diff --git a/src/algo/algo.go b/src/algo/algo.go
index c3a4ca7d..f4c128b6 100644
--- a/src/algo/algo.go
+++ b/src/algo/algo.go
@@ -347,15 +347,49 @@ func isAscii(runes []rune) bool {
 	return true
 }
 
-// runeFuzzyIndex is asciiFuzzyIndex for rune-mode input. Only valid when the
-// input cannot fold to ASCII, see the caller.
+// runePrefilterable reports whether scanning the rune array can decide this
+// pattern against this item without missing a match. Phase 2 lowercases an
+// uppercase text rune and then normalizes it, and the scan sees neither
+// transform, so every pattern rune must be unreachable by them.
+func runePrefilterable(input *util.Chars, pattern []rune, caseSensitive bool) bool {
+	if input.MayFoldToAscii() {
+		// A non-ASCII rune of this item could fold onto an ASCII pattern char
+		for _, r := range pattern {
+			if r < utf8.RuneSelf {
+				return false
+			}
+		}
+	}
+	if caseSensitive {
+		// No case transform is applied, and normalization only ever produces
+		// ASCII, so nothing can reach a non-ASCII pattern rune
+		return true
+	}
+	for _, r := range pattern {
+		// Another rune must not lowercase onto this one. Being uncased is not
+		// enough by itself: U+00DF has no simple uppercase yet U+1E9E
+		// lowercases to it. Excluding the foldable set covers that.
+		if r >= utf8.RuneSelf &&
+			(unicode.ToUpper(r) != r || unicode.ToLower(r) != r || util.MayFoldToAscii(r)) {
+			return false
+		}
+	}
+	return true
+}
+
+// runeFuzzyIndex is asciiFuzzyIndex for rune-mode input. Only valid when
+// runePrefilterable says so.
 func runeFuzzyIndex(input *util.Chars, pattern []rune, caseSensitive bool) (int, int) {
 	runes := input.Runes()
 	firstIdx, idx, lastIdx := 0, 0, 0
-	var b byte
+	var last rune
 	for pidx := range pattern {
-		b = byte(pattern[pidx])
-		idx = indexAsciiRune(runes, caseSensitive, b, idx)
+		last = pattern[pidx]
+		if last < utf8.RuneSelf {
+			idx = indexAsciiRune(runes, caseSensitive, byte(last), idx)
+		} else {
+			idx = indexRune(runes, last, idx)
+		}
 		if idx < 0 {
 			return -1, -1
 		}
@@ -370,7 +404,13 @@ func runeFuzzyIndex(input *util.Chars, pattern []rune, caseSensitive bool) (int,
 	// Find the last appearance of the last character of the pattern to limit
 	// the search scope
 	if lastIdx+1 < len(runes) {
-		if end := lastIndexAsciiRune(runes, caseSensitive, b, lastIdx+1); end >= 0 {
+		var end int
+		if last < utf8.RuneSelf {
+			end = lastIndexAsciiRune(runes, caseSensitive, byte(last), lastIdx+1)
+		} else {
+			end = lastIndexRune(runes, last, lastIdx+1)
+		}
+		if end >= 0 {
 			return firstIdx, end + 1
 		}
 	}
@@ -378,24 +418,26 @@ func runeFuzzyIndex(input *util.Chars, pattern []rune, caseSensitive bool) (int,
 }
 
 func asciiFuzzyIndex(input *util.Chars, pattern []rune, caseSensitive bool) (int, int) {
-	if !isAscii(pattern) {
-		// An ASCII string cannot contain a non-ASCII character
-		if input.IsBytes() {
-			return -1, -1
-		}
-		// Rune input with a non-ASCII pattern is not filtered yet
-		return 0, input.Length()
-	}
-
 	if !input.IsBytes() {
-		// Case folding or normalization can turn a non-ASCII rune into the
-		// ASCII character we are looking for, which the scan cannot see
-		if disableRunePrefilter || input.MayFoldToAscii() {
+		if disableRunePrefilter {
 			return 0, input.Length()
 		}
+		// runePrefilterable does not inline, so keep the common case out of
+		// it: an ASCII pattern against an item that cannot fold to ASCII is
+		// always scannable, and both of these checks do inline.
