Codex CLI's session management mechanism deserves deep study by agent engineering practitioners. When a user runs codex --session continue, the agent loads the previous session's context, including conversation history, file modification records, executed commands, and intermediate result states. Key design aspects include: Context Window Budget — Codex CLI doesn't simply dump all history to the model, but allocates token budget by priority: system instructions > critical file state > recent conversation turns > historical summaries. State Checkpoints — automatic state snapshots before and after each tool call, supporting rollback to any checkpoint. Cross-session Persistence — session data stored on the local filesystem, recoverable even after terminal restart.
In the Zero to Pro Agent Harness course, Codex CLI's session management serves as the core case study for the 'Agent State Management' chapter. Three takeaways for Harness designers. First: context windows are not unlimited — allocation strategies are essential. Second: state checkpoints are infrastructure for debugging and safety rollback, not optional — agent systems without checkpoints cannot roll back erroneous operations in production. Third: cross-session persistence is the architectural watershed from 'one-shot agents' to 'persistent agents' — only when an agent remembers the previous conversation state can it execute multi-turn workflows.