Fix non-deterministic engine validation error message

cmd/gh-aw/format_list_test.go

@@ -0,0 +1,55 @@
+//go:build !integration
+
+package main
+
+import (
+	"testing"
+)
+
+func TestFormatListWithOr(t *testing.T) {
+	tests := []struct {
+		name     string
+		items    []string
+		expected string
+	}{
+		{
+			name:     "empty list",
+			items:    []string{},
+			expected: "",
+		},
+		{
+			name:     "single item",
+			items:    []string{"apple"},
+			expected: "apple",
+		},
+		{
+			name:     "two items",
+			items:    []string{"apple", "banana"},
+			expected: "apple or banana",
+		},
+		{
+			name:     "three items",
+			items:    []string{"apple", "banana", "cherry"},
+			expected: "apple, banana, or cherry",
+		},
+		{
+			name:     "four items",
+			items:    []string{"apple", "banana", "cherry", "date"},
+			expected: "apple, banana, cherry, or date",
+		},
+		{
+			name:     "engine names with quotes",
+			items:    []string{"'claude'", "'codex'", "'copilot'", "'custom'"},
+			expected: "'claude', 'codex', 'copilot', or 'custom'",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := formatListWithOr(tt.items)
+			if result != tt.expected {
+				t.Errorf("formatListWithOr(%v) = %q, want %q", tt.items, result, tt.expected)
+			}
+		})
+	}
+}

cmd/gh-aw/main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"os"
+	"sort"
 	"strings"
 
 	"github.com/github/gh-aw/pkg/cli"
@@ -23,22 +24,49 @@ var (
 var verboseFlag bool
 var bannerFlag bool
 
+// formatListWithOr formats a list of strings with commas and "or" before the last item
+// Example: ["a", "b", "c"] -> "a, b, or c"
+func formatListWithOr(items []string) string {
+	if len(items) == 0 {
+		return ""
+	}
+	if len(items) == 1 {
+		return items[0]
+	}
+	if len(items) == 2 {
+		return items[0] + " or " + items[1]
+	}
+	// For 3+ items: "a, b, or c"
+	return strings.Join(items[:len(items)-1], ", ") + ", or " + items[len(items)-1]
+}
+
 // validateEngine validates the engine flag value
 func validateEngine(engine string) error {
 	// Get the global engine registry
 	registry := workflow.GetGlobalEngineRegistry()
 	validEngines := registry.GetSupportedEngines()
 
 	if engine != "" && !registry.IsValidEngine(engine) {
+		// Sort engines for deterministic output
+		sortedEngines := make([]string, len(validEngines))
+		copy(sortedEngines, validEngines)
+		sort.Strings(sortedEngines)
+
+		// Format engines with quotes and "or" conjunction
+		quotedEngines := make([]string, len(sortedEngines))
+		for i, e := range sortedEngines {
+			quotedEngines[i] = "'" + e + "'"
+		}
+		formattedList := formatListWithOr(quotedEngines)
+
 		// Try to find close matches for "did you mean" suggestion
 		suggestions := parser.FindClosestMatches(engine, validEngines, 1)
 
-		errMsg := fmt.Sprintf("invalid engine value '%s'. Must be '%s'",
-			engine, strings.Join(validEngines, "', '"))
+		errMsg := fmt.Sprintf("invalid engine value '%s'. Must be %s", engine, formattedList)
 
 		if len(suggestions) > 0 {
-			errMsg = fmt.Sprintf("invalid engine value '%s'. Must be '%s'.\n\nDid you mean: %s?",
-				engine, strings.Join(validEngines, "', '"), suggestions[0])
+			errMsg = fmt.Sprintf("invalid engine value '%s'. Must be %s.\n\nDid you mean: %s?",
+				engine, formattedList, suggestions[0])
 		}
 
 		return fmt.Errorf("%s", errMsg)

cmd/gh-aw/main_entry_test.go

@@ -87,8 +87,11 @@ func TestValidateEngine(t *testing.T) {
 					return
 				}
 
-				if tt.errMessage != "" && err.Error() != fmt.Sprintf("invalid engine value '%s'. Must be 'claude', 'codex', 'copilot', or 'custom'", tt.engine) {
-					t.Errorf("validateEngine(%q) error message = %v, want to contain %v", tt.engine, err.Error(), tt.errMessage)
+				// Check that error message contains the expected format
+				// Error may include "Did you mean" suggestions, so we check if it starts with the base message
+				expectedMsg := fmt.Sprintf("invalid engine value '%s'. Must be 'claude', 'codex', 'copilot', or 'custom'", tt.engine)
+				if tt.errMessage != "" && !strings.HasPrefix(err.Error(), expectedMsg) {
+					t.Errorf("validateEngine(%q) error message = %v, want to start with %v", tt.engine, err.Error(), expectedMsg)
 				}
 			} else {
 				if err != nil {