The Summer '26 drop is the one where Salesforce's agent tooling goes from click-to-configure toward code-first. Agent Script is a new scripting language for AI agents, open-sourced under Apache 2.0 — meaning you can inspect, fork, and ship it outside the sandbox. Agent Builder API adds a REST and Apex surface for creating and managing Agentforce agents programmatically, which matters if you're generating or versioning agent configs rather than hand-editing them through a UI. The Apex user-mode default change is quieter but has real teeth: it tightens sharing enforcement on database operations that previously ran in system mode unless you said otherwise. Review any Apex that relies on implicit elevated access — this one could surface bugs that were hiding behind permissive defaults.