cannot unmarshal %v into "buildFlags" of type []string or st

Error message

cannot unmarshal %v into "buildFlags" of type []string or string

What it means

The buildFlags launch argument accepts either a single string or an array of strings (custom flexType). If the JSON value is neither (e.g. a number, boolean, or object), the custom UnmarshalJSON returns this error, dropping the previous value.

Source

Thrown at service/dap/types.go:339

type BuildFlags struct {
	value any
}

func (s *BuildFlags) UnmarshalJSON(b []byte) error {
	if v := string(b); v == "" || v == "null" {
		s.value = nil
		return nil
	}
	var strs []string
	if err := json.Unmarshal(b, &strs); err == nil {
		s.value = strs
		return nil
	}
	var str string
	if err := json.Unmarshal(b, &str); err != nil {
		s.value = nil
		if uerr, ok := err.(*json.UnmarshalTypeError); ok {
			return fmt.Errorf(`cannot unmarshal %v into "buildFlags" of type []string or string`, uerr.Value)
		}
		return err
	}
	s.value = str
	return nil
}

View on GitHub (pinned to a23773e6c3)

Solutions

Set buildFlags to a single string: "buildFlags": "-tags=integration"
Or an array of strings: "buildFlags": ["-tags=integration","-v"]
Do not pass numbers, booleans or objects
Split multiple flags into array elements rather than one object

Example fix

// before
"buildFlags": true
// after
"buildFlags": ["-tags=integration"]

Defensive patterns

Strategy: type-guard

Validate before calling

func validBuildFlags(v any) bool { switch v.(type) { case string, []string, []any, nil: return true; default: return false } }

Type guard

func asBuildFlags(v any) ([]string, bool) {
  switch t := v.(type) {
  case string: return []string{t}, true
  case []string: return t, true
  case []any: out := []string{}; for _, e := range t { s, ok := e.(string); if !ok { return nil, false }; out = append(out, s) }; return out, true
  default: return nil, false
  }
}

Try / catch

if err := launch(cfg); err != nil { if strings.Contains(err.Error(), "buildFlags") { /* coerce to string or []string and retry */ } return err }

Prevention

Send buildFlags only as a string or array of strings
Never use booleans/objects for buildFlags
Split multiple flags into separate array elements

When it happens

Trigger: onLaunchRequest/onAttachRequest with e.g. "buildFlags": 42 or "buildFlags": {"a":1} — an UnmarshalTypeError from both the []string and string decode attempts.

Common situations: Users setting buildFlags to a boolean (true) intending 'enable', or nesting flags as an object of key/value pairs.

Related errors

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/2eec399e0038fc25. Report an issue: GitHub.