+		if input.MayFoldToAscii() || !isAscii(pattern) {
+			if !runePrefilterable(input, pattern, caseSensitive) {
+				return 0, input.Length()
+			}
+		}
 		return runeFuzzyIndex(input, pattern, caseSensitive)
 	}
 
+	// Not possible
+	if !isAscii(pattern) {
+		return -1, -1
+	}
+
 	firstIdx, idx, lastIdx := 0, 0, 0
 	var b byte
 	for pidx := range pattern {
diff --git a/src/algo/runeindex_others.go b/src/algo/runeindex_others.go
index 4a0b73d9..c6961447 100644
--- a/src/algo/runeindex_others.go
+++ b/src/algo/runeindex_others.go
@@ -13,3 +13,11 @@ func indexAsciiRune(runes []rune, caseSensitive bool, b byte, from int) int {
 func lastIndexAsciiRune(runes []rune, caseSensitive bool, b byte, from int) int {
 	return lastIndexAsciiRuneRef(runes, caseSensitive, b, from)
 }
+
+func indexRune(runes []rune, r rune, from int) int {
+	return indexRuneRef(runes, r, from)
+}
+
+func lastIndexRune(runes []rune, r rune, from int) int {
+	return lastIndexRuneRef(runes, r, from)
+}
diff --git a/src/algo/runeindex_ref.go b/src/algo/runeindex_ref.go
index 53bcd038..8bc07399 100644
--- a/src/algo/runeindex_ref.go
+++ b/src/algo/runeindex_ref.go
@@ -35,3 +35,21 @@ func lastIndexAsciiRuneRef(runes []rune, caseSensitive bool, b byte, from int) i
 	}
 	return -1
 }
+
+func indexRuneRef(runes []rune, r rune, from int) int {
+	for i := from; i < len(runes); i++ {
+		if runes[i] == r {
+			return i
+		}
+	}
+	return -1
+}
+
+func lastIndexRuneRef(runes []rune, r rune, from int) int {
+	for i := len(runes) - 1; i >= from; i-- {
+		if runes[i] == r {
+			return i
+		}
+	}
+	return -1
+}
diff --git a/src/algo/runeindex_x86.go b/src/algo/runeindex_x86.go
index 56d31db3..aea5c335 100644
--- a/src/algo/runeindex_x86.go
+++ b/src/algo/runeindex_x86.go
@@ -42,6 +42,69 @@ func indexAsciiRune(runes []rune, caseSensitive bool, b byte, from int) int {
 	return -1
 }
 
+// runeNeedle picks which of the rune's four bytes to scan for, and returns its
+// lane index and value. A zero byte is a useless needle because every ASCII
+// rune contributes three of them, so U+AE00 scanned by its low byte would hit
+// on almost every character of an ASCII-heavy line. Prefer a byte that cannot
+// occur in an ASCII rune at all, then any non-zero byte.
+func runeNeedle(r rune) (int, byte) {
+	var b [4]byte
+	b[0], b[1], b[2], b[3] = byte(r), byte(r>>8), byte(r>>16), byte(r>>24)
+	for i, v := range b {
+		if v >= 0x80 {
+			return i, v
+		}
+	}
+	for i, v := range b {
+		if v != 0 {
+			return i, v
+		}
+	}
+	return 0, 0
+}
+
+func runeAt(view []byte, start int) rune {
+	return rune(view[start]) | rune(view[start+1])<<8 | rune(view[start+2])<<16 | rune(view[start+3])<<24
+}
+
+// indexRune returns the index of the first rune equal to r at or after rune
+// index from. Case is not folded, so the caller must have established that no
+// other rune can transform into r.
