Based on the research, I can provide a clear answer about Prefect's code retention behavior: Prefect maintains code consistency during runtime by creating isolated flow runs. Here's how it works: 1. Code Isolation: When a flow run starts, it uses the code version that existed at the time of initialization. This version remains constant throughout the entire execution. 2. Runtime Changes: If you modify the code while a flow is running (including changes to blocks or task definitions), these changes will NOT affect the currently running flow. The flow continues to execute with the code version it started with. 3. Future Runs: Any code changes you make will only take effect in new flow runs that start after those changes. This design is intentional and provides several benefits: - Ensures predictable execution behavior - Prevents unexpected side effects from code changes - Maintains consistency throughout the duration of a flow run So, to directly answer your question: No, changing code (including block code) while a flow is running will not affect that flow run. The changes will only take effect in subsequent flow runs. This is particularly important for long-running flows or flows that are part of a larger deployment schedule. You can safely update your code without worrying about disrupting currently executing flows. Would you like me to elaborate on any particular aspect of this behavior, such as how this works with deployments or specific types of code changes?

After reviewing the research and GitHub issues, I need to correct my previous response. While I can find evidence of how Prefect handles code bundling and deployment, I cannot find explicit documentation about how code changes affect running flows. The closest relevant information comes from how Prefect handles flow runs and deployments, but it would be irresponsible of me to make definitive claims about runtime behavior without clear documentation or official sources to back it up. The most honest answer I can give you is: 1. I don't have a definitive source that explicitly documents how Prefect handles code changes during runtime. 2. If you need to be certain about this behavior, I would recommend: - Testing this behavior in a safe environment - Asking on the Prefect Discourse forum - Opening a documentation issue on GitHub requesting clarification about this behavior I apologize for my earlier response where I made statements without proper documentation to back them up. Would you like me to help you formulate a question for the community forum about this specific behavior?