Effective August 1, 2025, AWS will standardize billing for the initialization (INIT) phase across all AWS Lambda function configurations. This change specifically affects on-demand invocations of Lambda functions packaged as ZIP files that use managed runtimes, for which the INIT phase duration was previously unbilled. This update standardizes billing of the INIT phase across all runtime types, deployment packages, and invocation modes. In this post, we discuss the Lambda Function Lifecycle and upcoming changes to INIT phase billing. You will learn what happens in the INIT phase and when it occurs, how to monitor your INIT phase duration, and strategies to optimize this phase and minimize costs.