+func indexRune(runes []rune, r rune, from int) int {
+	view := runeBytes(runes)
+	lane, needle := runeNeedle(r)
+	for off := from*4 + lane; off < len(view); {
+		idx := bytes.IndexByte(view[off:], needle)
+		if idx < 0 {
+			return -1
+		}
+		pos := off + idx
+		if start := pos - lane; start&3 == 0 && runeAt(view, start) == r {
+			return start >> 2
+		}
+		off = pos + 1
+	}
+	return -1
+}
+
+// lastIndexRune is indexRune scanning backwards from the end.
+func lastIndexRune(runes []rune, r rune, from int) int {
+	view := runeBytes(runes)
+	lane, needle := runeNeedle(r)
+	for end := len(view); end > from*4+lane; {
+		idx := bytes.LastIndexByte(view[from*4+lane:end], needle)
+		if idx < 0 {
+			return -1
+		}
+		pos := from*4 + lane + idx
+		if start := pos - lane; start&3 == 0 && runeAt(view, start) == r {
+			return start >> 2
+		}
+		end = pos
+	}
+	return -1
+}
+
 // lastIndexAsciiRune is indexAsciiRune scanning backwards from the end.
 func lastIndexAsciiRune(runes []rune, caseSensitive bool, b byte, from int) int {
 	view := runeBytes(runes)[from*4:]
diff --git a/src/algo/runeprefilter_test.go b/src/algo/runeprefilter_test.go
index 59c2d283..fa566364 100644
--- a/src/algo/runeprefilter_test.go
+++ b/src/algo/runeprefilter_test.go
@@ -255,6 +255,131 @@ func TestNormalizeRuneUnchangedByGuard(t *testing.T) {
 	}
 }
 
+// Step G lets non-ASCII pattern runes use the scan, but only when no other
+// rune can transform into them. Being uncased is not sufficient: U+00DF has no
+// simple uppercase yet U+1E9E lowercases onto it. This pins the guard against
+// the full preimage relation over all of Unicode.
+func TestRunePrefilterableGuardIsSound(t *testing.T) {
+	preimage := map[rune][]rune{}
+	for r := rune(0); r <= unicode.MaxRune; r++ {
+		if r >= 0xD800 && r <= 0xDFFF {
+			continue
+		}
+		if charClassOfNonAscii(r) == charUpper {
+			if l := unicode.To(unicode.LowerCase, r); l != r {
+				preimage[l] = append(preimage[l], r)
+			}
+		}
+	}
+
+	clean := util.ToChars([]byte("漢字")) // rune mode, fold bit clear
+	admitted, violations := 0, 0
+	for r := rune(utf8.RuneSelf); r <= unicode.MaxRune; r++ {
+		if r >= 0xD800 && r <= 0xDFFF {
+			continue
+		}
+		if !runePrefilterable(&clean, []rune{r}, false) {
+			continue
+		}
+		admitted++
+		if extra := preimage[r]; len(extra) > 0 {
+			violations++
+			if violations <= 5 {
+				t.Errorf("guard admits U+%04X but %U lowercases onto it", r, extra)
+			}
+		}
+	}
+	t.Logf("guard admits %d non-ASCII pattern runes, unsound for %d", admitted, violations)
+
+	// The scripts this step exists for must be fully admitted.
+	for _, s := range []struct {
+		name   string
+		lo, hi rune
+	}{{"CJK", 0x4E00, 0x9FFF}, {"Hangul", 0xAC00, 0xD7A3}, {"kana", 0x3040, 0x30FF},
+		{"Thai", 0x0E00, 0x0E7F}, {"emoji", 0x1F300, 0x1FAFF}} {
+		for r := s.lo; r <= s.hi; r++ {
+			if !runePrefilterable(&clean, []rune{r}, false) {
+				t.Errorf("%s U+%04X should be admitted", s.name, r)
+				break
+			}
+		}
+	}
+}
+
+// The Step G path must actually engage and reject, otherwise the equivalence
+// test above proves nothing about non-ASCII patterns.
