A 429 is returned when an app (API Key) is attempting to make more than one call to the Rosette API at one time. The Rosette API accepts one call per Application at a time. If you are using a client binding (or in whatever language or platform you are working), refer to the best practices for queueing http calls in that language to ensure there are no simultaneous calls being made to our API. For an example, see our sample Chrome extensions in https://github.com/rosette-api-community. Our client bindings at https://github.com/rosette-api also provide basic retries when encountering 429s, so we recommend using a Rosette client binding where possible.
Or, ask about a custom plan to have concurrency with the Rosette API or try our local-deploy SDKs.