Context Eval Build Contract: COMPLETE
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Date: 2026-03-02
All deliverables (D1-D4) implemented and validated.

## What Was Built

### D1: context-eval CLI Command (core + CLI)
- `coderace/context_eval.py`: Core A/B evaluation engine
  - ContextEvalResult and TrialResult data classes
  - Context file backup/restore/placement/removal for baseline vs treatment
  - KNOWN_CONTEXT_FILES list (CLAUDE.md, AGENTS.md, .cursorrules, etc.)
  - run_context_eval() orchestrator running N trials per condition
- `coderace/commands/context_eval.py`: CLI subcommand with:
  --context-file PATH, --task PATH, --benchmark, --agents, --trials N,
  --output PATH, --task-dir PATH
- Full input validation (missing files, invalid agents, trials < 2, etc.)

### D2: Statistical Comparison Report
- `coderace/context_eval_report.py`: Statistical analysis and rendering
  - Delta with 95% CI using Welch's t-test
  - Cohen's d effect size
  - Per-agent summary: baseline vs treatment pass rates and scores
  - Per-task breakdown: which tasks improved, which degraded
  - Summary verdict: "improved", "degraded", or "no significant improvement"
  - Rich terminal table output
  - JSON output format

### D3: Dashboard Integration
- Extended `coderace/dashboard.py` with context-eval A/B section:
  - Bar chart: baseline vs treatment scores per agent
  - Delta table with CI (95%) and effect size
  - Verdict display
  - CSS for A/B visualization (.ab-baseline, .ab-treatment, .positive, .negative)
- Added --context-eval PATH flag to `coderace dashboard` command

### D4: Documentation + Examples
- README.md: Added "Context Evaluation" and "Measuring Context Engineering Impact" sections
  with usage examples, output format, CLI flags table, and effect size interpretation guide
- examples/context-eval-demo.sh: Executable demo script
- Clear help text on `coderace context-eval --help` and `coderace --help`

## Test Results

- 58 new tests added (41 for D1+D2, 17 for D3)
- All 505 tests pass (447 original + 58 new)
- No regressions in existing test suite

## Commits

1. feat(context-eval): add context-eval command with A/B statistical comparison (D1+D2)
2. feat(context-eval): add dashboard A/B comparison section (D3)
3. docs(context-eval): add README section, examples, and interpretation guide (D4)

## Files Created/Modified

New files:
- coderace/context_eval.py
- coderace/commands/context_eval.py
- coderace/context_eval_report.py
- tests/test_context_eval.py
- tests/test_context_eval_dashboard.py
- examples/context-eval-demo.sh

Modified files:
- coderace/cli.py (registered context-eval subcommand + --context-eval dashboard flag)
- coderace/dashboard.py (added A/B comparison section)
- README.md (added context-eval documentation)
- progress-log.md (added D1-D4 progress entries)
