "phrases": [
    {
      "term": "x402 protocol",
      "count": 4
    }
  ]
}

Try it - see the 402 challenge (free)

curl -i -X POST https://base-sentinel-production.up.railway.app/api/keywords \
  -H "Content-Type: application/json" \
  -d '{"text":"Long article text…","limit":10}'

The response is HTTP 402 Payment Required with exact payment requirements. Any x402 v2 client pays automatically and retries:

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://base-sentinel-production.up.railway.app/api/keywords", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "text": "Long article text…",
    "limit": 10
  }),
});

No wallet? Pay with compute

This is a pure-CPU tool, so an agent without a wallet can pay with proof-of-work instead of USDC: fetch a challenge, solve the sha256 puzzle (16 leading zero bits - a fraction of a second of CPU, no money, no AI tokens), and resend with the X-Pow-Solution header.

import { createHash } from "node:crypto";
const lz = (b) => { let t = 0; for (const x of b) { if (!x) { t += 8; continue; } t += Math.clz32(x) - 24; break; } return t; };
const c = await (await fetch("https://base-sentinel-production.up.railway.app/api/pow/challenge?slug=keywords")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://base-sentinel-production.up.railway.app/api/keywords", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"text":"Long article text…","limit":10}) });

Part of these workflows

This tool is one step in 2 curated multi-tool workflows - agents can fetch the whole sequence as an MCP prompt or call https://base-sentinel-production.up.railway.app/api/skill-packs/{slug}/prompt.

• Content grade - Grade a page's content quality - extract the readable content then analyze keyword density.
• Feed watch - Monitor an RSS/Atom feed in one call: parse the feed, read the top story in full, extract the keywords driving the cycle, and diff the item list against your last run to isolate what's new.

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