diff --git a/tools/etl/build.ts b/tools/etl/build.ts
index bc82657..53e49ef 100644
--- a/tools/etl/build.ts
+++ b/tools/etl/build.ts
@@ -49,6 +49,71 @@ function validateStars(stars: StarRecord[]): void {
   }
 }
 
+/**
+ * What a good merge looks like, in two numbers the unit suite cannot see.
+ *
+ * The catalogues are regenerated by a scheduled job that pushes straight to `main` once the unit
+ * tests and a production build pass — and both passed, for weeks, on a catalogue carrying 23 000
+ * stars twice: the suite tests code against fixtures, and no fixture is 400 000 real stars. The
+ * two ways the merge has actually failed both show up here.
+ *
+ * A star kept twice leaves its two entries near each other on the sky, from *different* sources —
+ * one catalogue does not list a star twice. Under an arcsecond that is never two stars at this
+ * depth, so every such pair is a miss. Nineteen survive today, all of them a second HYG row
+ * wanting a Gaia entry that already absorbed one (Gliese lists some doubles twice); the merge
+ * that trusted a Hipparcos parallax over direction left 1 112.
+ *
+ * The other failure leaves no close pair at all, because proper motion had already carried the
+ * two entries tens of arcseconds apart — the 2026-08-24 refresh, where HYG sat at epoch 2000.0
+ * and Gaia at J2016.0. What it does leave is HYG rows that found no counterpart: 36 056 of them
+ * against the 10 876 Gaia genuinely lacks (bright stars it saturates on, red dwarfs past its
+ * magnitude cut).
+ */
+const MAX_UNMERGED_TWINS = 100;
+const MAX_HYG_SURVIVORS = 15_000;
+const TWIN_TOLERANCE_RAD = (1 / 3600) * (Math.PI / 180);
+
+function validateMerge(stars: StarRecord[]): void {
+  const survivors = stars.filter((star) => star.source === 'hyg').length;
+  assertCondition(
+    survivors <= MAX_HYG_SURVIVORS,
+    `${survivors} HYG stars found no Gaia counterpart (at most ${MAX_HYG_SURVIVORS} expected) — the two catalogues are not being matched.`
+  );
+
+  // Sorted by declination, so each star is only compared against the handful sharing its
+  // parallel — an arcsecond of declination holds one or two of 400 000 stars.
+  const byDec = stars
+    .map((star) => {
+      const distance = Math.hypot(star.x, star.y, star.z);
+      return { star, distance, dec: distance === 0 ? 0 : Math.asin(Math.max(-1, Math.min(1, star.z / distance))) };
+    })
+    .filter((entry) => entry.distance > 0)
+    .sort((a, b) => a.dec - b.dec);
+
+  const cosTolerance = Math.cos(TWIN_TOLERANCE_RAD);
+  let twins = 0;
+  let example = '';
+  for (let i = 0; i < byDec.length; i++) {
+    const a = byDec[i];
+    for (let j = i + 1; j < byDec.length && byDec[j].dec - a.dec <= TWIN_TOLERANCE_RAD; j++) {
+      const b = byDec[j];
+      if (a.star.source === b.star.source) {
+        continue;
+      }
+      const cosine = (a.star.x * b.star.x + a.star.y * b.star.y + a.star.z * b.star.z) / (a.distance * b.distance);
+      if (cosine >= cosTolerance) {
+        twins++;
+        example ||= `${a.star.name} (${a.star.source}) and ${b.star.name} (${b.star.source})`;
+      }
+    }
+  }
+  assertCondition(
+    twins <= MAX_UNMERGED_TWINS,
+    `${twins} stars from different catalogues sit within an arcsecond of each other (at most ${MAX_UNMERGED_TWINS} expected), starting with ${example} — the merge is keeping the same star twice.`
+  );
+  console.log(`  ${survivors} HYG stars have no Gaia counterpart; ${twins} unmerged cross-catalogue pairs within an arcsecond.`);
+}
+
 function validateBodies(bodies: BodyRecord[]): void {
   assertCondition(bodies.length > 0, 'No solar-system bodies were produced.');
 
@@ -162,6 +227,7 @@ async function build(): Promise<void> {
 
   console.log('Validating output...');
   validateStars(stars);
+  validateMerge(stars);
   validateBodies(bodies);
   validateExoplanets(exoplanets, new Set(stars.map((star) => star.id)));
   validateDeepSky(deepSky);