What causes "cannot marshal data: %v" in vitessio/vitess?

Inspect the wrapped marshal error to identify the offending field/type.

cannot marshal data: %v

Error message

cannot marshal data: %v

What it means

In the same vtgate HTTP API handler, after successfully fetching the object, json.MarshalIndent of the response fails and returns `cannot marshal data: <err>`. This is an internal serialization failure — the fetched object could not be encoded to JSON.

Source

Thrown at go/vt/vtgate/api.go:68

		}()
		if err := handlerFunc(w, r); err != nil {
			httpErrorf(w, r, "%v", err)
		}
	})
}

func handleCollection(collection string, getFunc func(*http.Request) (any, error)) {
	handleAPI(collection+"/", func(w http.ResponseWriter, r *http.Request) error {
		// Get the requested object.
		obj, err := getFunc(r)
		if err != nil {
			return fmt.Errorf("can't get %v: %v", collection, err)
		}

		// JSON encode response.
		data, err := json.MarshalIndent(obj, "", "  ")
		if err != nil {
			return fmt.Errorf("cannot marshal data: %v", err)
		}
		w.Header().Set("Content-Type", jsonContentType)
		w.Write(data)
		return nil
	})
}

func getItemPath(url string) string {
	// Strip API prefix.
	if !strings.HasPrefix(url, apiPrefix) {
		return ""
	}
	url = url[len(apiPrefix):]

	// Strip collection name.
	parts := strings.SplitN(url, "/", 2)
	if len(parts) != 2 {
		return ""

View on GitHub (pinned to 01a25a7d17)

Solutions

Inspect the wrapped marshal error to identify the offending field/type.
Report/fix the data type producing unmarshalable values in the API handler.
Retry the request after upgrading Vitess if it is a known bug.

Defensive patterns

Strategy: try-catch

Try / catch

data, err := doAPICall(path)
if err != nil && strings.Contains(err, "cannot marshal data") {
    // retry or report the Vitess bug with the offending endpoint
}

Prevention

Report occurrences with the endpoint URL — it's usually a Vitess bug.
Keep Vitess updated to a version where the marshal issue is fixed.

When it happens

Trigger: A vtgate debug API object contains a value that json cannot encode (e.g. unsupported type, NaN, or marshal error inside the collected data) while serving /api/<collection>/ requests.

Common situations: Rare; typically indicates a bug in the data collected by the API (invalid types) rather than user error.

Understand the failure class

Background: "JSON serialization failed", "not JSON serializable", "Failed to serialize": why JSON marshaling errors happen and how to fix them — this error's family across 46 libraries.

Related errors

AI-assisted analysis of vitessio/vitess@01a25a7d17 (2026-09-01). Data as JSON: /api/errors/eb35a222d03195ae. Report an issue: GitHub.