diff --git a/README.md b/README.md
index 12de544..ada2699 100644
--- a/README.md
+++ b/README.md
@@ -68,7 +68,11 @@ A broad, practical subset of Ruby 4.0, all differential-tested against MRI:
   `(a, b)` destructuring group params, stabby lambdas `->(){}`, numbered params
   (`_1`) and `it`, `yield`, `super`, **multiple-value `return a, b`**.
 - **Classes/modules/metaprogramming:** `class`/`module`, inheritance, `@ivars`,
-  **`@@class variables`**, constants, singleton method defs
+  **`@@class variables`**, constants — including **non-ASCII** ones, since Ruby
+  decides constant-ness from the first character's *case* and not from `A-Z`, so
+  `Ἅbb` and the titlecase `Džx` are constants while `μm`, `日x` and `אx` are
+  local variables (`token.IsConstStart` carries the rule and its citations) —
+  singleton method defs
   (`def self.foo`/`def obj.foo`/`def Const.foo`), **global-variable assignment**
   (`$g = …`), multiple assignment / destructuring, **adjacent string-literal
   concatenation** (`"a" "b"`).
diff --git a/nonascii_constant_test.go b/nonascii_constant_test.go
new file mode 100644
index 0000000..3a5a710
--- /dev/null
+++ b/nonascii_constant_test.go
@@ -0,0 +1,87 @@
+package parser
+
+import (
+	"testing"
+
+	"github.com/go-ruby-parser/parser/ast"
+)
+
+// TestNonASCIIConstantIdentifier covers the parse-level consequence of Ruby
+// deciding constant-ness from the first character's CASE rather than from its
+// being in 'A'..'Z': a name beginning with a non-ASCII uppercase or titlecase
+// letter is a CONSTANT, and one beginning with a lowercase or CASELESS letter is
+// a local variable.
+//
+// ⛔ `LookupIdent` tested `s[0]`, a BYTE, so every multibyte name came back
+// IDENT. `ἍBB = 1` parsed as a local-variable assignment, which is why
+// `defined?(ἍBB)` answered nil in rbgo where MRI answers "constant", and why
+// ruby/spec's `language/variables_spec.rb` "parses a non-ASCII upcased
+// character as a constant identifier" could not pass
+// (go-embedded-ruby/ruby#799).
+//
+// Each classification was measured against /usr/bin/ruby; the per-character
+// table and its citations live in token.IsConstStart.
+func TestNonASCIIConstantIdentifier(t *testing.T) {
+	for _, tc := range []struct {
+		src  string
+		kind string // "const" or "local"
+	}{
+		{"ἍBB = 1", "const"},  // GREEK CAPITAL ALPHA WITH PSILI AND OXIA (Lu)
+		{"Дeta = 1", "const"}, // CYRILLIC CAPITAL DE (Lu)
+		{"École = 1", "const"},
+		{"Džx = 1", "const"}, // titlecase digraph (Lt)
+		{"μ = 1", "local"},  // GREEK SMALL MU (Ll)
+		{"日x = 1", "local"}, // CJK, caseless (Lo)
+		{"אx = 1", "local"}, // HEBREW ALEF, caseless (Lo)
+		{"Ascii = 1", "const"},
+		{"ascii = 1", "local"},
+	} {
+		t.Run(tc.src, func(t *testing.T) {
+			prog, err := Parse(tc.src)
+			if err != nil {
+				t.Fatalf("Parse(%q) error: %v", tc.src, err)
+			}
+			if len(prog.Body) != 1 {
+				t.Fatalf("Parse(%q) gave %d statements, want 1", tc.src, len(prog.Body))
+			}
+			switch node := prog.Body[0].(type) {
+			case *ast.ConstAssign:
+				if tc.kind != "const" {
+					t.Errorf("Parse(%q) gave *ast.ConstAssign, want a local assignment", tc.src)
+				}
+			case *ast.Assign:
+				if tc.kind != "local" {
+					t.Errorf("Parse(%q) gave *ast.Assign, want *ast.ConstAssign", tc.src)
+				}
+			default:
+				t.Errorf("Parse(%q) gave %T, want *ast.ConstAssign or *ast.Assign", tc.src, node)
+			}
+		})
+	}
+}
+
+// A non-ASCII constant must work everywhere an ASCII one does, or the fix would
+// only have moved the problem into the parser's other constant positions.
