Key Signature

{
  "asOf": "2026-10-11T14:26:13.995Z",
  "circleOfFifths": [
    {
      "accidentals": "0 sharps",
      "clock": 0,
      "major": "C",
      "minor": "Am"
    },
    {
      "accidentals": "1 sharp",
      "clock": 1,
      "major": "G",
      "minor": "Em"
    }
  ],
  "circleOfFifthsClockPosition": 9,
  "degraded": false,
  "disclaimer": "Screening signal computed from the named public source. Not financial, legal or investment advice; provided as is. Verify at the source before acting.",
  "dominantKey": "Bb major",
  "enharmonicKey": null,
  "fifthsPosition": -3,
  "key": "Eb major",
  "notChecked": [
    "modal key signatures (dorian and so on use the signature of their parent major key)",
    "key changes inside a piece",
    "…1 more"
  ],
  "parallelKey": "Eb minor",
  "parallelSignature": {
    "accidentals": [
      "Bb",
      "Eb",
      "…4 more"
    ],
    "count": 6,
    "theoretical": false,
    "type": "flats"
  },
  "reasons": [],
  "relativeKey": "C minor",
  "scaleNotes": [
    "Eb",
    "F",
    "…5 more"
  ],
  "signature": {
    "accidentals": [
      "Bb",
      "Eb",
      "…1 more"
    ],
    "count": 3,
    "theoretical": false,
    "type": "flats"
  },
  "source": "computed locally",
  "subdominantKey": "Ab major",
  "summary": "Eb major: 3 flats (Bb Eb Ab); relative Cm, parallel Ebm.",
  "verdict": "COMPUTED"
}

# bash or zsh: see the payment challenge. Nothing is charged.
curl -sS -g -i 'https://music-theory.x402tookit.com/key-signature?key=Eb'

# x402 v2 sends the challenge base64-encoded in the PAYMENT-REQUIRED header (v1 sends it in the body).
# To read the header:
curl -sS -g -o /dev/null -D - 'https://music-theory.x402tookit.com/key-signature?key=Eb' \
  | grep -i '^payment-required:' | cut -d' ' -f2 | tr -d '\r' | base64 --decode

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

// Pays at most $0.02 per call.
const client = new x402Client().setSpendControls({ maxAmountPerPayment: "$0.02" });
registerExactEvmScheme(client, { signer: privateKeyToAccount(process.env.EVM_PRIVATE_KEY) });
const pay = wrapFetchWithPayment(fetch, client);

// Not sure it's safe to pay? ToolVet checks it first for $0.005. Pay for that from a separate
// client: this one is capped at $0.02 per payment. GET https://toolvet.app/api/v1/check?url=https%3A%2F%2Fmusic-theory.x402tookit.com%2Fkey-signature
const res = await pay("https://music-theory.x402tookit.com/key-signature?key=Eb");
console.log(res.status, await res.text());

# pip install "x402[requests,evm]"
import os
from eth_account import Account
from x402 import x402ClientSync
from x402.http.clients import x402_requests
from x402.mechanisms.evm import EthAccountSigner
from x402.mechanisms.evm.exact.register import register_exact_evm_client

# Pays at most $0.02 per call.
client = x402ClientSync().set_spend_controls({"max_amount_per_payment": "$0.02"})
register_exact_evm_client(client, EthAccountSigner(Account.from_key(os.environ["EVM_PRIVATE_KEY"])))

with x402_requests(client) as session:
    res = session.get("https://music-theory.x402tookit.com/key-signature?key=Eb")
    print(res.status_code, res.text)