fluent/fluentd · error · Fluent::ConfigError

'[' found but ']' not found. Invalid syntax: #{param}

Error message

'[' found but ']' not found. Invalid syntax: #{param}

What it means

Fluent::ConfigError from Fluent::RecordAccessor's parse_dot_array_op. Once '[' is seen in dot notation, the parser scans for the matching ']'; if the remainder of the path ends before one is found, the path is syntactically incomplete and rejected with the full offending parameter in the message.

Source

Thrown at lib/fluent/plugin_helper/record_accessor.rb:147

          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

        def self.parse_bracket_notation(param)
          orig_param = param
          result = []
          param = param[1..-1]

View on GitHub (pinned to dd45c6e18d)

Solutions

1. Close the bracket: $.array[1]
2. When composing paths from fragments, assert each '[' has a matching ']'
3. Prefer bracket notation $['key'][0] when generating paths programmatically - it is easier to build correctly

Example fix

# before
accessor = record_accessor_create('$.items[1')

# after
accessor = record_accessor_create('$.items[1]')

# assert brackets balance before creating the accessor
raise ArgumentError, 'unbalanced brackets' if param.count('[') != param.count(']')
accessor = record_accessor_create(param)

begin
  Fluent::RecordAccessor::Accessor.new(param)
rescue Fluent::ConfigError => e
  log.error "malformed accessor #{param.inspect}: #{e.message}"
  raise
end

When it happens

Trigger: record_accessor_create with a dot-notation path having an unclosed bracket: $.array[1, $.a[0.b, or any path ending in '[' plus digits without ']'.

Common situations: Truncation of long paths by shell quoting or templating; hand-joining path fragments and dropping the closing bracket; regex-extracted keys that cut off at '['.

Related errors

• found more characters after ']'. Invalid syntax: #{param}
• empty keys in dot notation
• whitespace character is not allowed in dot notation. Use bra
• missing array index in '[]'. Invalid syntax: #{param}
• Incomplete bracket. Invalid syntax: #{orig_param}

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/dd13781d672664a6. Report an issue: GitHub.