Support
Find answers to common questions or get in touch.
Getting Started
What do I need to use Castwyn?
A Mac running macOS 14 (Sonoma) or later. Apple Silicon is recommended for best on-device transcription performance. No account required.
Do I need an API key?
Not for on-device transcription — that's completely free after downloading a model. API keys are only needed for cloud transcription (OpenAI) or AI transcript improvement (OpenAI or Anthropic).
Which transcription model should I choose?
For non-English podcasts, use large-v3 (~3 GB) — it has the best accuracy. For English-only content, large-v3 turbo (~1.6 GB) is faster with similar quality. Smaller models (tiny, small) are faster but less accurate.
Transcription
What audio formats are supported?
MP3, M4A, WAV, AAC, AIFF, OGG, and WebM. You can drag and drop files or use the file picker. Multiple files are queued automatically.
How long does transcription take?
On-device with large-v3: roughly real-time on Apple Silicon (a 1-hour podcast takes about 1 hour). Cloud transcription is significantly faster — typically 2-5 minutes for a 1-hour episode.
What's the difference between on-device and cloud?
On-device is free and private — no audio leaves your Mac. Cloud is faster and uses OpenAI's API, but audio is sent to their servers and costs approximately $0.36 per hour of audio.
AI Improvement
What does AI improvement do?
It sends the transcript text to Claude (Anthropic) or GPT (OpenAI) to fix spelling, punctuation, proper names, and formatting. The result is a cleaner, more readable transcript split into proper paragraphs.
Which improvement model should I use?
GPT-5.4 offers the best balance of speed, cost (~$0.24/hour), and quality. Claude Opus 4.6 is the most accurate for proper names but slower and more expensive (~$0.60/hour).
Can I customize the improvement prompt?
Yes. Go to Settings → Improvement Prompt. You can provide custom instructions for how the AI should process your transcripts.
Privacy & Data
Does the app collect any data?
No. Castwyn does not collect analytics, usage data, or personal information. There are no tracking SDKs or user accounts. See our Privacy Policy for full details.
Where are my transcripts stored?
Locally on your Mac in the app's sandboxed Application Support directory. Nothing is synced to any external server.
Are my API keys safe?
Keys are stored locally in your app preferences. They are only sent to the respective API provider (OpenAI or Anthropic) when making requests — never anywhere else.