Validation

Check content, text length, and media against platform rules before a post exists.

Preflight checks a saved post. The validation endpoints run the same checks on content you have not saved yet, so a form can tell a user what will fail before anything is created. Nothing is stored, and every endpoint needs the posts scope.

The three endpoints answer three different questions:

/validate/post runs the full preflight on a body, its media and a list of platforms
/validate/length counts text the way each platform counts it and compares it with the limit
/validate/media fetches a file by URL and runs the upload checks on it

Validate a post

POST /v1/validate/post

curl -X POST https://api.fopost.com/v1/validate/post \
  -H "X-API-Key: $FOPOST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "content": "Shipping something new today. https://yourbrand.com/launch",
    "media": [
      { "url": "https://yourbrand.com/launch.png", "mime_type": "image/png", "size": 240000 }
    ],
    "platforms": ["twitter", "instagram", "linkedin"]
  }'

Field	Type	Required	Description
`content`	string	No	The post text. Defaults to empty
`media`	object[]	No	Files the post will carry. Up to 20
`media[].url`	string	Yes	Public http(s) URL of the file
`media[].mime_type`	string	Yes	MIME type of the file, for example `image/png`
`media[].size`	integer	No	File size in bytes. Size checks are skipped without it
`platforms`	string[]	Yes	Platform slugs to check against

The response has one entry per platform, in the shape preflight uses per account:

{
  "data": {
    "ready": false,
    "platforms": [
      {
        "platform": "twitter",
        "ready": true,
        "issues": [],
        "score": 97,
        "signals": [
          { "level": "info", "code": "no_emoji", "message": "Adding a relevant emoji can lift engagement on twitter." }
        ]
      },
      {
        "platform": "instagram",
        "ready": true,
        "issues": [],
        "score": 94,
        "signals": [
          { "level": "info", "code": "link_in_caption", "message": "instagram doesn't render links in captions. Consider \"link in bio\" instead." }
        ]
      },
      {
        "platform": "linkedin",
        "ready": false,
        "issues": ["Unsupported media type: image/webp"],
        "score": 95,
        "signals": []
      }
    ]
  }
}

issues are hard blockers: publishing this content to that platform would fail. signals are advisory and never block publishing. ready at the top is true only when every platform is ready.

Check length

POST /v1/validate/length

curl -X POST https://api.fopost.com/v1/validate/length \
  -H "X-API-Key: $FOPOST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "text": "A long announcement with a link https://yourbrand.com/launch",
    "platforms": ["twitter", "bluesky", "youtube"]
  }'

Field	Type	Required	Description
`text`	string	Yes	The text to count
`platforms`	string[]	Yes	Platform slugs to check against

{
  "data": {
    "ok": true,
    "platforms": [
      { "platform": "twitter", "length": 55, "limit": 25000, "unit": "chars", "ok": true, "signals": [] },
      { "platform": "bluesky", "length": 61, "limit": 300, "unit": "chars", "ok": true, "signals": [] },
      { "platform": "youtube", "length": 61, "limit": 5000, "unit": "bytes", "ok": true, "signals": [] }
    ]
  }
}

length is what the platform counts, which is not always the character count: some platforms bill every link at a flat rate however long it is, and some count bytes, so unit says which. limit is null for a platform with no text limit. signals carries only the length signals, over_length and near_length_limit.

Check a media file

POST /v1/validate/media

curl -X POST https://api.fopost.com/v1/validate/media \
  -H "X-API-Key: $FOPOST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{ "url": "https://yourbrand.com/launch.png" }'

Field	Type	Required	Description
`url`	string	Yes	Public http(s) URL of the file

The file is fetched and put through the same checks as an upload: the type has to be on the allowed list, the bytes have to match the declared type, and the file has to be under the size ceiling. It is not stored.

{
  "data": {
    "ok": true,
    "issues": [],
    "name": "launch.png",
    "size": 240000,
    "mime_type": "image/png",
    "type": "image"
  }
}

A file that fails a check still answers 200, with ok: false and the reason in issues. A URL that cannot be fetched at all is an error: 400 for a link that is not public, including one that points at a private address, 404 when the link answers not found, and 401 when it needs credentials.

Errors

Status	Code	Meaning
`400`	`validation_error`	A field is missing, or a platform slug is unknown
`400`	`unsupported`	The media URL is not fetchable from here
`403`	`forbidden`	The API key lacks the `posts` scope

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