+func TestNonASCIIConstantInEveryPosition(t *testing.T) {
+	for _, src := range []string{
+		"ἍBB",                   // a read
+		"ἍBB = 1",               // an assignment
+		"ἍBB ||= 1",             // an op-assign
+		"ἍBB::Inner",            // as a scope
+		"Outer::ἍBB",            // as a scoped name
+		"ἍBB, x = 1, 2",         // a masgn target
+		"class ἍBB; end",        // a class name
+		"module ἍBB; end",       // a module name
+		"class ἍBB < Base; end", // and with a superclass
+		"case x; in ἍBB; end",   // a pattern
+		"def ἍBB.m; end",        // a singleton definee
+		"ἍBB.new",               // a receiver
+	} {
+		if _, err := Parse(src); err != nil {
+			t.Errorf("Parse(%q) error: %v", src, err)
+		}
+	}
+	if _, err := Parse("begin\n  x\nrescue ἍBB => e\nend"); err != nil {
+		t.Errorf("non-ASCII constant as a rescue class: %v", err)
+	}
+}
diff --git a/token/const_start_test.go b/token/const_start_test.go
new file mode 100644
index 0000000..ba8df57
--- /dev/null
+++ b/token/const_start_test.go
@@ -0,0 +1,94 @@
+package token
+
+import "testing"
+
+// Ruby decides constant-ness from the first character's CASE, not from its being
+// in 'A'..'Z'. Every row here was measured against /usr/bin/ruby through
+// `Object.const_defined?(name.to_sym)`, which raises NameError for a name that
+// is not a constant name — that is rb_is_const_name (symbol.c:1287), which is
+// the same predicate parse.y consults.
+//
+// ⛔ Before this, the test was `s[0] >= 'A' && s[0] <= 'Z'` on a BYTE, so every
+// multibyte name read as a local: `ἍBB = 1` created a local variable and no
+// constant existed afterwards.
+func TestIsConstStartFollowsRubysCaseRule(t *testing.T) {
+	for _, tc := range []struct {
+		name  string
+		first string // the leading character, by name and category
+		konst bool
+	}{
+		// ASCII, unchanged.
+		{"A", "ASCII upper", true},
+		{"Z", "ASCII upper", true},
+		{"a", "ASCII lower", false},
+		{"z", "ASCII lower", false},
+		{"_", "ASCII underscore", false},
+		{"_Foo", "ASCII underscore", false},
+
+		// Lu — uppercase letters outside ASCII.
+		{"ἍBB", "GREEK CAPITAL ALPHA WITH PSILI AND OXIA (Lu)", true},
+		{"Дa", "CYRILLIC CAPITAL DE (Lu)", true},
+		{"İstanbul", "LATIN CAPITAL I WITH DOT ABOVE (Lu)", true},
+		{"École", "LATIN CAPITAL E WITH ACUTE (Lu)", true},
+
+		// ⛔ Lt — titlecase letters. rb_sym_constant_char_p tests the
+		// "titlecaseletter" Onigmo property explicitly, AFTER isupper and
+		// islower have both said no, so these are constants although
+		// unicode.IsUpper alone reports false for them.
+		{"Džx", "LATIN CAPITAL D WITH SMALL Z WITH CARON (Lt)", true},
+		{"Ljx", "LATIN CAPITAL L WITH SMALL J (Lt)", true},
+
+		// Ll — lowercase letters outside ASCII.
+		{"μm", "GREEK SMALL MU (Ll)", false},
+		{"école", "LATIN SMALL E WITH ACUTE (Ll)", false},
+
+		// ⛔ Lo — CASELESS letters start a LOCAL variable, not a constant.
+		// This is why neither clause may be written as "not lowercase".
+		{"日x", "CJK UNIFIED IDEOGRAPH 65E5 (Lo)", false},
+		{"אx", "HEBREW ALEF (Lo)", false},
+		{"بx", "ARABIC BEH (Lo)", false},
+
+		// Degenerate inputs: no character to read is not an uppercase one.
+		{"", "empty", false},
+		{"\xff\xfe", "invalid UTF-8", false},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := IsConstStart(tc.name); got != tc.konst {
+				t.Errorf("IsConstStart(%q) = %v, want %v — %s", tc.name, got, tc.konst, tc.first)
+			}
+			// LookupIdent must agree, since that is what the lexer calls. A
+			// keyword is neither, and no keyword is non-ASCII, so every row here
+			// reaches the same branch.
