[benbrandt]

Summary

• Extend the existing recovery path's before/after configuration comparison into one shared configuration-change helper. Publish a complete config_option_update when the advertised list changes, rather than duplicating publication in individual callers.
• Use it for session/set_config_option, the legacy mode/model setters, command-driven changes, approved-plan implementation, and provider/recovery model refreshes. The requesting client's duplicate notification is written before the setter response.
• Do not publish for unchanged lists, rejected changes, the existing JetBrains 2026.1 disabled-config workaround, or a session state superseded by replay.
• Build setter responses from the current session state after publication. If replay replaces the session while a native settings request is pending, the old request does not advertise stale options in its notification or response.
• Keep v2 field conversion in the existing conversion layer. Both ACP versions receive full lists, including dependent changes to reasoning choices and removal of fast mode after a model switch.

Checked against ACP commit 854e6003. docs/protocol/v2/session-config-options.mdx:375-379 recommends configuration notifications whenever configuration changes, including changes requested through session/set_config_option, with the requesting client's duplicate before its response. Responses and notifications replace the complete list.

Scope

Based on the merged #714, not stacked on its feature branch. No review/checklist Markdown file is added. This covers configuration changes owned by the adapter; it does not add new native settings-event synchronization or change active-resume environment replacement. Remaining session/entity metadata reconstruction is separate.

Validation

• npm run typecheck
• npm run build
• npm test: 2,332 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.

New wire tests cover notification-before-response ordering for every root config option on both protocol versions, complete dependent model changes, no-op and rejected/native-failure cases, one notification for /plan, legacy setters, and replay replacing the session during a pending native settings request. Existing command, plan, provider/recovery, and child configuration tests remain passing without snapshot updates.