> ## Documentation Index > Fetch the complete documentation index at: https://docs.worldlabs.ai/llms.txt > Use this file to discover all available pages before exploring further. # Frequently asked questions > Common questions and answers about the World API ### Why does `worlds:generate` return `400 Bad request`? A `400 Bad request` means the request was rejected before generation started. The response body is generic and includes a `request_id`, but not the exact cause. Other status codes narrow things down: `401` is an authentication problem, `402` is insufficient API credits, `422` is a schema validation error, and `429` is a rate limit. The most common cause is a request body that is not valid JSON. This is the usual reason a request that works in `curl` fails from another HTTP client. Some libraries send an object's string form instead of JSON. For example, a Java `Map.toString()` produces `key=value` pairs with unquoted keys: ```text theme={null} # Rejected: a stringified map, not JSON {model=marble-1.0, world_prompt={type=text, text_prompt=A sunlit atrium}} # Accepted: serialized JSON {"model":"marble-1.0","world_prompt":{"type":"text","text_prompt":"A sunlit atrium"}} ``` Serialize the payload with a JSON library (Jackson, Gson, `encoding/json`, and similar) before sending it, and set `Content-Type: application/json`. Setting that header while sending non-JSON bytes still fails. To confirm, log the exact bytes you send and compare them against a working `curl` payload. Other causes to check: * An input URL that World Labs cannot fetch. For an `image_prompt` or `video_prompt` with `source: "uri"`, the URL must be reachable with no cookies, referer, or auth. Hosts that block hotlinking or use expiring links fail. Upload the file as a [media asset](/api/reference/media-assets/prepare-upload) or inline it with `source: "data_base64"` instead. * Input that was rejected by content policy. * An invalid `model` value. Use one of `marble-1.0-draft`, `marble-1.0`, `marble-1.1`, or `marble-1.1-plus`. * A field constraint, such as `display_name` over 64 characters or more than 10 `tags`. Keep the `request_id` from the response so support can trace the exact failure. ### Can I retrieve PLY files from the API? Yes. Use [Export a world](/api/reference/worlds/export) to convert a generated world's SPZ splats into a cached `.ply` download: ```bash theme={null} curl -X POST "https://api.worldlabs.ai/marble/v1/worlds/:export" \ -H "WLT-Api-Key: $WLT_API_KEY" \ -H "Content-Type: application/json" \ -d '{"asset_type":"splats","format":"ply","resolution":"full_res"}' ``` The response is an operation. PLY conversions usually return with `done: true` and a signed download URL in `response.url`. You can also export a high-quality mesh with `{"asset_type":"mesh","format":"glb"}`; mesh exports are asynchronous, so poll the returned operation until it completes. ### How do I convert a generated world to real-world (metric) scale? Generated splat assets are exported in arbitrary model units, not meters. The `semantics_metadata` object on the world response tells you how to convert to a metric, ground-aligned frame: * `metric_scale_factor`: multiply every XYZ coordinate (and isotropic scale) by this to get meters. * `ground_plane_offset`: after scaling, subtract this from the Y coordinate to place the ground plane at y=0. ```python theme={null} def to_metric(xyz, semantics): scaled = xyz * semantics.metric_scale_factor scaled[..., 1] -= semantics.ground_plane_offset return scaled ``` For Gaussian splat sizes, renderer coordinate systems, and log-scale `scale_0` fields, see [Rendering Marble SPZ files in third-party engines](/api/rendering-spz). ### How does API billing work? Billing for the World API is separate from billing for the Marble web app. * Credits purchased for the Marble app cannot be used with the API * API usage requires credits purchased through the World Labs Platform If you plan to use the API, make sure you purchase credits on the World Labs Platform, NOT in the Marble app. A Marble subscription — including a Free plan — says nothing about your API billing. The two are separate accounts even when you sign in with the same email, so being on the Marble Free plan does not mean your API usage is free. ### Why was I charged when I have not used the API recently? Almost certainly this is an overage charge, and it covers an earlier billing period. API credits are prepaid: you buy them up front and requests draw down that balance, so normal usage produces no charge at all. But requests are admitted based on a low-balance threshold rather than a per-request cost estimate, so a request that starts while your balance is low can cost more than you have left and take your balance below zero. That uncovered usage — your overage — is the only part that gets invoiced, and it settles at the end of the monthly billing period rather than at the time of the request. So the charge can arrive weeks after your last request: a charge dated early August covers overage from July. The credits are converted to USD at your account's credit rate (\$1.00 USD per 1,250 credits on standard pay-as-you-go accounts). Note that having auto-refill disabled does not prevent this. Auto-refill only controls whether your balance is topped up automatically; it does not cap usage. To reconcile a charge, compare the billing period on the invoice against your usage events for that period on the [usage page](https://platform.worldlabs.ai/usage). See [Pricing](/api/pricing#overage) for details. ### How do I get a panorama image from my world generation? Every world generation includes a panorama image in the response, accessible via `assets.imagery.pano_url`. This panorama is automatically generated as part of the world creation process. The panorama URL will be available in the response at `operation.response.assets.imagery.pano_url` or when you fetch the world via `GET /marble/v1/worlds/{world_id}`. ### What is the difference between `marble-1.0`, `marble-1.1`, and `marble-1.1-plus`? World Labs currently offers three API model variants for world generation: * `marble-1.0` provides standard world generation at a fixed cost. * `marble-1.1` provides newer standard world generation at a fixed cost. * `marble-1.1-plus` provides the most expansive generation path and may add 0-1,500 variable world generation cost for larger worlds, determined automatically by the system during inference. For the latest pricing details, see the [API pricing page](/api/pricing). ### Where can I read more about World Labs policies? Please view our [Terms of Service](/terms-of-service) and [Privacy Policy](/privacy-policy) for details. This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.