What causes "column 'object_details' is not a string, but %T" in googleapis/mcp-toolbox?

Inspect the column types in your query result and ensure the JSON payload column is a Spanner STRING

column 'object_details' is not a string, but %T

Error message

column 'object_details' is not a string, but %T

What it means

In processRows (internal/sources/spanner/spanner.go:153), the Spanner source expects a column's value to convert to a Go string via val.AsInterface().(string) — this is used to parse JSON 'object_details' columns into maps. When the Spanner value's underlying Go type is not a string (e.g. []byte, int64, float64, bool), the type assertion fails and this error is returned. It indicates the query returned a non-JSON/non-string value in a column the code path treats as a JSON string payload.

Source

Thrown at internal/sources/spanner/spanner.go:153

		row, err := iter.Next()
		if err == iterator.Done {
			break
		}
		if err != nil {
			return nil, fmt.Errorf("unable to parse row: %w", err)
		}

		rowMap := orderedmap.Row{}
		cols := row.ColumnNames()
		for i, c := range cols {
			if c == "object_details" { // for list graphs or list tables
				val := row.ColumnValue(i)
				if val == nil { // ColumnValue returns the Cloud Spanner Value of column i, or nil for invalid column.
					rowMap.Add(c, nil)
				} else {
					jsonString, ok := val.AsInterface().(string)
					if !ok {
						return nil, fmt.Errorf("column 'object_details' is not a string, but %T", val.AsInterface())
					}
					var details map[string]any
					if err := json.Unmarshal([]byte(jsonString), &details); err != nil {
						return nil, fmt.Errorf("unable to unmarshal JSON: %w", err)
					}
					rowMap.Add(c, details)
				}
			} else {
				rowMap.Add(c, row.ColumnValue(i))
			}
		}
		out = append(out, rowMap)
	}
	return out, nil
}

func (s *Source) RunSQL(ctx context.Context, readOnly bool, statement string, params map[string]any) (any, error) {
	var results []any

View on GitHub (pinned to 8cc6e09de2)

Solutions

Inspect the column types in your query result and ensure the JSON payload column is a Spanner STRING
Cast the column in SQL: SELECT CAST(object_details AS STRING) AS object_details ...
If the value arrives as []byte, convert with string(val) before unmarshalling in processRows
Check for recent schema migrations that changed the column's type away from STRING

Example fix

// before
SELECT object_details FROM objects;
// after
SELECT CAST(object_details AS STRING) AS object_details FROM objects;

Defensive patterns

Strategy: type-guard

Validate before calling

// verify the column is STRING before running the tool
// in Spanner: SELECT column_name, spanner_type FROM information_schema.columns WHERE table_name = 'objects';

Type guard

func asJSONString(v any) (string, bool) {
	s, ok := v.(string)
	if !ok {
		if b, ok2 := v.([]byte); ok2 {
			return string(b), true
		}
		return "", false
	}
	return s, true
}

Prevention

Keep JSON payload columns as Spanner STRING type
CAST in SQL when column type may vary
Assert the schema in CI with information_schema checks

When it happens

Trigger: Calling RunSQL/InvokeSearchCatalog through the Spanner source where a result column's value from row.ColumnValue(i).AsInterface() is not a string — e.g. the search/catalog query returns numeric or []byte columns instead of JSON text.

Common situations: Spanner returning JSON columns as []byte with certain drivers/settings; a schema change altered a column from STRING to NUMERIC/BOOL; using a custom SQL query that selects non-string columns into a code path expecting 'object_details' JSON.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/70a4218e2c9089ac. Report an issue: GitHub.