+func TestNonAsciiPatternPrefilterEngages(t *testing.T) {
+	rng := rand.New(rand.NewSource(6))
+	parts := []string{"漢字", "한글", "src", "conf", "/", "мир", "🎉"}
+	engaged, rejected, narrowed, bypassed := 0, 0, 0, 0
+	for range 5000 {
+		var sb strings.Builder
+		for range 1 + rng.Intn(6) {
+			sb.WriteString(parts[rng.Intn(len(parts))])
+		}
+		chars := util.ToChars([]byte(sb.String()))
+		if chars.IsBytes() {
+			continue
+		}
+		pattern := []rune([]string{"漢", "漢字", "한글", "мир", "🎉", "é", "ß"}[rng.Intn(7)])
+		if !runePrefilterable(&chars, pattern, false) {
+			bypassed++
+			continue
+		}
+		engaged++
+		lo, hi := asciiFuzzyIndex(&chars, pattern, false)
+		switch {
+		case lo < 0:
+			rejected++
+		case hi-lo < chars.Length():
+			narrowed++
+		}
+	}
+	t.Logf("non-ASCII patterns: engaged %d, bypassed %d, rejected %d, narrowed %d",
+		engaged, bypassed, rejected, narrowed)
+	if engaged == 0 || rejected == 0 {
+		t.Fatalf("non-ASCII pattern path not exercised (engaged=%d rejected=%d)", engaged, rejected)
+	}
+	if bypassed == 0 {
+		t.Fatal("cased and foldable patterns should still bypass")
+	}
+}
+
+// indexRune picks which byte lane to scan, and is checked against the shipped
+// reference scanners rather than a copy of them. Zero-low-byte runes (U+AE00) and
+// runes whose lanes collide with common ASCII bytes are the cases that break a
+// naive low-byte scan, so the alphabet includes both.
+func TestIndexRuneMatchesReference(t *testing.T) {
+
+	alphabet := []rune{
+		'a', 'e', 'N', '/', 0x00,
+		0xAE00, 0xAC00, 0xD55C, // Hangul, low byte zero for U+AE00
+		0x4E00, 0x6587, 0x9FFF, // CJK, U+4E00 has zero low byte
+		0x3040, 0x0E00, // kana, Thai with zero low byte
+		0x1F389, 0x1F300, // emoji, 3 significant bytes
+		0x0100, 0x0165, 0x00E9, // low byte zero / ASCII-colliding lanes
+	}
+	rng := rand.New(rand.NewSource(7))
+	for trial := range 30000 {
+		n := rng.Intn(20)
+		runes := make([]rune, n)
+		for i := range runes {
+			runes[i] = alphabet[rng.Intn(len(alphabet))]
+		}
+		r := alphabet[rng.Intn(len(alphabet))]
+		from := 0
+		if n > 0 {
+			from = rng.Intn(n)
+		}
+		if got, exp := indexRune(runes, r, from), indexRuneRef(runes, r, from); got != exp {
+			t.Fatalf("trial %d: indexRune(%U, U+%04X, %d) = %d, expected %d", trial, runes, r, from, got, exp)
+		}
+		if got, exp := lastIndexRune(runes, r, from), lastIndexRuneRef(runes, r, from); got != exp {
+			t.Fatalf("trial %d: lastIndexRune(%U, U+%04X, %d) = %d, expected %d", trial, runes, r, from, got, exp)
+		}
+	}
+}
+
 // preparePattern mirrors what pattern.go guarantees the algo functions:
 // lowercased when case-insensitive, normalized when normalize is on.
 func preparePattern(pat string, caseSensitive, normalize bool) []rune {