# Key Takeaways Stack data query opcodes—**OP\_DEPTH** and **OP\_SIZE**—provide essential introspection capabilities for Bitcoin Script development. These non-destructive operations enable scripts to measure stack state and make intelligent decisions based on input patterns and data characteristics. **OP\_DEPTH** enables adaptive validation logic that responds to different input structures, while **OP\_SIZE** enforces data format requirements and protocol compliance. Together, they form the foundation for robust input validation and flexible script design. Effective implementation combines stack queries with conditional logic to create layered validation: checking structure, then format, then content before performing expensive operations. This defensive approach improves security, efficiency, and maintainability. --- # 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://hub.bsvblockchain.org/higher-learning/bsv-academy/bsv-opcodes/stack-data-queries/key-takeaways.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.