Rendering API
Fill Generative Negative Prompt fill-gen-neg-prompt
Provides a negative text suggestion to the output of fill=gen. It accepts URL-safe string values but can also be base64 encoded (fill-gen-neg-prompt64).
Generative fill using fill-gen-prompt=eagle%20flying%20around: 
https://assets.imgix.net/example-images/generative-fill/man-standing-on-cliff.jpg?fill=gen&fill-gen-pos=left&fill-gen-prompt=eagle%20flying%20around&fit=fill&w=600&h=900&auto=compress,format
Suggest that clouds not be used with fill-gen-neg-prompt=clouds: 
https://assets.imgix.net/example-images/generative-fill/man-standing-on-cliff.jpg?fill=gen&fill-gen-pos=left&fill-gen-prompt=eagle%20flying%20around&fit=fill&w=600&h=900&auto=compress,format&fill-gen-neg-prompt=clouds
imgix recommends using the Base64 variant version of this parameter to make encoding values easier.
Limits
Guarantee of keyword output
While fill-gen-neg-prompt suggests what not to display in the final output, it does not guarantee those keywords will not be used in the final generative fill.
For example, using the negative prompt keywords fill-gen-neg-prompt=clouds,trees,sky,black%20bird outputs an image that does not have clouds or trees, but it still has a black-colored bird even if we suggested that black-colored birds not be included.
https://assets.imgix.net/example-images/generative-fill/man-standing-on-cliff.jpg?fill=gen&fill-gen-pos=left&fill-gen-prompt=eagle%20flying%20around&fit=fill&w=600&h=900&auto=compress,format&fill-gen-neg-prompt=clouds,trees,sky,black%20bird
Character limits
There is no direct character limit for prompt values, though we recommend keeping your image URLs under 1024 characters.
See the API specification for more information about this parameter's value ranges and format.