diff --git a/gorilla-merger/internal/merger/ingest.go b/gorilla-merger/internal/merger/ingest.go
index 8999a1d5..bb1eb280 100644
--- a/gorilla-merger/internal/merger/ingest.go
+++ b/gorilla-merger/internal/merger/ingest.go
@@ -47,15 +47,17 @@ func NewIngester(s *Storage, logger *slog.Logger) *Ingester {
 // labelsFor builds the series labels for a fragment: __name__ = MetricName,
 // plus the fragment Attributes, plus the merger's external labels. External
 // labels win on conflict (they identify this merger/agent and must be stable).
-func labelsFor(metricName string, attrs map[string]string, external labels.Labels) labels.Labels {
+func labelsFor(metricName string, attrs map[string]string) labels.Labels {
 	bld := labels.NewBuilder(labels.EmptyLabels())
 	bld.Set(labels.MetricName, metricName)
 	for k, v := range attrs {
 		bld.Set(k, v)
 	}
-	external.Range(func(l labels.Label) {
-		bld.Set(l.Name, l.Value)
-	})
+	// External labels (cluster/merger) are NOT stamped into the stored series:
+	// the Thanos TSDBStore appends them at query time, and the shipper writes
+	// them into each block's meta. Stamping them here too produced DUPLICATE
+	// cluster/merger labels, so thanos TSDBStore.Series -> ReAllocZLabelsStrings
+	// read a malformed label set and crashed (corrupt-length OOM).
 	return bld.Labels()
 }
 
@@ -92,7 +94,7 @@ func (i *Ingester) IngestBatch(ctx context.Context, raw []byte) (ingestResult, e
 			_ = app.Rollback()
 			return ingestResult{}, fmt.Errorf("fragment %d: decode xor chunk: %w", fi, cerr)
 		}
-		ls := labelsFor(f.MetricName, f.Attributes, i.externalLabels)
+		ls := labelsFor(f.MetricName, f.Attributes)
 
 		it := chunk.Iterator(nil)
 		var ref storage.SeriesRef
diff --git a/gorilla-merger/internal/merger/ingest_test.go b/gorilla-merger/internal/merger/ingest_test.go
index 3800f757..08e76736 100644
--- a/gorilla-merger/internal/merger/ingest_test.go
+++ b/gorilla-merger/internal/merger/ingest_test.go
@@ -138,10 +138,11 @@ func TestIngestRoundTrip(t *testing.T) {
 		t.Fatalf("expected 200, got %d", resp.StatusCode)
 	}
 
-	// Series A: __name__ + its attrs + external labels.
+	// Series A: __name__ + its attrs only (external labels are added by the
+	// TSDBStore at query time, NOT stamped into the stored series).
 	wantA := labels.FromStrings(
 		labels.MetricName, "http_requests_total",
-		"job", "api", "instance", "a", "merger", "test-merger")
+		"job", "api", "instance", "a")
 	gotA := readBack(t, storage, wantA)
 	wantSamplesA := []sample{{base, 1}, {base + 1000, 2}, {base + 2000, 3}}
 	assertSamples(t, "A", gotA, wantSamplesA)
@@ -149,7 +150,7 @@ func TestIngestRoundTrip(t *testing.T) {
 	// Series B: a distinct instance is a distinct series.
 	wantB := labels.FromStrings(
 		labels.MetricName, "http_requests_total",
-		"job", "api", "instance", "b", "merger", "test-merger")
+		"job", "api", "instance", "b")
 	gotB := readBack(t, storage, wantB)
 	wantSamplesB := []sample{{base, 10}, {base + 1000, 20}}
 	assertSamples(t, "B", gotB, wantSamplesB)
@@ -178,12 +179,14 @@ func TestIngestBadBodyReturns400(t *testing.T) {
 	}
 }
 
-func TestLabelsForExternalWins(t *testing.T) {
-	ext := labels.FromStrings("merger", "m1", "shared", "external")
-	got := labelsFor("metric", map[string]string{"shared": "frag", "job": "x"}, ext)
+func TestLabelsForNoExternalStamping(t *testing.T) {
+	// External labels (cluster/merger) must NOT be stamped at ingest — the
+	// Thanos TSDBStore appends them at query time. Stamping them here too
+	// produced duplicate labels and crashed TSDBStore.Series.
+	got := labelsFor("metric", map[string]string{"shared": "frag", "job": "x"})
 	want := labels.FromStrings(
 		labels.MetricName, "metric",
-		"job", "x", "merger", "m1", "shared", "external")
+		"job", "x", "shared", "frag")
 	if labels.Compare(got, want) != 0 {
 		t.Fatalf("labelsFor mismatch:\n got  %s\n want %s", got.String(), want.String())
 	}
diff --git a/gorilla-merger/internal/merger/storeapi.go b/gorilla-merger/internal/merger/storeapi.go
index 075c9e9b..c8f5d92a 100644
--- a/gorilla-merger/internal/merger/storeapi.go
+++ b/gorilla-merger/internal/merger/storeapi.go
@@ -7,7 +7,10 @@ import (
 	kitlog "github.com/go-kit/log"
 	"github.com/prometheus/prometheus/model/labels"
 	"github.com/thanos-io/thanos/pkg/component"
+	"github.com/thanos-io/thanos/pkg/info"
+	"github.com/thanos-io/thanos/pkg/info/infopb"
 	"github.com/thanos-io/thanos/pkg/store"
+	"github.com/thanos-io/thanos/pkg/store/labelpb"
 	"github.com/thanos-io/thanos/pkg/store/storepb"
 	"google.golang.org/grpc"
 )
@@ -35,6 +38,25 @@ func NewStoreAPI(s *Storage, extLset labels.Labels, logger kitlog.Logger, addr s
 	grpcSrv := grpc.NewServer()
 	storepb.RegisterStoreServer(grpcSrv, tsdbStore)
 
+	// thanos-query (v0.41) discovers an endpoint via the Info service; a
+	// Store-only server is reachable but undiscoverable ("neither info nor
+	// store client found"), so register Info too — mirroring the sidecar.
+	infoSrv := info.NewInfoServer(
+		component.Receive.String(),
+		info.WithLabelSetFunc(func() []labelpb.ZLabelSet { return tsdbStore.LabelSet() }),
+		info.WithStoreInfoFunc(func() (*infopb.StoreInfo, error) {
+			mint, maxt := tsdbStore.TimeRange()
+			return &infopb.StoreInfo{
+				MinTime:                      mint,
+				MaxTime:                      maxt,
+				SupportsSharding:             true,
+				SupportsWithoutReplicaLabels: true,
+				TsdbInfos:                    tsdbStore.TSDBInfos(),
+			}, nil
+		}),
+	)
+	info.RegisterInfoServer(infoSrv)(grpcSrv)
+
 	return &StoreAPI{
 		srv:     grpcSrv,
 		tsdbStr: tsdbStore,