diff --git a/src/routes/internal.js b/src/routes/internal.js
index d960e91..e968e90 100644
--- a/src/routes/internal.js
+++ b/src/routes/internal.js
@@ -629,4 +629,27 @@ router.get('/diagnose-refusals', async (req, res) => {
   }
 });
 
+/**
+ * GET /api/internal/feature-coverage (Connect-layers Step 0)
+ *
+ * READ-ONLY input check: before wiring any layer into the grade, measure
+ * whether its inputs are actually populated on real props. Writes nothing.
+ *
+ *   ?sport=mlb&sample=60
+ */
+router.get('/feature-coverage', async (req, res) => {
+  try {
+    const { coverage } = require('../services/featureCoverage');
+    const out = await coverage({
+      sport: req.query.sport || 'mlb',
+      sample: parseInt(req.query.sample, 10) || undefined,
+      concurrency: parseInt(req.query.concurrency, 10) || undefined,
+    });
+    res.set('Cache-Control', 'no-store');
+    return res.json({ ok: true, ...out });
+  } catch (err) {
+    return res.status(500).json({ ok: false, error: err && err.message });
+  }
+});
+
 module.exports = router;
diff --git a/src/services/featureCoverage.js b/src/services/featureCoverage.js
new file mode 100644
index 0000000..5e484ec
--- /dev/null
+++ b/src/services/featureCoverage.js
@@ -0,0 +1,142 @@
+'use strict';
+
+/**
+ * featureCoverage — Order: CONNECT PROJECTION LAYERS, Step 0. READ-ONLY.
+ *
+ * Before wiring ANY layer into the grade, measure whether its inputs are
+ * actually populated on real props. A layer wired onto sparse inputs does not
+ * degrade gracefully by default — `Number(null) === 0` turns a missing
+ * opportunity into "zero opportunity", which is a fabricated input, not an
+ * absent one.
+ *
+ * Reports, per feature, over a real slate:
+ *   populated / total, and the population rate, split by stat_type — because a
+ *   feature can be 100% present for batters and 0% for pitchers, and a pooled
+ *   number would hide exactly that.
+ */
+
+const DEFAULT_SAMPLE = 60;
+const DEFAULT_CONCURRENCY = 5;
+
+// The opportunity/usage inputs the order is about, plus the projection inputs
+// they would sit alongside (so the report shows relative coverage, not an
+// isolated number that looks fine until you compare it).
+const TRACKED = Object.freeze([
+  'ab_per_game',        // MLB "usage" — season atBats / games
+  'rest_days',
+  'l5_avg',
+  'l20_avg',
+  'l10_stddev',
+  'opp_rank_stat',
+  'minutes_per_game',   // NBA/WNBA usage equivalent
+  'usage_rate',
+  'game_count_in_7d',
+]);
+
+async function mapLimit(items, limit, fn) {
+  const out = new Array(items.length);
+  let cursor = 0;
+  const workers = Array.from({ length: Math.max(1, limit) }, async () => {
+    for (;;) {
+      const idx = cursor;
+      if (idx >= items.length) return;
+      cursor += 1;
+      out[idx] = await fn(items[idx], idx);
+    }
+  });
+  await Promise.all(workers);
+  return out;
+}
+
+/** Strict: a feature counts as populated only when it is a finite number. */
+const populated = (v) => Number.isFinite(Number(v)) && v !== null && v !== '';
+
+async function coverage(opts = {}) {
+  const sport = String(opts.sport || 'mlb').toLowerCase();
+  const sample = Math.max(1, Math.min(300, opts.sample || DEFAULT_SAMPLE));
+  const concurrency = Math.max(1, Math.min(10, opts.concurrency || DEFAULT_CONCURRENCY));
+
+  const getOdds = opts.getOdds || require('./oddsService').getOdds;
+  const getFeatures = opts.getFeatures || require('./intelligence/featureCache').getFeatures;
+  const isModelBook = opts.isModelBook || require('../config/bookRoles').isModelBook;
+
+  const odds = await getOdds(sport);
+  const rows = (odds && odds.props) || [];
+
+  const seen = new Set();
+  const unique = [];
+  for (const p of rows) {
+    if (!p || !p.player || !p.stat_type || p.line == null) continue;
+    if (!isModelBook(p.book)) continue;
+    const k = `${p.player}::${p.stat_type}::${p.line}`;
+    if (seen.has(k)) continue;
+    seen.add(k);
+    unique.push(p);
+  }
+  const batch = unique.slice(0, sample);
+
+  const feats = await mapLimit(batch, concurrency, async (p) => {
+    try {
+      const f = await getFeatures({
+        player: p.player, stat_type: p.stat_type, sport,
+        home_team: p.home_team, away_team: p.away_team, game_time: p.game_time,
+      });
+      return { p, f: f || {} };
+    } catch (err) {
+      return { p, f: {}, error: (err && err.message) || String(err) };
+    }
+  });
+
+  const overall = {};
+  const byStat = {};
+  const distinctValuesPerPlayer = {};   // is the feature prop-specific or per-player constant?
+  for (const key of TRACKED) overall[key] = 0;
+
+  for (const { p, f } of feats) {
+    const stat = String(p.stat_type || '?');
+    byStat[stat] = byStat[stat] || { n: 0 };
+    byStat[stat].n += 1;
+    for (const key of TRACKED) {
+      const ok = populated(f[key]);
+      if (ok) overall[key] += 1;
+      byStat[stat][key] = (byStat[stat][key] || 0) + (ok ? 1 : 0);
+    }
+    if (populated(f.ab_per_game)) {
+      const pk = String(p.player).toLowerCase();
+      distinctValuesPerPlayer[pk] = distinctValuesPerPlayer[pk] || new Set();
+      distinctValuesPerPlayer[pk].add(Math.round(Number(f.ab_per_game) * 1000));
+    }
+  }
+
+  const n = batch.length || 1;
+  const rate = (v) => Math.round((1000 * v) / n) / 10;
+
+  // A per-player CONSTANT cannot separate that player's props from each other.
+  // If ab_per_game takes one value across every prop a player has, it can only
+  // shift all of his props together — which is a very different thing from a
+  // per-prop opportunity signal, and worth knowing before wiring it.
+  const multiPropPlayers = Object.values(distinctValuesPerPlayer).filter((s) => s.size > 0);
+  const playersWithVaryingValue = multiPropPlayers.filter((s) => s.size > 1).length;
+
+  return {
+    read_only: true,
+    sport,
+    generated_at: new Date().toISOString(),
+    sampled: batch.length,
+    unique_gradeable: unique.length,
+    coverage: Object.fromEntries(TRACKED.map((k) => [k, { populated: overall[k], pct: rate(overall[k]) }])),
+    by_stat: Object.fromEntries(Object.entries(byStat).map(([stat, v]) => [stat, {
+      n: v.n,
+      ...Object.fromEntries(TRACKED.map((k) => [k, v.n ? Math.round((1000 * (v[k] || 0)) / v.n) / 10 : 0])),
+    }])),
+    ab_per_game_shape: {
+      players_with_value: multiPropPlayers.length,
+      players_where_it_varies_across_their_props: playersWithVaryingValue,
+      note: playersWithVaryingValue === 0
+        ? 'CONSTANT per player across all of that player\'s props — it can only move all of a player\'s props together, not separate them.'
+        : 'varies across a player\'s props',
+    },
+  };
+}
+
+module.exports = { coverage, __internals: { TRACKED, populated, mapLimit } };