What causes "expected item at index %d to be integer, got %T" in googleapis/mcp-toolbox?

Ensure the input slice holds Go `int` values before calling, or pre-convert with a normalization loop

expected item at index %d to be integer, got %T

Error message

expected item at index %d to be integer, got %T

What it means

ConvertAnySliceToTyped converts a []any into []int64 when itemType is "integer". Each element must be an `int` (Go native); if any element is not an int (e.g. float64 from JSON, or a string), the function aborts with this error naming the failing index.

Source

Thrown at internal/util/parameters/common.go:43

func ConvertAnySliceToTyped(s []any, itemType string) (any, error) {
	var typedSlice any
	switch itemType {
	case "string":
		tempSlice := make([]string, len(s))
		for j, item := range s {
			s, ok := item.(string)
			if !ok {
				return nil, fmt.Errorf("expected item at index %d to be string, got %T", j, item)
			}
			tempSlice[j] = s
		}
		typedSlice = tempSlice
	case "integer":
		tempSlice := make([]int64, len(s))
		for j, item := range s {
			i, ok := item.(int)
			if !ok {
				return nil, fmt.Errorf("expected item at index %d to be integer, got %T", j, item)
			}
			tempSlice[j] = int64(i)
		}
		typedSlice = tempSlice
	case "float":
		tempSlice := make([]float64, len(s))
		for j, item := range s {
			f, ok := item.(float64)
			if !ok {
				return nil, fmt.Errorf("expected item at index %d to be float, got %T", j, item)
			}
			tempSlice[j] = f
		}
		typedSlice = tempSlice
	case "boolean":
		tempSlice := make([]bool, len(s))
		for j, item := range s {
			b, ok := item.(bool)

View on GitHub (pinned to 8cc6e09de2)

Solutions

Ensure the input slice holds Go `int` values before calling, or pre-convert with a normalization loop
If the source is JSON-decoded data, convert float64 elements (whole numbers) to int first
Change the declared parameter type to 'float' if decimals are possible
Have clients send unquoted integer literals in the tool call arguments

Example fix

// before
ConvertAnySliceToTyped([]any{float64(1), float64(2)}, "integer") // fails: float64 not int
// after
ConvertAnySliceToTyped([]any{1, 2}, "integer")

Defensive patterns

Strategy: type-guard

Validate before calling

func allInts(v []any) bool {
    for _, item := range v {
        if _, ok := item.(int); !ok {
            return false
        }
    }
    return true
}

Type guard

func isIntArray(v []any) bool {
    for _, item := range v {
        switch item.(type) {
        case int, int64:
        default:
            return false
        }
    }
    return true
}

Try / catch

typed, err := ConvertAnySliceToTyped(s, "integer")
if err != nil {
    return nil, fmt.Errorf("invalid integer array parameter: %w", err)
}

Prevention

Remember JSON decoding yields float64; convert to int before passing
Normalize numeric strings ("5") to ints at the ingestion boundary
Declare the schema element type as 'integer' so clients send real integers

When it happens

Trigger: Passing a slice containing float64, string, or bool values to ConvertAnySliceToTyped with itemType="integer", e.g. JSON-decoded arrays where numbers are float64, or clients sending quoted numbers ["1","2"].

Common situations: JSON unmarshalling makes all numbers float64, so integer array params fail; LLM sends "5" instead of 5; mixed-type arrays generated by an agent.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/f1f9e8fd2f305a53. Report an issue: GitHub.