What causes "missing array index in '[]'. Invalid syntax: #{param}" in fluent/fluentd?

Replace [] with a concrete index: $.array[0]

fluent/fluentd · error · Fluent::ConfigError

missing array index in '[]'. Invalid syntax: #{param}

Error message

missing array index in '[]'. Invalid syntax: #{param}

What it means

Fluent::ConfigError from Fluent::RecordAccessor's parse_dot_array_op. Inside dot notation an array index must be an integer between brackets; when the bracket content is empty (index_value == ']'), i.e. '[]', the parser raises because no index was supplied.

Source

Thrown at lib/fluent/plugin_helper/record_accessor.rb:142

            end
          }
        end

        def self.parse_dot_array_op(key, param)
          start = key.index('[')
          result = if start.zero?
                     []
                   else
                     [key[0..start - 1]]
                   end
          key = key[start + 1..-1]
          in_bracket = true

          until key.empty?
            if in_bracket
              if i = key.index(']')
                index_value = key[0..i - 1]
                raise Fluent::ConfigError, "missing array index in '[]'. Invalid syntax: #{param}" if index_value == ']'
                result << Integer(index_value)
                key = key[i + 1..-1]
                in_bracket = false
              else
                raise Fluent::ConfigError, "'[' found but ']' not found. Invalid syntax: #{param}"
              end
            else
              if i = key.index('[')
                key = key[i + 1..-1]
                in_bracket = true
              else
                raise Fluent::ConfigError, "found more characters after ']'. Invalid syntax: #{param}"
              end
            end
          end

          result
        end

View on GitHub (pinned to dd45c6e18d)

Solutions

Replace [] with a concrete index: $.array[0]
If you meant 'iterate all elements', do the iteration in a filter/record_transformer ruby block instead of the accessor path
Use pure bracket notation $[0] for top-level array indexing

Example fix

# before
accessor = record_accessor_create('$.items[]')

# after
accessor = record_accessor_create('$.items[0]')

Defensive patterns

Strategy: try-catch

Validate before calling

# reject empty index brackets up front
raise ArgumentError, 'array index required in path' if param.include?('[]')
accessor = record_accessor_create(param)

Try / catch

begin
  accessor = Fluent::RecordAccessor::Accessor.new(param)
rescue Fluent::ConfigError => e
  raise ArgumentError, "invalid record path #{param.inspect}: #{e.message}"
end

Prevention

Remember record_accessor has no 'all elements' wildcard
Always emit a concrete integer inside [] when generating paths

When it happens

Trigger: record_accessor_create with a dot-notation path containing an empty bracket: $.array[], $.list[].field or $.[].

Common situations: Writing a wildcard-style '[]' expecting 'all elements' (record_accessor has no wildcard; use a filter with record_accessor per element); typos when converting $[0]-style paths to dot notation.

Related errors

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/2602fc4b3ebacaa3. Report an issue: GitHub.