caddyserver/caddy · error

unsupported value type in matcher input: %T

Error message

unsupported value type in matcher input: %T

What it means

Thrown when converting CEL matcher input maps to map[string][]string: a value is a []ref.Val (a CEL list) but one of its elements is not a types.String (e.g. an int or bool CEL value). Since header values must be strings, the element cannot be converted.

Source

Thrown at modules/caddyhttp/celmatcher.go:707

		}
	} else {
		mapStrIface = mapStrRaw.(map[string]any)
	}
	mapStrListStr := make(map[string][]string, len(mapStrIface))
	for k, v := range mapStrIface {
		switch val := v.(type) {
		case string:
			mapStrListStr[k] = []string{val}
		case types.String:
			mapStrListStr[k] = []string{string(val)}
		case []string:
			mapStrListStr[k] = val
		case []ref.Val:
			convVals := make([]string, len(val))
			for i, elem := range val {
				strVal, ok := elem.(types.String)
				if !ok {
					return nil, fmt.Errorf("unsupported value type in matcher input: %T", val)
				}
				convVals[i] = string(strVal)
			}
			mapStrListStr[k] = convVals
		case []any:
			convVals := make([]string, len(val))
			for i, elem := range val {
				switch e := elem.(type) {
				case string:
					convVals[i] = e
				case types.String:
					convVals[i] = string(e)
				default:
					return nil, fmt.Errorf("unsupported element type in matcher input list: %T", elem)
				}
			}
			mapStrListStr[k] = convVals
		default:

View on GitHub (pinned to 50e54ee279)

Solutions

1. Make all list elements in the matcher map strings (quote numbers/booleans)
2. Validate the matcher input map is map[string][]string-shaped before passing it to the CEL matcher
3. Simplify the expression to use explicit string literals

Example fix

// before
{'accept': [1, 'gzip']}
// after
{'accept': ['1', 'gzip']}

// Ensure every header matcher value is []string of strings before serializing
func isStringSlice(v any) bool {
    s, ok := v.([]string)
    return ok && len(s) >= 0
}

When it happens

Trigger: A CEL matcher whose map value is a heterogeneous list, e.g. {'x': [1, 'a']} — the []ref.Val case iterates elements and requires each to be types.String; an int element fails the type assertion.

Common situations: CEL expressions mixing types in lists used as header matcher input; configs written by tools that emit JSON numbers where header values are expected.

Related errors

• unsupported element type in matcher input list: %T
• unsupported map key type in header match: %T
• unknown try policy %s
• %s
• loading matcher sets: %v

AI-assisted analysis of caddyserver/caddy@50e54ee279 (2026-08-15). Data as JSON: /api/errors/dbc3fa50137c13ad. Report an issue: GitHub.