mikefarah/yq · error

cannot change case with %v, can only operate on strings.

Error message

cannot change case with %v, can only operate on strings. 

What it means

The upcase/downcase operators share changeCaseOperator, which — like other string operators — requires each matched node's resolved tag to be !!str (custom types unwrapped by guessTagFromCustomType). The first non-string node aborts with its tag in the message; case folding is simply undefined for numbers, bools, maps.

Source

Thrown at pkg/yqlib/operator_strings.go:192

			newStringNode = node.CreateReplacement(ScalarNode, "!!str", result)
			newStringNode.Style = DoubleQuotedStyle
		}
		newStringNode.Tag = "!!str"
		results.PushBack(newStringNode)
	}

	return context.ChildContext(results), nil
}

func changeCaseOperator(_ *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
	results := list.New()
	prefs := expressionNode.Operation.Preferences.(changeCasePrefs)

	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
		node := el.Value.(*CandidateNode)

		if node.guessTagFromCustomType() != "!!str" {
			return Context{}, fmt.Errorf("cannot change case with %v, can only operate on strings. ", node.Tag)
		}

		value := ""
		if prefs.ToUpperCase {
			value = strings.ToUpper(node.Value)
		} else {
			value = strings.ToLower(node.Value)
		}
		newStringNode := node.CreateReplacement(ScalarNode, node.Tag, value)
		newStringNode.Style = node.Style
		results.PushBack(newStringNode)

	}

	return context.ChildContext(results), nil

}

View on GitHub (pinned to 8b5af0694b)

Solutions

1. Convert to string first: . | tostring | upcase
2. Target the specific string field, not the whole node
3. Filter out nulls/non-strings with select(tag == "!!str")

When it happens

Trigger: Thrown at pkg/yqlib/operator_strings.go:192 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/5ef160b2ae5536b0. Report an issue: GitHub.