# AUR4205 ## Error Message `AUR4205: Unknown expression of type {{type}}` Where `{{type}}` is the runtime expression type encountered by the validation controller. ## Description This error occurs when the validation controller encounters a binding/expression shape that it does not understand or cannot work with. This typically happens when using validation with an unsupported binding expression type or a custom/extended expression that the controller cannot interpret. ## Common Scenarios * Using validation APIs against a binding that is not a standard property binding. * Custom expression transforms/plugins that produce expression nodes the validation controller does not recognize. ## Solution * Use validation with standard property bindings, or validate programmatically using a supported API. * If you are extending expression parsing/compilation, ensure your extensions are compatible with validation-html. ## Troubleshooting * Capture/log the expression type shown in `{{type}}` and locate where it originates. * Reduce the binding to a simple `value.bind`/`checked.bind` and re-introduce complexity to find the incompatible part. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.aurelia.io/developer-guides/error-messages/4200-to-4206/aur4205.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.