diff --git a/execution/binary/utils.go b/execution/binary/utils.go
index a11557ca..db086930 100644
--- a/execution/binary/utils.go
+++ b/execution/binary/utils.go
@@ -42,9 +42,18 @@ func newManyToManyMatchError(matching *parser.VectorMatching, original, duplicat
 
 func (e *errManyToManyMatch) Error() string {
 	group := e.original.MatchLabels(e.matching.On, e.matching.MatchingLabels...)
+	// The choice of which series is reported as "original" vs "duplicate" is
+	// driven by upstream StepVector / map iteration order and is therefore
+	// non-deterministic across runs. Sort the rendered label-set strings so
+	// the error message is stable; downstream consumers (e.g. Cortex,
+	// see cortexproject/cortex#7546) compare these messages.
+	original, duplicate := e.original.String(), e.duplicate.String()
+	if duplicate < original {
+		original, duplicate = duplicate, original
+	}
 	msg := "found duplicate series for the match group %s on the %s hand-side of the operation: [%s, %s]" +
 		";many-to-many matching not allowed: matching labels must be unique on one side"
-	return fmt.Sprintf(msg, group, e.side, e.original.String(), e.duplicate.String())
+	return fmt.Sprintf(msg, group, e.side, original, duplicate)
 }
 
 func shouldDropMetricName(op parser.ItemType, returnBool bool) bool {
diff --git a/execution/binary/utils_test.go b/execution/binary/utils_test.go
new file mode 100644
index 00000000..e8098a1b
--- /dev/null
+++ b/execution/binary/utils_test.go
@@ -0,0 +1,96 @@
+// Copyright (c) The Thanos Community Authors.
+// Licensed under the Apache License 2.0.
+
+package binary
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/efficientgo/core/testutil"
+	"github.com/prometheus/prometheus/model/labels"
+	"github.com/prometheus/prometheus/promql/parser"
+)
+
+// TestErrManyToManyMatchDeterministicOrder verifies that errManyToManyMatch.Error()
+// produces a byte-identical error string regardless of which of the two duplicate
+// series happened to land in the "original" vs "duplicate" slot.
+//
+// Background: the choice of which series becomes original vs duplicate is driven
+// by upstream StepVector ordering, which is in turn fed by map iteration (e.g.
+// in execution/scan/subquery.go). That made the error message non-deterministic
+// across runs and across parallel engine workers, breaking error-string equality
+// checks in downstream consumers (see cortexproject/cortex#7546).
+//
+// Fix: lexicographically sort the two rendered label-set strings before
+// formatting the message.
+func TestErrManyToManyMatchDeterministicOrder(t *testing.T) {
+	matching := &parser.VectorMatching{
+		On:             true,
+		MatchingLabels: []string{"job"},
+	}
+
+	for _, tc := range []struct {
+		name string
+		side binOpSide
+		a    labels.Labels
+		b    labels.Labels
+	}{
+		{
+			name: "simple labels right side",
+			side: rhBinOpSide,
+			a:    labels.FromStrings("__name__", "requests_total", "instance", "a", "job", "api"),
+			b:    labels.FromStrings("__name__", "requests_total", "instance", "b", "job", "api"),
+		},
+		{
+			name: "simple labels left side",
+			side: lhBinOpSide,
+			a:    labels.FromStrings("zone", "us-east-1", "job", "api"),
+			b:    labels.FromStrings("zone", "us-west-2", "job", "api"),
+		},
+		{
+			// Adversarial values that include the same punctuation we use to
+			// frame the rendered list ([, ], comma, braces, quotes). Sorting
+			// the rendered label-set strings — not the labels themselves —
+			// keeps the comparison robust against these characters.
+			name: "adversarial label values",
+			side: rhBinOpSide,
+			a: labels.FromStrings(
+				"detail", `left[,{}"]`,
+				"instance", `a[,{}"]`,
+				"job", `api[,{}"]`,
+			),
+			b: labels.FromStrings(
+				"detail", `right[,{}"]`,
+				"instance", `b[,{}"]`,
+				"job", `api[,{}"]`,
+			),
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			aFirst := (&errManyToManyMatch{
+				matching:  matching,
+				side:      tc.side,
+				original:  tc.a,
+				duplicate: tc.b,
+			}).Error()
+			bFirst := (&errManyToManyMatch{
+				matching:  matching,
+				side:      tc.side,
+				original:  tc.b,
+				duplicate: tc.a,
+			}).Error()
+
+			testutil.Equals(t, aFirst, bFirst)
+
+			// Sanity-check that both rendered label sets are still present and
+			// that the existing prefix/format expected by downstream consumers
+			// (e.g. Prometheus-compatible error matchers) is preserved.
+			testutil.Assert(t, strings.Contains(aFirst, tc.a.String()), "expected %q to contain %q", aFirst, tc.a.String())
+			testutil.Assert(t, strings.Contains(aFirst, tc.b.String()), "expected %q to contain %q", aFirst, tc.b.String())
+			testutil.Assert(t, strings.Contains(aFirst, "found duplicate series for the match group"), "missing expected prefix in %q", aFirst)
+			testutil.Assert(t, strings.Contains(aFirst, "many-to-many matching not allowed"), "missing expected suffix in %q", aFirst)
+			testutil.Assert(t, strings.Contains(aFirst, string(tc.side)+" hand-side"), "missing expected side marker in %q", aFirst)
+		})
+	}
+}