# Music Theory Key Signature Lookup

> Music Theory Key Signature Lookup is a paid API for AI agents from music-theory.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-11-03).

Returns key signature details (sharps/flats, accidentals), circle-of-fifths position and neighbours, and relative/parallel keys for any major or minor key.

## Facts

- Endpoint: GET https://music-theory.x402tookit.com/key-signature?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-11-03
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/music-theory-key-signature-lookup-38ebacc0
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_ubF1EfvuThNgshY41ret7

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 music-theory-key-signature-lookup-38ebacc0
```

Example prompt: What's the key signature for F# minor — how many sharps or flats does it have, what are the accidentals, and what are its relative and parallel keys?

## When to prefer this

Use this endpoint when you need authoritative, structured key signature data for any standard or theoretical major/minor key, including accidentals, circle-of-fifths relationships, and relative/parallel key lookups — especially when building music theory tools, educational apps, or composition assistants that need reliable harmonic context rather than generating that information from an LLM.

## Known failure modes

- Missing 'key' query parameter returns an error
- Unrecognized key name or format returns null/error fields
- Ambiguous key names without major/minor qualifier may default to major
- Theoretical keys (e.g. G# major) may be flagged with a 'theoretical' flag rather than rejected

## How this service works

Key signature (sharps or flats), circle-of-fifths neighbours, relative and parallel keys for any major or minor key.

## Output

A JSON object containing: the key signature type (sharp or flat), count of accidentals, list of accidental note names, whether it is a theoretical key, all scale notes, the relative key, parallel key, dominant key, subdominant key, enharmonic equivalent key, the key's position on the circle of fifths (clock position and fifths index), and the full circle-of-fifths array with clock positions and accidental counts for each key.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [
      "key"
     ],
     "properties": {
      "key": {
       "type": "string",
       "description": "Key: C, G, Bb, F# minor, Am, Ebm"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "key": {
       "type": [
        "null",
        "string"
       ]
      },
      "asOf": {
       "type": [
        "null",
        "string"
       ]
      },
      "source": {
       "type": [
        "null",
        "string"
       ]
      },
      "reasons": {
       "type": [
        "array",
        "null"
       ]
      },
      "summary": {
       "type": [
        "null",
        "string"
       ]
      },
      "verdict": {
       "type": [
        "null",
        "string"
       ]
      },
      "degraded": {
       "type": [
        "boolean",
        "null"
       ]
      },
      "signature": {
       "type": [
        "null",
        "object"
       ],
       "properties": {
        "type": {
         "type": [
          "null",
          "string"
         ]
        },
        "count": {
         "type": [
          "null",
          "number"
         ]
        },
        "accidentals": {
         "type": [
          "array",
          "null"
         ],
         "items": {
          "type": "string"
         }
        },
        "theoretical": {
         "type": [
          "boolean",
          "null"
         ]
        }
       }
      },
      "disclaimer": {
       "type": [
        "null",
        "string"
       ]
      },
      "notChecked": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "scaleNotes": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "dominantKey": {
       "type": [
        "null",
        "string"
       ]
      },
      "parallelKey": {

… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/music-theory-key-signature-lookup-38ebacc0/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from music-theory.x402tookit.com](https://www.zero.xyz/host/music-theory.x402tookit.com/llms.txt)