diff --git a/tools/etl/build.ts b/tools/etl/build.ts
index 6f11c09..5ca1709 100644
--- a/tools/etl/build.ts
+++ b/tools/etl/build.ts
@@ -73,8 +73,11 @@ function validateStars(stars: StarRecord[]): void {
  * lacks: bright stars it saturates on, red dwarfs past its magnitude cut. So the headroom left to
  * the ceiling tracks the gap between those two cutoffs as much as Gaia's completeness.
  *
- * This bounds a merge that went wrong. It cannot bound a Gaia download that came back short: that
- * makes *fewer* survivors, not more, and is guarded where it can be seen, in `fetchGaiaStars`.
+ * This bounds a merge that went wrong, and — loosely — a Gaia download that came back short: a
+ * truncated answer leaves the HYG rows whose counterpart it dropped without one, so survivors go
+ * *up*, not down. Measured against the published catalogue: 10 886 today, 11 004 at nine tenths of
+ * the rows, 12 711 at half, 16 258 at a third. So this ceiling only catches a truncation past about
+ * two thirds, and `fetchGaiaStars` catches the shallower ones with its own row floor.
  */
 const MAX_UNMERGED_TWINS = 100;
 const MAX_HYG_SURVIVORS = 15_000;
diff --git a/tools/etl/fetchStars.ts b/tools/etl/fetchStars.ts
index 1f2eb66..0225fba 100644
--- a/tools/etl/fetchStars.ts
+++ b/tools/etl/fetchStars.ts
@@ -3,7 +3,7 @@ import { writeFileSync } from 'node:fs';
 import { mergeStarCatalogues, placementDistancePc } from '../../src/app/shared/astro/star-merge';
 import { encodeStarCatalog } from '../../src/app/shared/models/star-catalog';
 import { StarRecord, SUN_STAR_ID } from '../../src/app/shared/models/star.model';
-import { fetchGaiaDistancesByHip } from './sources/gaia';
+import { fetchGaiaDistancesByHip, GaiaAnswerError } from './sources/gaia';
 import { positionalSources } from './sources/registry';
 import { PARALLAX_PRECISION_MAS } from './sources/star-sources';
 import { parseCsvObjects, parseOptionalNumber } from './lib/csv';
@@ -142,7 +142,9 @@ export async function fetchStars(): Promise<StarRecord[]> {
  *
  * A source that cannot be reached is reported and skipped here rather than thrown, so a run still
  * gets as far as validation and says what it has. Whether that may be published is decided
- * there: `validateMerge` in build.ts refuses a catalogue Gaia contributed nothing to.
+ * there: `validateMerge` in build.ts refuses a catalogue Gaia contributed nothing to. A source
+ * that answered with something unusable ({@link GaiaAnswerError}) is a different matter, and stops
+ * the run where it happened rather than being reported later as an outage.
  */
 async function mergeWithOtherSources(hygStars: StarRecord[]): Promise<StarRecord[]> {
   const others = positionalSources().filter((source) => source.id !== 'hyg');
@@ -160,6 +162,11 @@ async function mergeWithOtherSources(hygStars: StarRecord[]): Promise<StarRecord
         stars: await source.fetch!()
       });
     } catch (error) {
+      // An answer that cannot be worked with is not an outage: skipping it would write a
+      // half-catalogue over the published assets before the merge gate got to say so.
+      if (error instanceof GaiaAnswerError) {
+        throw error;
+      }
       console.log(`  skipping ${source.name}: ${error instanceof Error ? error.message : error}`);
     }
   }
diff --git a/tools/etl/sources/gaia.ts b/tools/etl/sources/gaia.ts
index 6d02dc9..1df52bc 100644
--- a/tools/etl/sources/gaia.ts
+++ b/tools/etl/sources/gaia.ts
@@ -45,23 +45,6 @@ const DISTANCE_CUTOFF_PC = Number(process.env['ETL_GAIA_DISTANCE_PC'] ?? DEFAULT
 const MAGNITUDE_LIMIT = Number(process.env['ETL_GAIA_MAGNITUDE_LIMIT'] ?? DEFAULT_MAGNITUDE_LIMIT);
 const ROW_LIMIT = Number(process.env['ETL_GAIA_ROW_LIMIT'] ?? DEFAULT_ROW_LIMIT);
 
-/**
- * How many rows the scheduled job's own query holds: 412 765, and DR3 is a finished data release,
- * so that number only moves when the query does.
- *
- * Checked because a short answer looks exactly like a complete one. The TAP service truncates on
- * its own timeout and still serves a well-formed CSV with a 200, and the rows are ordered by
- * magnitude, so what comes back is the bright half — the half HYG overlaps. The merge gate in
- * `build.ts` would then see Gaia stars present, fewer HYG survivors and fewer unmerged twins, and
- * pass a catalogue missing two hundred thousand stars, which the weekly job would publish and the
- * runner would cache for the weeks after it. Same failure, and same guard, as
- * {@link MIN_USABLE_HIP_DISTANCES} below.
