diff --git a/internal/tools/foldkey_internal_test.go b/internal/tools/foldkey_internal_test.go
new file mode 100644
index 0000000..da7d057
--- /dev/null
+++ b/internal/tools/foldkey_internal_test.go
@@ -0,0 +1,42 @@
+package tools
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+)
+
+// foldKey must agree with strings.EqualFold, which is how encoding/json matches.
+func TestFoldKeyAgreesWithEqualFold(t *testing.T) {
+	pairs := [][2]string{{"branch", "Branch"}, {"sha", "ſha"}, {"k", "K"}, {"ss", "ß"}, {"a", "b"}, {"Σ", "ς"}}
+	for _, p := range pairs {
+		if got, want := foldKey(p[0]) == foldKey(p[1]), strings.EqualFold(p[0], p[1]); got != want {
+			t.Errorf("%q vs %q: foldKey equal=%v, EqualFold=%v", p[0], p[1], got, want)
+		}
+	}
+}
+
+// The duplicate scan is linear in the number of keys: 90k keys (about the
+// 1 MiB body cap) must scan well under a second.
+func TestRejectAmbiguousKeysScalesLinearly(t *testing.T) {
+	var b strings.Builder
+	b.WriteString("{")
+	for i := 0; i < 90_000; i++ {
+		if i > 0 {
+			b.WriteString(",")
+		}
+		fmt.Fprintf(&b, `"k%d":1`, i)
+	}
+	b.WriteString("}")
+	var dst struct {
+		Owner string `json:"owner"`
+	}
+	start := time.Now()
+	if err := rejectAmbiguousKeys([]byte(b.String()), &dst); err != nil {
+		t.Fatal(err)
+	}
+	if d := time.Since(start); d > 2*time.Second {
+		t.Errorf("scan of 90k keys took %v", d)
+	}
+}
diff --git a/internal/tools/tool.go b/internal/tools/tool.go
index 52eb640..ca79e9e 100644
--- a/internal/tools/tool.go
+++ b/internal/tools/tool.go
@@ -8,6 +8,7 @@ import (
 	"reflect"
 	"regexp"
 	"strings"
+	"unicode"
 
 	"git.d-ma.be/mathias/gitea-mcp/internal/gitea"
 	"git.d-ma.be/mathias/gitea-mcp/internal/registry"
@@ -84,21 +85,18 @@ func scanJSONValue(dec *json.Decoder) error {
 	}
 	switch d {
 	case '{':
-		var seen []string
+		seen := map[string]string{}
 		for dec.More() {
 			kt, err := dec.Token()
 			if err != nil {
 				return err
 			}
 			k, _ := kt.(string)
-			// EqualFold, not ToLower: encoding/json folds the same way, so
-			// long s (U+017F) matches s and the Kelvin sign matches k.
-			for _, prev := range seen {
-				if strings.EqualFold(prev, k) {
-					return fmt.Errorf("duplicate argument keys %q and %q (keys must be unique ignoring case): %w", prev, k, gitea.ErrValidation)
-				}
+			fk := foldKey(k)
+			if prev, dup := seen[fk]; dup {
+				return fmt.Errorf("duplicate argument keys %q and %q (keys must be unique ignoring case): %w", prev, k, gitea.ErrValidation)
 			}
-			seen = append(seen, k)
+			seen[fk] = k
 			if err := scanJSONValue(dec); err != nil {
 				return err
 			}
@@ -114,6 +112,25 @@ func scanJSONValue(dec *json.Decoder) error {
 	return err
 }
 
+// foldKey maps every rune to the smallest member of its unicode.SimpleFold
+// orbit, so foldKey(a) == foldKey(b) exactly when strings.EqualFold(a, b) —
+// the folding encoding/json uses (long s == s, Kelvin sign == k). A map keyed
+// on it keeps the duplicate scan linear (PR #66 review: a pairwise EqualFold
+// loop cost 7s of CPU on ~90k keys).
+func foldKey(s string) string {
+	var b strings.Builder
+	for _, r := range s {
+		min := r
+		for f := unicode.SimpleFold(r); f != r; f = unicode.SimpleFold(f) {
+			if f < min {
+				min = f
+			}
+		}
+		b.WriteRune(min)
+	}
+	return b.String()
+}
+
 // jsonFieldNames returns the json tag names of dst's top-level struct fields.
 func jsonFieldNames(dst any) []string {
 	v := reflect.ValueOf(dst)