# RelayStation Subtitle Burn (libass)

> RelayStation Subtitle Burn (libass) is a paid API for AI agents from api.relaystation.ai, paid per call via x402, $0.01/call, status unknown (last checked 2026-09-14).

Permanently burns styled subtitle/caption tracks into a video file using libass, so captions are hardcoded into every copy of the video.

## Facts

- Endpoint: POST https://api.relaystation.ai/v1/media/subtitle-burn
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/relaystation-subtitle-burn-libass-3ac9a5c3
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_Jk5pbOyzxjJAyUYbDXYxg

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 relaystation-subtitle-burn-libass-3ac9a5c3 -d '<json body>'
```

Example prompt: Take my video file and permanently burn in the subtitle file I have — I want the styled captions hardcoded into the video so they show up no matter where the file is played.

## When to prefer this

Choose this endpoint when you need subtitles permanently embedded (hardcoded) into video frames — not as a removable soft track — especially when distributing to platforms or devices that don't support external subtitle files. Ideal when styled captions (custom fonts, colors, positioning via libass/ASS format) must be preserved exactly as authored. Prefer over FFmpeg-direct or generic video APIs when you want a pay-per-MB microservice with no infrastructure to manage.

## Known failure modes

- Unsupported video or subtitle format returns a 4xx error
- Subtitle file and video duration mismatch may result in partial or missing captions
- Malformed ASS/SRT subtitle syntax causes libass rendering failure
- Video file too large or exceeds processing limits returns an error
- Network timeout for very long or high-bitrate videos
- Invalid or inaccessible input URL returns a 4xx error

## How this service works

$0.0005/MB. Burn subtitles permanently into video via libass — styled captions everywhere the file goes. 1¢ x402 min; remainder auto-credits — relaystation.ai/penny

## Output

Returns a video file (or download URL) with subtitles permanently composited into the video frames using libass rendering — the output video has hardcoded, styled captions visible on any player without needing a separate subtitle track.

## 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",
    "bodyType",
    "body"
   ],
   "properties": {
    "body": {
     "type": "object",
     "properties": {
      "ext": {
       "enum": [
        "mp4",
        "mov",
        "mkv",
        "mp3",
        "m4a",
        "aac",
        "wav",
        "ogg",
        "flac"
       ],
       "type": "string"
      },
      "file": {
       "type": "object",
       "description": "cputools input-source: { inline: <base64 ≤ 4 MiB> } or { inputKey: <scratch key ≤ 50 MB> }. The input container must sniff (ffprobe) to the live allowlist (operator-tunable cputools.media.input_formats): 3g2, 3gp, aac, asf, avi, flac, flv, gif, m4a, m4v, matroska, mj2, mov, mov,mp4,m4a,3gp,3g2,mj2, mp3, mp4, mpeg, mpegts, mpegvideo, ogg, wav, webm, wmv. An off-list input is a free 422 UNSUPPORTED_FORMAT."
      },
      "subtitle": {
       "type": "object",
       "description": "cputools input-source: { inline: <base64 ≤ 4 MiB> } or { inputKey: <scratch key from /v1/cputools/upload-url, ≤ 50 MB> }."
      }
     }
    },
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "POST",
      "PUT",
      "PATCH"
     ],
     "type": "string"
    },
    "bodyType": {
     "enum": [
      "json",
      "form-data",
      "text"
     ],
     "type": "string"
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object"
    }
   }
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/relaystation-subtitle-burn-libass-3ac9a5c3/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.relaystation.ai](https://www.zero.xyz/host/api.relaystation.ai/llms.txt)