What causes "'explore' must be a string, got %T" in googleapis/mcp-toolbox?

Send 'explore' as a JSON string, e.g. {"explore": "order_items"}.

'explore' must be a string, got %T

Error message

'explore' must be a string, got %T

What it means

Runtime type check in ProcessFieldArgs: the invocation's `explore` parameter is not a string (the message reports the actual Go type received, e.g. map or float64), so the target Looker explore cannot be identified.

Source

Thrown at internal/tools/looker/lookercommon/lookercommon.go:179

		filtersParameter,
		pivotsParameter,
		sortsParameter,
		limitParameter,
		tzParameter,
		filterExpressionParameter,
		dynamicFieldsParameter,
	}
}

func ProcessFieldArgs(ctx context.Context, params parameters.ParamValues) (*string, *string, error) {
	mapParams := params.AsMap()
	model, ok := mapParams["model"].(string)
	if !ok {
		return nil, nil, fmt.Errorf("'model' must be a string, got %T", mapParams["model"])
	}
	explore, ok := mapParams["explore"].(string)
	if !ok {
		return nil, nil, fmt.Errorf("'explore' must be a string, got %T", mapParams["explore"])
	}
	return &model, &explore, nil
}

// escapeUnquotedParameterValue escapes Looker filter-expression metacharacters
// so the value reaches a type: unquoted parameter without being interpreted as
// a wildcard pattern. Looker treats `_` as a single-character wildcard and `%`
// as a multi-character wildcard, and rejects either inside unquoted-parameter
// values; `,` is the filter-expression value separator; `^` is the escape
// character itself. Already-escaped sequences (`^_`, `^%`, `^,`, `^^`) pass
// through unchanged, which keeps the function idempotent for callers that pass
// pre-escaped forms (e.g. round-tripping `default_filter_value` from the
// explore metadata). Lone metacharacters get a `^` prefix; lone `^` is doubled
// to `^^`. Walking rune-by-rune (not whole-string scanning) means a value with
// both an already-escaped sequence and an unescaped metacharacter gets each
// half handled correctly — e.g. `first^_touch_v2` becomes `first^_touch^_v2`.
func escapeUnquotedParameterValue(value string) string {
	var sb strings.Builder

View on GitHub (pinned to 8cc6e09de2)

Solutions

Send 'explore' as a JSON string, e.g. {"explore": "order_items"}.
Ensure both 'model' and 'explore' are present in the tool call.
Validate the request payload types client-side before invoking.

Example fix

// before
{"model": "ecommerce", "explore": true}
// after
{"model": "ecommerce", "explore": "order_items"}

Defensive patterns

Strategy: validation

Validate before calling

// Client-side check before invoking
const explore = "order_items";
if (typeof explore !== "string" || explore.length === 0) {
    throw new Error("'explore' must be a non-empty string");
}

Type guard

e, ok := mapParams["explore"]
explore, isStr := e.(string)
if !ok || !isStr {
    return fmt.Errorf("explore must be string, got %T", e)
}

Try / catch

model, explore, err := lookercommon.ProcessFieldArgs(ctx, params)
if err != nil {
    return nil, util.NewInvalidArgumentError("check 'explore' parameter: %v", err)
}

Prevention

Always send both model and explore as strings in the same payload.
Validate parameter types before invoking the tool programmatically.
Use typed SDK wrappers that enforce string arguments at compile time.

When it happens

Trigger: Invoke receives params where mapParams["explore"] is nil (missing) or a non-string dynamic value (number, bool, array).

Common situations: Client omitting 'explore'; JSON payload typing explore as a number; programmatic invocation passing the wrong type; template interpolation injecting a non-string.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/eed41fd903a06763. Report an issue: GitHub.