---
id: host-finalize-instance
name: Add finalizeInstance to HostConfig
status: pending
depends_on: [fiber-disposal]
created: 2026-05-18T16:22:57.323634873Z
modified: 2026-05-18T16:22:57.323635316Z
scope: single
risk: low
impact: component
level: implementation
---

# Description

Add the optional `finalizeInstance?(instance: Instance, ctx: RootCtx): void` method to the `HostConfig` interface. This method allows hosts to perform per-instance cleanup when an instance is removed (e.g., releasing GPU buffer slots, closing database connections, removing graphology nodes).

This is a backward-compatible change: the method is optional, so existing `HostConfig` implementations continue to work without modification. The reconciler calls `finalizeInstance` during `disposeFiber` after children are disposed but before the fiber's own state is cleared.

## Acceptance Criteria

- [ ] `finalizeInstance?(instance: Instance, ctx: RootCtx): void` added to `HostConfig` interface
- [ ] Method is optional (existing hosts don't need to implement it)
- [ ] Called by `disposeFiber` during fiber tree disposal
- [ ] Called bottom-up: children's `finalizeInstance` before parent's
- [ ] Updated in `src/host/config.ts` interface definition
- [ ] TypeScript compiles (`npm run build:tsc`)
- [ ] Existing tests pass (existing HostConfig implementations don't break)
- [ ] New test: host implementing `finalizeInstance` receives calls during disposal

## References

- docs/architecture/lifecycle.md — Host Notification section, finalizeInstance interface
- docs/architecture/host-config.md — Open Question 3 (should HostConfig include finalizeInstance)

## Notes

> To be filled by implementation agent

## Summary

> To be filled on completion