feat(mcp): only set variables for set flags (#1234)

Previously we had code which tried to detect nil values. But for example
if a flag is an integer that wouldn't work. Instead we use a more robust
approach of only setting variables if the flag was passed in as an
argument.

Test Plan: this command actually returns results now. Previously it
wouldn't since it would have limit set to 0.

  go run ./cmd/src -v mcp list-repos -query 'cloud'

Author: keegancsmith

cmd/src/mcp.go

@@ -76,7 +76,7 @@ func mcpMain(args []string) error {
 	if err := flags.Parse(flagArgs); err != nil {
 		return err
 	}
-	mcp.DerefFlagValues(vars)
+	mcp.DerefFlagValues(flags, vars)
 
 	if err := validateToolArgs(tool.InputSchema, args, vars); err != nil {
 		return err

internal/mcp/mcp_args.go

@@ -24,36 +24,28 @@ func (s *strSliceFlag) String() string {
 	return strings.Join(s.vals, ",")
 }
 
-func DerefFlagValues(vars map[string]any) {
+func DerefFlagValues(fs *flag.FlagSet, vars map[string]any) {
+	setFlags := make(map[string]bool)
+	fs.Visit(func(f *flag.Flag) {
+		setFlags[f.Name] = true
+	})
+
 	for k, v := range vars {
+		if !setFlags[k] {
+			delete(vars, k)
+			continue
+		}
 		rfl := reflect.ValueOf(v)
 		if rfl.Kind() == reflect.Pointer {
 			vv := rfl.Elem().Interface()
 			if slice, ok := vv.(strSliceFlag); ok {
 				vv = slice.vals
 			}
-			if isNil(vv) {
-				delete(vars, k)
-			} else {
-				vars[k] = vv
-			}
+			vars[k] = vv
 		}
 	}
 }
 
-func isNil(v any) bool {
-	if v == nil {
-		return true
-	}
-	rv := reflect.ValueOf(v)
-	switch rv.Kind() {
-	case reflect.Slice, reflect.Map, reflect.Pointer, reflect.Interface:
-		return rv.IsNil()
-	default:
-		return false
-	}
-}
-
 func BuildArgFlagSet(tool *ToolDef) (*flag.FlagSet, map[string]any, error) {
 	if tool == nil {
 		return nil, nil, errors.New("cannot build flagset on nil Tool Definition")

internal/mcp/mcp_args_test.go

@@ -63,7 +63,7 @@ func TestFlagSetParse(t *testing.T) {
 	if err := flagSet.Parse(args); err != nil {
 		t.Fatalf("flagset parsing failed: %v", err)
 	}
-	DerefFlagValues(vars)
+	DerefFlagValues(flagSet, vars)
 
 	if v, ok := vars["repos"].([]string); ok {
 		if len(v) != 2 {