---
id: review-key-field
name: Review key field implementation
status: pending
depends_on: [key-on-uelement, key-extraction-in-h]
created: 2026-05-18T16:22:57.125725833Z
modified: 2026-05-18T16:22:57.125726289Z
scope: narrow
risk: low
impact: phase
level: review
---

# Description

Review the implementation of the `key` field on `UElement` before proceeding to the fiber tree and reconciler phases. This is a critical checkpoint because every downstream task depends on `key` being correctly added to the schema and extracted by `h()`.

Verify schema/type consistency, backward compatibility, and that ADR-004's consequences are properly handled.

## Acceptance Criteria

- [ ] `UElement` TypeScript type matches TypeBox Module schema for `key` field
- [ ] `key` never appears in component function props
- [ ] Backward compatibility: unkeyed elements work identically to pre-key behavior
- [ ] TypeBox `Value.Check` passes for both keyed and unkeyed elements
- [ ] No unintended `key: undefined` serialization on unkeyed elements (verify JSON output)
- [ ] All existing + new tests pass
- [ ] `isUElement` guard still correctly discriminates from `URoot`

## References

- docs/architecture/schema.md
- docs/architecture/decisions/004-key-as-first-class-field.md
- docs/architecture/element-factory.md

## Notes

> To be filled on completion of review

## Summary

> To be filled on completion