mikefarah/yq · error

can only use merge anchors with maps (!!map) or sequences (!

Error message

can only use merge anchors with maps (!!map) or sequences (!!seq) of maps, but got sequence containing %v

What it means

While exploding a merge-key ('<<') anchor whose value is a sequence, fixedReconstructAliasedMap found an element of that sequence that is not itself a map. YAML merge semantics require each merged entry to be a mapping (or a single map), so a sequence containing scalars/arrays cannot be merged into the parent map and the explode fails.

Source

Thrown at pkg/yqlib/operator_anchors_aliases.go:178

			sequence := valueNode
			if sequence.Kind == AliasNode {
				sequence = sequence.Alias
			}
			if sequence.Kind != SequenceNode {
				sequence = &CandidateNode{Content: []*CandidateNode{sequence}}
			}
			for index := 0; index < len(sequence.Content); index = index + 1 {
				// for merge anchors, we only set them if the key is not already in node or the newContent
				mergeNodeSeq := sequence.Content[index]
				if mergeNodeSeq.Kind == AliasNode {
					mergeNodeSeq = mergeNodeSeq.Alias
				}
				mergeNodeSeq = mergeNodeSeq.Copy()
				if err := explodeNode(mergeNodeSeq, Context{}); err != nil {
					return err
				}
				if mergeNodeSeq.Kind != MappingNode {
					return fmt.Errorf("can only use merge anchors with maps (!!map) or sequences (!!seq) of maps, but got sequence containing %v", mergeNodeSeq.Tag)
				}
				itemsToAdd := mergeNodeSeq.FilterMapContentByKey(func(keyNode *CandidateNode) bool {
					return getContentValueByKey(node.Content, keyNode.Value) == nil &&
						getContentValueByKey(newContent, keyNode.Value) == nil
				})

				for _, item := range itemsToAdd {
					copied := item.Copy()
					copied.Parent = node
					newContent = append(newContent, copied)
				}
			}
		}
	}
	node.Content = newContent
	return nil
}

View on GitHub (pinned to 8b5af0694b)

Solutions

1. Make every element of the merge sequence a mapping
2. Use a single map as the merge value: <<: *anchor
3. If the sequence of non-maps is intentional, merge it manually with e.g. *a + *b instead of '<<'

When it happens

Trigger: Thrown at pkg/yqlib/operator_anchors_aliases.go:178 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of mikefarah/yq@8b5af0694b (2026-09-05). Data as JSON: /api/errors/8bd48c0e5013b700. Report an issue: GitHub.