What causes "invalid '%s' parameter; expected a string" in googleapis/mcp-toolbox?

Quote the value so it is a JSON string, e.g. "19991231-20201231".

invalid '%s' parameter; expected a string

Error message

invalid '%s' parameter; expected a string

What it means

Every other recognized DICOM search parameter (keys listed in paramKeys, e.g. patient name, date ranges) must be a string. If a value arrives as a number, bool, array or object, the assertion fails and this error is returned.

Source

Thrown at internal/tools/cloudhealthcare/common/util.go:83

			if _, ok := v.([]any); !ok {
				return nil, fmt.Errorf("invalid '%s' parameter; expected a string array", k)
			}
			attributeIDsSlice, err := parameters.ConvertAnySliceToTyped(v.([]any), "string")
			if err != nil {
				return nil, fmt.Errorf("can't convert '%s' to array of strings: %s", k, err)
			}
			attributeIDs := attributeIDsSlice.([]string)
			if len(attributeIDs) != 0 {
				opts = append(opts, googleapi.QueryParameter(k, strings.Join(attributeIDs, ",")))
			}
		} else if k == EnablePatientNameFuzzyMatchingKey {
			if _, ok := v.(bool); !ok {
				return nil, fmt.Errorf("invalid '%s' parameter; expected a boolean", k)
			}
			opts = append(opts, googleapi.QueryParameter(k, fmt.Sprintf("%t", v.(bool))))
		} else if slices.Contains(paramKeys, k) {
			if _, ok := v.(string); !ok {
				return nil, fmt.Errorf("invalid '%s' parameter; expected a string", k)
			}
			if v.(string) != "" {
				opts = append(opts, googleapi.QueryParameter(k, v.(string)))
			}
		}
	}
	return opts, nil
}

View on GitHub (pinned to 8cc6e09de2)

Solutions

Quote the value so it is a JSON string, e.g. "19991231-20201231".
Convert numbers to their DICOM string representation before sending.
Check the tool's parameter schema for which keys expect strings.
Validate payload types before invoking.

Example fix

// before
{"studydate": 20201231}
// after
{"studydate": "20201231"}

Defensive patterns

Strategy: validation

Validate before calling

function assertStringParams(obj, keys) {
  for (const k of keys) {
    if (k in obj && typeof obj[k] !== 'string') {
      throw new Error(`parameter ${k} must be a string`);
    }
  }
}
// e.g. assertStringParams(params, ['patientid','studydate','modality'])

Type guard

function isString(v) { return typeof v === 'string'; }

Prevention

Format dates as DICOM strings (YYYYMMDD) before sending
Never rely on JSON numbers for string-typed search params
Validate payloads against the tool's declared parameter schema
Wrap LLM tool-call arguments in a type-checking adapter

When it happens

Trigger: Invoking a DICOM search tool with a search parameter (one of paramKeys) whose value is not a string, e.g. {"limit": 10} on a string-typed key or a numeric patient ID.

Common situations: LLM sending unquoted numbers or dates as JSON numbers; clients serializing dates as timestamps instead of DICOM date strings (e.g. "19991231"); mixed payloads from dynamic UIs.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/dcb6191f3e7148c5. Report an issue: GitHub.