# MLB Grade Pipeline Degradation — Backend Finding (2026-07-17)

Source: phone-audit P1-7 (broken edge board) + P2-9 (B grades at 45% confidence).
Diagnosed against LIVE `GET /api/snapshot/mlb` on 2026-07-17. This is a BACKEND
grading/feature-pipeline issue, NOT a frontend display bug. The frontend guards
(sane-edge cap, statAbbrev) are damage control, not the fix.

## The evidence (25 live MLB grades)
- **projection == 0 for 9/25.** The model has no projection reference for a third
  of props — a degraded feature path (the S58 insufficient-data refusal should
  have caught these; instead they graded with projection 0).
- **edge_pct on a broken scale.** Distinct values {20, 60, 100, 140}. A real
  prop-market edge is single-digit %, never past ~40. These are not a market edge
  (frontend now shows them absent — slateAdapter EDGE_BOARD_SANE_MAX / EdgeCell).
- **grade <-> confidence mismatch on 25/25.** Every grade's letter disagrees with
  its own surfaced `confidence` vs `grade_thresholds.json`:
    - B shown at confidence 55 -> 55 is the B- band (55-59), not B (60-65).
    - Systematic ~5-point (one sub-tier) gap on every prop.
  The surfaced `confidence` is NOT the probability that derived the letter.

## Likely root cause (to investigate, not yet fixed)
1. `confidence` field is post-`apply_data_sufficiency_modifier` (bayesian.py),
   while the grade letter was assigned from the PRE-penalty prob -> the two
   disagree by one sub-tier on every prop. Either surface the same value, or
   re-letter from the penalized confidence. Decide which is the source of truth.
2. projection==0 for 9/25: the MLB feature/projection path is feeding 0 instead
   of refusing (S58 `insufficient_data`). Trace `projectionFor` for MLB props;
   projection 0 also breaks `computeEdge` (=> the 20/60/100/140 garbage).

## Do NOT
- Do not re-letter or shift thresholds on the frontend (would hide the bug).
- Do not "fix" edge display by rescaling 140 -> 14 (guessing the scale is its own
  fabrication). The number must come out of the pipeline correct.

## Frontend already shipped (honest guards)
- Leaderboard stat labels: `web/src/lib/statAbbrev.js` (raw snake_case -> SB/ER/TB).
- Edge board: impossible |edge|>40 rendered absent AND excluded from ranking.