diff --git a/README.md b/README.md
index 9cceed8..0e3ccb6 100644
--- a/README.md
+++ b/README.md
@@ -116,7 +116,12 @@ path list.
 
 `Supported(filename string) bool` reports whether a file's extension maps to a
 language with an outlining query. `SupportedSource(src, filename)` also checks
-source-based detection such as Ruby shebangs.
+source-based detection such as Ruby shebangs. Ruby interpreter names must be
+`ruby` or `ruby` followed by a numeric version, such as `ruby3.3`. Detection
+checks the interpreter, not later arguments. For `env`, it recognizes literal
+assignments and the `-i`, `-u`, `-C`, and `-S` options (including their long
+forms), then checks the selected command. Unknown options, quoted words,
+escapes, and variable expansion before the command are left unsupported.
 
 `SetParseTimeout(d time.Duration)` overrides the per-file parse timeout
 (default 1s). Must be called before the first `Outline` or `Pack` call.
diff --git a/outline.go b/outline.go
index 2d190a9..9523255 100644
--- a/outline.go
+++ b/outline.go
@@ -208,18 +208,8 @@ func detectSource(src []byte, filename string) (*lang, bool) {
 	if l, ok := detect(filename); ok {
 		return l, true
 	}
-	line, _, _ := bytes.Cut(src, []byte{'\n'})
-	if !bytes.HasPrefix(line, []byte("#!")) {
-		return nil, false
-	}
-	for field := range bytes.FieldsSeq(line[2:]) {
-		name := field
-		if i := bytes.LastIndexByte(name, '/'); i >= 0 {
-			name = name[i+1:]
-		}
-		if bytes.HasPrefix(name, []byte("ruby")) {
-			return langs["ruby"], true
-		}
+	if rubyShebang(src) {
+		return langs["ruby"], true
 	}
 	return nil, false
 }
diff --git a/shebang.go b/shebang.go
new file mode 100644
index 0000000..6f27805
--- /dev/null
+++ b/shebang.go
@@ -0,0 +1,80 @@
+package outline
+
+import (
+	"bytes"
+	"path"
+	"regexp"
+	"strings"
+)
+
+var rubyInterpreter = regexp.MustCompile(`^ruby(?:[0-9]+(?:\.[0-9]+)*)?$`)
+
+func rubyShebang(src []byte) bool {
+	line, _, _ := bytes.Cut(src, []byte{'\n'})
+	if !bytes.HasPrefix(line, []byte("#!")) {
+		return false
+	}
+	fields := strings.Fields(string(line[2:]))
+	if len(fields) == 0 || strings.HasSuffix(fields[0], "/") {
+		return false
+	}
+	interpreter := fields[0]
+	if path.Base(interpreter) == "env" {
+		interpreter = envInterpreter(fields[1:])
+	}
+	return !strings.HasSuffix(interpreter, "/") && rubyInterpreter.MatchString(path.Base(interpreter))
+}
+
+func envInterpreter(fields []string) string {
+	options := true
+	for len(fields) != 0 {
+		field := fields[0]
+		fields = fields[1:]
+		if !literalEnvWord(field) {
+			return ""
+		}
+		if options {
+			switch field {
+			case "--":
+				options = false
+				continue
+			case "-", "-i", "--ignore-environment", "-S", "--split-string":
+				continue
+			case "-u", "--unset", "-C", "--chdir":
+				if len(fields) == 0 || !literalEnvWord(fields[0]) {
+					return ""
+				}
+				fields = fields[1:]
+				continue
+			}
+			if strings.HasPrefix(field, "--unset=") || strings.HasPrefix(field, "--chdir=") {
+				continue
+			}
+			if split, ok := envSplitString(field); ok {
+				fields = append([]string{split}, fields...)
+				continue
+			}
+		}
+		if strings.HasPrefix(field, "-") {
+			return ""
+		}
+		if index := strings.IndexByte(field, '='); index > 0 {
+			options = false
+			continue
+		}
+		return field
+	}
+	return ""
+}
+
+func envSplitString(field string) (string, bool) {
+	if value, ok := strings.CutPrefix(field, "--split-string="); ok {
+		return value, true
+	}
+	return strings.CutPrefix(field, "-S")
+}
+
+func literalEnvWord(field string) bool {
+	// Quoting, escapes and expansion require more than whitespace splitting.
