Add pre_compact and post_compact hooks for context compaction

What feature would you like to see?

Please add two compaction lifecycle hooks:

• pre_compact: fires immediately before Codex compacts the conversation context
• post_compact: fires immediately after compaction completes

Today Codex appears to expose hook surfaces around tool execution and session lifecycle. It also appears to emit compaction-related notifications/events such as thread/compacted / ContextCompacted after compaction. But there does not appear to be a first-class pair of user-configurable compaction hooks that users can rely on as stable automation points.

That gap makes it hard to build robust long-running workflows.

Why pre_compact matters:

• persist high-value session state before any context is discarded
• generate project-specific summaries or checkpoints while the full pre-compaction context is still available
• snapshot external state right before compaction, instead of trying to guess when compaction is about to happen

Why post_compact matters:

• react immediately after compaction succeeds
• rebuild derived state, refresh prompts, or append audit logs using the new compacted session state
• distinguish successful compaction from attempted compaction

Suggested behavior:

• pre_compact triggers once per compaction attempt, right before compaction begins
• post_compact triggers once after successful compaction finishes
• provide thread/session identifiers and working directory
• provide a short reason or trigger source when available, such as manual /compact vs automatic compaction
• command hooks would already be useful; prompt hooks would also be welcome if they fit the current hook model
• if pre_compact fails, either continue with surfaced warning or support an explicit block/abort policy
• if compaction fails, post_compact should not fire, or should include an explicit failure status if the design prefers a single post-event path

Why this matters:

• post-compaction notifications/events alone are too late for workflows that need the full pre-compaction context
• stop is not a substitute, because compaction can happen long before a task is done
• external wrappers have to approximate compaction timing, which is brittle
• the pair of hooks would make compaction automation symmetrical with other lifecycle boundaries

A minimal first version would already be valuable even if these hooks are command-only.

Are you interested in implementing this feature?

No

Additional information

Local verification on codex-cli 0.117.0:

• hook/event names visible in the installed binary include pre_tool_use, post_tool_use, session_start, user_prompt_submit, and stop
• compaction-related post notifications/events such as thread/compacted and ContextCompacted are present
• I could not find corresponding first-class pre_compact / post_compact hooks

This seems like a natural addition for the CLI and would fit well with existing context-management behavior.