# Music Theory – Chord Progression Transposer

> Music Theory – Chord Progression Transposer is a paid API for AI agents from music-theory.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-29).

Transposes a chord progression by a given number of semitones or between two keys, returning correctly spelled output chords for the target key.

## Facts

- Endpoint: GET https://music-theory.x402tookit.com/transpose?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-29
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/music-theory-chord-progression-transposer-af30dffb
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_4GENlvSwl1qqs8X6ZJB1V

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-chord-progression-transposer-af30dffb
```

Example prompt: Can you transpose the chord progression C Am F G7 from C major to G major and make sure the chords are spelled correctly for the new key?

## When to prefer this

Use this endpoint when you need to move an entire chord progression to a new key with musically correct chord spelling — e.g. Bb instead of A# in flat keys. Prefer it over manual interval arithmetic when key context matters for enharmonic spelling correctness. Ideal for song arrangement, vocal range matching, instrument transposition, and capo calculations.

## Known failure modes

- Missing required 'progression' parameter returns an error
- Ambiguous or unrecognised chord symbols may result in null output entries or a 'notChecked' list
- Providing both 'semitones' and 'fromKey'/'toKey' may cause conflict; only one method should be used
- Out-of-range semitone values (beyond ±24) are rejected
- Misspelled key names (e.g. 'Csharp' instead of 'C#') may fail to parse

## How this service works

Transpose a chord progression by semitones or from one key to another, with spelling that fits the target key.

## Output

Returns a list of transposed chord names (one per input chord), the detected or specified source key, the target key, the semitone distance applied, a summary string, and per-chord input-to-output mappings. May also include spelling rationale and a disclaimer if any chords could not be resolved.

## 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": [
      "progression"
     ],
     "properties": {
      "toKey": {
       "type": "string",
       "description": "Target key, e.g. G or Em; the shift is the upward distance between tonics"
      },
      "prefer": {
       "type": "string",
       "description": "sharps or flats to force the spelling of out-of-key chords"
      },
      "fromKey": {
       "type": "string",
       "description": "Original key, e.g. C or Am"
      },
      "semitones": {
       "type": "integer",
       "description": "Shift -24 to 24 semitones (use this, or fromKey and toKey)"
      },
      "progression": {
       "type": "string",
       "description": "Chords separated by spaces, commas or bars: C Am F G7"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "asOf": {
       "type": [
        "null",
        "string"
       ]
      },
      "toKey": {
       "type": [
        "null",
        "string"
       ]
      },
      "chords": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "object",
        "properties": {
         "input": {
          "type": [
           "null",
           "string"
          ]
         },
         "output": {
          "type": [
           "null",
           "string"
          ]
         }
        }
       }
      },
      "source": {
       "type": [
        "null",
        "string"
       ]
      },
      "fromKey": {
       "type": [
        "null",
        "string"
       ]
      },
      "reasons": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "summary": {
       "type": [
        "null",
        "string"
       ]
      },
      "verdict": {
       "type": [
        "null",
        "string"
       ]
      },
      "degraded": {
       "type": [
        "boolean",
        "null"

… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/music-theory-chord-progression-transposer-af30dffb/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)