# Signed Extensions {% embed url="" %} Substrate provides the concept of **signed extensions** to extend an extrinsic with additional data, provided by the [`SignedExtension`](https://paritytech.github.io/substrate/master/sp_runtime/traits/trait.SignedExtension.html) trait. The transaction queue regularly calls signed extensions to keep checking that a transaction is valid before it gets put in the ready queue. This is a useful safeguard for verifying that transactions won't fail in a block. They are commonly used to enforce validation logic to protect the transaction pool from spam and replay attacks. ### Reference {% embed url="" %} --- # 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://bootcamp.openguild.wtf/building-a-blockchain-with-polkadot-sdk/polkadot-sdk/substrate/adding-a-custom-logic-to-runtime/extensions/signed-extensions.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.