- *
- * Only checked for that query: the environment overrides exist to fetch a smaller slice on purpose.
- */
-const DEFAULT_QUERY_ROWS = 412_765;
-const MIN_ROW_SHARE = 0.95;
-
 /**
  * Relative parallax error above which a star is dropped: a parallax measured to worse than 20%
  * gives a distance that is not worth plotting, and inverting a noisy parallax biases it badly.
@@ -87,6 +70,35 @@ function buildQuery(): string {
   ].join(' ');
 }
 
+/**
+ * How many rows the query above holds when nothing is overridden: 412 765, and DR3 is a finished
+ * data release, so that number only moves when the query does. It lives here, under the query, so
+ * that an edit to any of its filters is made with the count it invalidates in view.
+ *
+ * Checked because a short answer looks exactly like a complete one. The TAP service truncates on
+ * its own timeout and still serves a well-formed CSV with a 200, and the rows are ordered by
+ * magnitude, so what comes back is the bright half — the half HYG overlaps. The merge gate in
+ * `build.ts` would then see Gaia stars present and a survivor count barely moved, and pass a
+ * catalogue missing two hundred thousand stars, which the weekly job would publish and the runner
+ * would cache for the weeks after it. Same failure, and same guard, as
+ * {@link MIN_USABLE_HIP_DISTANCES} below.
+ *
+ * Only checked when nothing is overridden: the environment overrides exist to fetch a smaller
+ * slice on purpose.
+ */
+const DEFAULT_QUERY_ROWS = 412_765;
+const MIN_ROW_SHARE = 0.95;
+
+/**
+ * An answer the archive gave that cannot be worked with, as against an archive that gave none.
+ *
+ * `fetchStars` skips a source it cannot reach and leaves the merge gate to judge the result. That
+ * is right for an outage and wrong for a truncated CSV, which would be skipped, cached, and land
+ * as "the archive was unreachable" long after the assets had been overwritten — so these throws
+ * are marked, and rethrown there.
+ */
+export class GaiaAnswerError extends Error {}
+
 /**
  * Gaia publishes no spectral classifications, but `bp_rp` is a colour index on the same footing
  * as HYG's `ci` — so the app's existing colour and spectral-class handling works unchanged, and
@@ -110,17 +122,18 @@ export async function fetchGaiaStars(): Promise<StarRecord[]> {
   // Keyed by the whole request, so a response cached for other columns, another order, or
   // another endpoint can never be mistaken for this one — the cache records only that some
   // response arrived, not what it answered.
-  const csv = await fetchTextCached(url, `gaia-dr3-${createHash('sha1').update(url).digest('hex').slice(0, 8)}.csv`);
+  const cacheKey = `gaia-dr3-${createHash('sha1').update(url).digest('hex').slice(0, 8)}.csv`;
+  const csv = await fetchTextCached(url, cacheKey);
   const rows = parseCsvObjects(csv);
   const jobsQuery = DISTANCE_CUTOFF_PC === DEFAULT_DISTANCE_CUTOFF_PC && MAGNITUDE_LIMIT === DEFAULT_MAGNITUDE_LIMIT && ROW_LIMIT === DEFAULT_ROW_LIMIT;
   if (jobsQuery && rows.length < DEFAULT_QUERY_ROWS * MIN_ROW_SHARE) {
-    throw new Error(
-      `Gaia returned ${rows.length} rows, not the ~${DEFAULT_QUERY_ROWS} this query holds — the answer was cut short, ` +
-        'or was an error page served with a 200; delete tools/etl/.cache/gaia-dr3-*.csv once the archive answers properly'
+    throw new GaiaAnswerError(
+      `Gaia returned ${rows.length} rows, not the ~${DEFAULT_QUERY_ROWS} this query holds — the answer was cut short, it was an error page ` +
+        `served with a 200, or the query was edited without updating DEFAULT_QUERY_ROWS; delete tools/etl/.cache/${cacheKey} once the archive answers properly`
     );
   }
-  if (rows.length >= ROW_LIMIT) {
-    throw new Error(`Gaia returned the query's own ${ROW_LIMIT}-row limit, so it is the limit deciding what the map holds; raise ETL_GAIA_ROW_LIMIT.`);
+  if (jobsQuery && rows.length >= ROW_LIMIT) {
+    throw new GaiaAnswerError(`Gaia returned the query's own ${ROW_LIMIT}-row limit, so it is the limit deciding what the map holds; raise ETL_GAIA_ROW_LIMIT.`);
   }
   const stars: StarRecord[] = [];