[benbrandt]

Summary

• Add one v2 session-info metadata owner at the common send boundary. Shared v1-shaped and direct v2 publications use the same shallow patch state. Omission leaves values unchanged, top-level nulls remove keys, and nested objects replace whole values.
• On every successful resume, after replay/live-event draining and before the response, clear retained client metadata with _meta: null, then restore the agent's currently known metadata when nonempty. Serialize both frames with live session-info updates so an older restoration cannot overwrite a newer patch.
• Keep metadata for the active session lifetime, including full replay. Resume reinstalls SessionState, so the metadata lifetime must not follow that object's identity. Seed known native thread status before releasing new live events. Close/reopen starts a fresh metadata lifetime.
• Carry the current failure reference across active reinstallation. Clear published failure metadata when that incident resolves, preserving sibling metadata and checking the ID/revision so a late resolution cannot remove a newer incident.
• Preserve notification params._meta through the shared v2 conversion path, but do not store or replay it as entity metadata. Keep v1 wire behavior unchanged.

Checked against ACP a965f1f1: docs/protocol/v2/extensibility.mdx:103-150 defines metadata replacement/removal and explicit resynchronization clears. docs/protocol/v2/session-setup.mdx:355-358 recommends current session info after replay and before the response, including omitted/null replayFrom.

Scope

This synchronizes currently known session_info_update metadata. It does not invent a deep merge for extension namespaces, copy raw session-list snapshot metadata into entity patches, or reconstruct every metadata field from persistent storage. Writers sharing a top-level namespace still need to compose complete values; that cleanup is separate, with the new serialized domain-patch API available for it. Newer upstream auth/proxy/background schema additions are not adopted here. No review/checklist Markdown file is included.

Validation

• npm run typecheck
• npm run build
• npm test: 2,358 passed, 39 skipped
• npm run test:integration: all 19 passed
• npm run codex-test -- -p "Reply with exactly OK. Do not use tools or modify any files." -o summary: completed with end_turn
• git diff --check
• Opt-in E2E tests were not run.

Coverage includes shallow replacement and removals, nested null values, clearing keys the agent has never seen, JSON copies, notification-only metadata, serialization under backpressure, queued patches, disposal during restoration, write-failure recovery, failure revision matching, active resumes with/without replay, close/reopen, current failure continuity, and post-close publication fencing. The real-Codex foreground-state integration covers all replay settings with the extended info synchronization sequence.