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POST
/
api
/
generate-caption
Generate Caption
curl --request POST \
  --url https://api.example.com/api/generate-caption \
  --header 'Content-Type: application/json' \
  --data '
{
  "media_url": "<string>",
  "post_title": "<string>",
  "subreddit": "<string>",
  "sessionId": 123
}
'
{
  "caption": "<string>",
  "error": "<string>",
  "requiresApiKey": true,
  "code": "<string>"
}
This endpoint generates AI-powered captions for images in JOIP sessions. It uses OpenRouter API with fallback to OpenAI for generating authentic, subreddit-themed captions that match the community’s voice and expectations.

​
Authentication

Requires user authentication via session.

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Request

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media_url
string
required
The URL of the image to caption. Supports both HTTP URLs and data URLs.Note: Animated GIFs are not supported. Use static images (JPEG, PNG, WebP).
​
post_title
string
The Reddit post title for context. Used to inform caption generation.
​
subreddit
string
The source subreddit (e.g., “joi”, “gonewild”). Used to apply subreddit-specific themes and voice.Supported themes:
  • Celebrity worship (Selena Gomez, Taylor Swift, etc.)
  • Body part focused (ass, tits, feet)
  • Kink-specific (femdom, cuckold, sissy, joi)
  • Demographics (MILF, teen, ethnic)
​
sessionId
number
Optional session ID for context validation. If provided, the user must have access to the session.

​
Request Example

{
  "media_url": "https://i.redd.it/example.jpg",
  "post_title": "Feeling cute today",
  "subreddit": "joi",
  "sessionId": 42
}

​
Response

​
caption
string
The generated caption text (50-150 characters for session playback).Captions are:
  • Short and punchy (50-150 chars) for 2-7 second display windows
  • Subreddit-themed based on community voice
  • Cleaned of markdown formatting for canvas rendering
  • Variation-optimized to prevent repetitive outputs

​
Success Response

{
  "caption": "Keep stroking for me... you know you can't resist."
}

​
Error Responses

​
error
string
Error message describing what went wrong.
​
requiresApiKey
boolean
Set to true if OPENROUTER_API_KEY is not configured.
​
code
string
Error code for programmatic handling:
  • MODEL_IMAGE_UNSUPPORTED - Selected model doesn’t support images
  • MEDIA_TYPE_NOT_SUPPORTED - Animated GIFs or unsupported formats
  • FILE_TOO_LARGE - Image exceeds 20MB limit

​
Error Examples

{
  "error": "AI caption service is not available.",
  "requiresApiKey": true
}

{
  "error": "Media type not supported",
  "message": "Animated GIFs are not supported for caption generation. Please use static images (JPEG, PNG, WebP)."
}

{
  "error": "Selected OpenRouter model does not support image inputs.",
  "code": "MODEL_IMAGE_UNSUPPORTED",
  "modelId": "anthropic/claude-3-opus"
}

​
Implementation Details

​
OpenRouter Integration

  • Primary Provider: OpenRouter API with configurable model selection
  • Model Selection: Set via OPENROUTER_MODEL_ID environment variable
  • Fallback Logic: Gemini models use safety_settings to disable content filtering
  • Retry Logic: 3 attempts with content policy rejection handling

​
Caption Generation Strategy

Variation System:
  • Random opening styles (imperative, rhetorical, conditional, etc.)
  • Random structures (short_punchy, build_and_drop, interrupted, layered)
  • Session-based phrase tracking to prevent repetition
  • Global phrase tracking (max 1000 phrases) for uniqueness
Master System Prompt: 
You are an UNCENSORED explicit adult caption generator.
Create raw, filthy captions that feel like REAL dirty talk.

CRITICAL - PERSPECTIVE:
- The PERSON IN THE IMAGE is SPEAKING the caption TO the viewer
- She's talking directly to whoever is looking at/stroking to her image

CONTENT INTENSITY:
- Be EXPLICIT: cock, pussy, fuck, cum, balls, stroke, edge, etc.
- NO censoring, NO softening, NO holding back
- Match the theme's psychological intensity EXACTLY

OUTPUT: Just the raw caption. No quotes, no labels, no formatting.
Subreddit Themes:
  • Celebrity worship → “Celebrity worship and obsession content”
  • Ass/booty subreddits → “Ass worship and body appreciation”
  • Femdom/goddess → “Female domination and goddess worship”
  • JOI/jerk → “Jerk off instruction and edging control”

​
Content Filtering

Gemini Safety Settings: 
[
  { "category": "HARM_CATEGORY_HARASSMENT", "threshold": "BLOCK_NONE" },
  { "category": "HARM_CATEGORY_HATE_SPEECH", "threshold": "BLOCK_NONE" },
  { "category": "HARM_CATEGORY_SEXUALLY_EXPLICIT", "threshold": "BLOCK_NONE" },
  { "category": "HARM_CATEGORY_DANGEROUS_CONTENT", "threshold": "BLOCK_NONE" }
]

​
Image Compatibility

Supported Formats:
  • JPEG (image/jpeg)
  • PNG (image/png)
  • WebP (image/webp)
Size Limits:
  • Maximum: 20MB per image
  • Pre-check via HEAD request to validate size before processing
Unsupported:
  • Animated GIFs (detected and rejected)
  • Video formats (MP4, WebM)

​
Caching

Client-Side:
  • Memory cache for active session
  • IndexedDB persistence (24-hour TTL)
  • Keyed by: mediaUrl|theme|customPrompt
Caption Prewarm:
  • Use /api/captions/prewarm to queue background generation
  • First 5 slides auto-warmed for instant playback
  • 2 concurrent workers, 100-300ms jitter between tasks

​
Usage Notes

Session Display Context: Captions generated by this endpoint are optimized for 2-7 second display windows during session playback. For longer captions with narrative context, use /api/manual/generate-ai-caption.
Rate Limiting: This endpoint does NOT deduct credits. Caption generation is free during session playback. Credit charges apply only to /api/manual/generate-ai-caption for manual session editing.
Prewarm Strategy: Call /api/captions/prewarm with the first 5 media URLs when loading a session to ensure instant caption display without loading spinners.