+	return field != "" && !strings.HasPrefix(field, "#") && !strings.ContainsAny(field, "'\"\\$`")
+}
diff --git a/shebang_test.go b/shebang_test.go
new file mode 100644
index 0000000..f694b74
--- /dev/null
+++ b/shebang_test.go
@@ -0,0 +1,78 @@
+package outline_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/git-pkgs/outline"
+)
+
+func TestBuildRubyShebangInterpreter(t *testing.T) {
+	for _, tc := range []struct {
+		name, line string
+		ruby       bool
+	}{
+		{"direct", "#!/usr/bin/ruby", true},
+		{"direct argument", "#!/usr/bin/ruby -w", true},
+		{"numeric suffix", "#!/usr/bin/ruby3", true},
+		{"dotted suffix", "#!/usr/bin/ruby3.3", true},
+		{"env", "#!/usr/bin/env ruby", true},
+		{"env split", "#!/usr/bin/env -S ruby -w", true},
+		{"env attached split", "#!/usr/bin/env -Sruby -w", true},
+		{"env long split", "#!/usr/bin/env --split-string=ruby -w", true},
+		{"env assignment", "#!/usr/bin/env -S RUBYOPT=-w ruby", true},
+		{"env unset", "#!/usr/bin/env -S -u RUBYOPT ruby", true},
+		{"env chdir", "#!/usr/bin/env -S -C /tmp ruby", true},
+		{"env flags", "#!/usr/bin/env -S -i -- ruby", true},
+		{"env long options", "#!/usr/bin/env -S --ignore-environment --unset=RUBYOPT --chdir=/tmp ruby", true},
+		{"shell argument", "#!/bin/sh ruby", false},
+		{"env shell argument", "#!/usr/bin/env -S sh ruby", false},
+		{"env unset operand", "#!/usr/bin/env -S -u ruby sh", false},
+		{"env chdir operand", "#!/usr/bin/env -S -C ruby sh", false},
+		{"env assignment value", "#!/usr/bin/env -S LANGUAGE=ruby sh", false},
+		{"env ended options", "#!/usr/bin/env -S -- -u ruby", false},
+		{"env missing operand", "#!/usr/bin/env -S -u ruby", false},
+		{"env unknown option", "#!/usr/bin/env --unknown ruby", false},
+		{"env help", "#!/usr/bin/env --help ruby", false},
+		{"env quoted operand", "#!/usr/bin/env -S -u 'x ruby y' sh", false},
+		{"env expansion", "#!/usr/bin/env -S ${INTERPRETER} ruby", false},
+		{"env comment operand", "#!/usr/bin/env -S -u # ruby", false},
+		{"env directory", "#!/usr/bin/env/ ruby", false},
+		{"ruby directory", "#!/usr/bin/ruby/", false},
+		{"lookalike", "#!/usr/bin/ruby-not-an-interpreter", false},
+		{"suffix letters", "#!/usr/bin/ruby3helper", false},
+		{"suffix trailing dot", "#!/usr/bin/ruby3.", false},
+		{"suffix double dot", "#!/usr/bin/ruby3..3", false},
+		{"empty", "#!", false},
+		{"not first line", "\n#!/usr/bin/ruby", false},
+		{"tabs and CRLF", "#!\t/usr/bin/env\t-S\truby\r", true},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			root := t.TempDir()
+			src := []byte(tc.line + "\nrequire 'json'\n")
+			if err := os.WriteFile(filepath.Join(root, "tool"), src, 0700); err != nil {
+				t.Fatal(err)
+			}
+			if got := outline.SupportedSource(src, "tool"); got != tc.ruby {
+				t.Errorf("SupportedSource = %v, want %v", got, tc.ruby)
+			}
+			graph, err := outline.Build(root, outline.Options{})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if graph.Node(outline.FileID("tool")) == nil {
+				t.Fatal("missing file node")
+			}
+			var loaded bool
+			for _, edge := range graph.Edges {
+				if edge.From == outline.FileID("tool") && edge.To == outline.ModID("ruby", "json") && edge.Rel == outline.RelLoads {
+					loaded = true
+				}
+			}
+			if loaded != tc.ruby {
+				t.Errorf("Ruby json load = %v, want %v", loaded, tc.ruby)
+			}
+		})
+	}
+}