+			want := IDENT
+			if tc.konst {
+				want = CONST
+			}
+			if tc.name == "" {
+				return // LookupIdent is never called with an empty identifier
+			}
+			if got := LookupIdent(tc.name); got != want {
+				t.Errorf("LookupIdent(%q) = %v, want %v — %s", tc.name, got, want, tc.first)
+			}
+		})
+	}
+}
+
+// The control: a keyword still wins over the case rule, and the rule has not
+// swallowed the ASCII path. Without this, a version of IsConstStart that always
+// answered false would still pass the `false` rows above.
+func TestIsConstStartControls(t *testing.T) {
+	if LookupIdent("def") != DEF {
+		t.Error("a keyword must still be looked up as a keyword")
+	}
+	if !IsConstStart("Foo") {
+		t.Fatal("the ASCII path is broken: Foo must be a constant start")
+	}
+	if IsConstStart("foo") {
+		t.Fatal("the ASCII path is broken: foo must not be a constant start")
+	}
+	// And the planted positive for the branch this change added: a name that is
+	// a constant ONLY because of the non-ASCII clause.
+	if !IsConstStart("ἍBB") {
+		t.Fatal("the non-ASCII clause is not reached")
+	}
+}
diff --git a/token/token.go b/token/token.go
index 2fa50f3..b66c73b 100644
--- a/token/token.go
+++ b/token/token.go
@@ -6,6 +6,11 @@
 // the grammar grows (plan-rbgo.md §10).
 package token
 
+import (
+	"unicode"
+	"unicode/utf8"
+)
+
 type Type int
 
 const (
@@ -167,8 +172,52 @@ func LookupIdent(s string) Type {
 	if kw, ok := Keywords[s]; ok {
 		return kw
 	}
-	if c := s[0]; c >= 'A' && c <= 'Z' {
+	if IsConstStart(s) {
 		return CONST
 	}
 	return IDENT
 }
+
+// IsConstStart reports whether s begins with a character Ruby treats as the
+// start of a CONSTANT, rather than of a local variable.
+//
+// ⛔ The test used to be `s[0] >= 'A' && s[0] <= 'Z'`, which reads a BYTE. For
+// any multibyte character s[0] is a UTF-8 lead byte of 0xC2 or more, never in
+// 'A'..'Z', so every non-ASCII name was classified as a local — `ἍBB = 1`
+// created a local variable and no constant existed afterwards, where MRI reports
+// `defined?(ἍBB)` as "constant".
+//
+// The rule is not the lexer's: parse.y assumes tCONSTANT provisionally and
+// DOWNGRADES it with `if (result == tCONSTANT && is_local_id(ident)) result =
+// tIDENTIFIER` (parse.y:10453 at v4.0.9), so the decision belongs to the ID
+// system. rb_sym_constant_char_p (symbol.c:508-542) is where it is made:
+//
+//	ASCII                     -> ISUPPER
+//	non-ASCII, rb_enc_isupper -> constant
+//	non-ASCII, rb_enc_islower -> local
+//	titlecase letter, Unicode -> constant
+//	anything else (caseless)  -> local
+//
+// Go's unicode.IsUpper is that isupper and unicode.IsTitle that titlecase
+// property, so the two clauses below are those four lines. Measured against
+// /usr/bin/ruby through `Object.const_defined?`, which raises NameError for a
+// name that is not a constant name (rb_is_const_name, symbol.c:1287): Greek Ἅ,
+// Cyrillic Д and Turkish İ are constants; the titlecase digraphs Dž and Lj are
+// constants too; Greek μ, CJK 日, Hebrew א and Arabic ب are not — a caseless
+// letter starts a LOCAL variable in Ruby, which is why neither clause may be
+// loosened to "not lower".
+func IsConstStart(s string) bool {
+	if s == "" {
+		return false
+	}
+	if c := s[0]; c < utf8.RuneSelf {
+		return c >= 'A' && c <= 'Z'
+	}
+	r, size := utf8.DecodeRuneInString(s)
+	if r == utf8.RuneError && size <= 1 {
+		// Invalid UTF-8: rb_sym_constant_char_p returns FALSE when
+		// rb_enc_precise_mbclen finds no character, so this is a local.
+		return false
+	}
+	return unicode.IsUpper(r) || unicode.IsTitle(r)
+}