fluent/fluentd · error · ArgumentError

tag must be a String: #{tag.class}

Error message

tag must be a String: #{tag.class}

What it means

Output#metadata (lib/fluent/plugin/output.rb:938) is the factory for Fluent::Plugin::Metadata objects used as buffer-chunk keys. It validates its arguments and raises ArgumentError unless tag is nil or a String. Plugin authors and test code call metadata directly; core fluentd normally passes the tag string emitted by inputs/routers, so seeing this error means a non-String tag object reached a buffered output's chunking layer.

Source

Thrown at lib/fluent/plugin/output.rb:938

        @emit_count_metrics.inc
        begin
          execute_chunking(tag, es, enqueue: (@flush_mode == :immediate))
          if !@retry && @buffer.queued?(nil, optimistic: true)
            submit_flush_once
          end
        rescue
          # TODO: separate number of errors into emit errors and write/flush errors
          @num_errors_metrics.inc
          raise
        end
      end

      # TODO: optimize this code
      def metadata(tag, time, record)
        # this arguments are ordered in output plugin's rule
        # Metadata 's argument order is different from this one (timekey, tag, variables)

        raise ArgumentError, "tag must be a String: #{tag.class}" unless tag.nil? || tag.is_a?(String)
        raise ArgumentError, "time must be a Fluent::EventTime (or Integer): #{time.class}" unless time.nil? || time.is_a?(Fluent::EventTime) || time.is_a?(Integer)
        raise ArgumentError, "record must be a Hash: #{record.class}" unless record.nil? || record.is_a?(Hash)

        if @chunk_keys.nil? && @chunk_key_time.nil? && @chunk_key_tag.nil?
          # for tests
          return Struct.new(:timekey, :tag, :variables).new
        end

        timekey = @chunk_key_time ? calculate_timekey(time) : nil
        @_metadata_cache ||= MetadataCache.new

        # timekey is int from epoch, and `timekey - timekey % 60` is assumed to mach with 0s of each minutes.
        # it's wrong if timezone is configured as one which supports leap second, but it's very rare and
        # we can ignore it (especially in production systems).
        if @chunk_keys.empty?
          return @_metadata_cache.metadata if @_metadata_cache.cached?(timekey: timekey, tag: tag)

          meta = if @chunk_key_time && @chunk_key_tag

View on GitHub (pinned to dd45c6e18d)

Solutions

1. Pass a real Fluentd tag String like 'app.logs' (dot-separated) or nil.
2. Check argument order against metadata(tag, time, record) — note it differs from Metadata.new(timekey, tag, variables).
3. In tests, provide a string tag when emitting events through the plugin test driver instead of nil/symbol shortcuts.

Example fix

# before
meta = output.metadata(record, time, nil) # record/Hash in tag slot -> ArgumentError
# after
meta = output.metadata('app.logs', time, record)

tag = tag.to_s unless tag.is_a?(String) || tag.nil?
meta = output.metadata(tag, time, record)

def valid_fluent_tag?(t)
  t.is_a?(String) || t.nil?
end

rescue ArgumentError => e
  raise if e.message !~ /tag must be a String/
  log.warn 'dropping event with malformed tag', tag_class: tag.class

When it happens

Trigger: A custom output/filter plugin calling router/output.metadata(:event, time, record) or metadata(Object.new, ...); passing an Integer/Array tag; calling metadata with argument order swapped (e.g. record first) so a Hash lands in the tag slot.

Common situations: Writing a custom buffered output plugin or plugin tests that construct Metadata manually; refactoring a filter so a variable holding nil-or-symbol is passed as tag; copy-pasting code that assumed the Metadata.new(timekey, tag, variables) argument order instead of the metadata(tag, time, record) order.

Understand the failure class

Background: Invalid argument type errors: "must be of type string", "expected X, got Y", and ERR_INVALID_ARG_TYPE explained — this error's family across 15 libraries.

Related errors

• <buffer> section is configured, but plugin '#{self.class}' d
• secondary plugin '#{self.class}' must support buffering, but
• Invalid path component detected in #{matched}: #{replace}
• Invalid path component detected, replaced to: #{rvalue}
• time must be a Fluent::EventTime (or Integer): #{time.class}

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/5b5e1066bdef6556. Report an issue: GitHub.