# Brief: diagnosis-eval-baseline-diff

## Background

The eval harness now has a complete saved baseline. This change adds the comparison layer that turns the baseline into an actionable regression signal.

## Goals

1. Compare baseline and current `DiagnosisEvalReport` objects.
2. Detect aggregate and per-case regressions.
3. Output JSON and Markdown diff reports.
4. Document how to read the diff in interview and engineering terms.

## Scope

- Diff data structures
- Deterministic report comparison
- JSON / Markdown diff output
- Focused tests and eval docs

## Non-Goals

- No live Agent execution
- No LLM-as-judge
- No evaluator scoring rule changes
- No production API changes

## Related OpenSpec

`openspec/changes/diagnosis-eval-baseline-diff/`