SpeechRequest schema

Field	Type and description
`model` Required	string TTS model identifier.
`input` Required	string The text to synthesize into speech.
`voice`	string Voice preset to use. `default: "aura-asteria-en"`
`response_format`	string Audio output format. Allowed values: `["mp3","opus","aac","flac","wav","pcm"]` `default: "mp3"`
`referenceAudioUrl`	string Public HTTPS URL to a reference audio file for voice cloning. Required only when the selected model uses voice-cloning mode (AUDIO_REFERENCE). Validated against SSRF rules before any fetch — must be a publicly reachable HTTPS URL. Has no effect on models that do not support AUDIO_REFERENCE. `format: "uri"`