What causes "cannot parse form: %s" in vitessio/vitess?

URL-encode query parameters correctly: /livequeryz?format=json

cannot parse form: %s

Error message

cannot parse form: %s

What it means

The /livequeryz handler calls r.ParseForm() and returns HTTP 500 'cannot parse form: <err>' when the request's form data is malformed and cannot be parsed. This guards the format=json query handling that follows.

Source

Thrown at go/vt/vttablet/tabletserver/livequeryz.go:68

			<td>{{.Duration}}</td>
			<td>{{.Start}}</td>
			<td>{{.ConnID}}</td>
			<td><a href='terminate?connID={{.ConnID}}'>Terminate</a></td>
		</tr>
	`))
)

func livequeryzHandler(queryLists []*QueryList, w http.ResponseWriter, r *http.Request) {
	if err := acl.CheckAccessHTTP(r, acl.DEBUGGING); err != nil {
		acl.SendError(w, err)
		return
	}
	var rows []QueryDetailzRow
	for _, ql := range queryLists {
		rows = ql.AppendQueryzRows(rows)
	}
	if err := r.ParseForm(); err != nil {
		http.Error(w, fmt.Sprintf("cannot parse form: %s", err), http.StatusInternalServerError)
		return
	}
	format := r.FormValue("format")
	if format == "json" {
		js, err := json.Marshal(rows)
		if err != nil {
			http.Error(w, err.Error(), http.StatusInternalServerError)
			return
		}
		w.Header().Set("Content-Type", "application/json")
		w.Write(js)
		return
	}
	logz.StartHTMLTable(w)
	defer logz.EndHTMLTable(w)
	w.Write(livequeryzHeader)
	for i := range rows {
		if err := livequeryzTmpl.Execute(w, rows[i]); err != nil {

View on GitHub (pinned to 01a25a7d17)

Solutions

URL-encode query parameters correctly: /livequeryz?format=json
Fix the proxy/client generating the malformed request
Retry with a simple well-formed request to confirm the endpoint itself is healthy

Example fix

// before (unencoded)
curl 'http://tablet:15100/livequeryz?format=json%'
// after
curl 'http://tablet:15100/livequeryz?format=json'

Defensive patterns

Strategy: validation

Validate before calling

u, err := url.Parse(baseURL + "/livequeryz")
if err != nil {
    return err
}
u.RawQuery = "format=json" // let net/url encode properly

Try / catch

resp, err := http.Get(u.String())
if resp != nil && resp.StatusCode == http.StatusInternalServerError {
    body, _ := io.ReadAll(resp.Body)
    if strings.Contains(string(body), "cannot parse form") {
        // fix request encoding and retry
    }
}

Prevention

Build URLs with net/url and url.Values instead of string concatenation
Avoid manual percent-encoding in query strings
Inspect proxy logs for request rewriting

When it happens

Trigger: GET/POST to /livequeryz with a malformed query string or body (bad percent-encoding, invalid URL syntax) causing http.Request.ParseForm to fail.

Common situations: Proxies or clients double-encoding query strings; truncated requests; hand-rolled URLs with unescaped special characters like & or %.

Related errors

AI-assisted analysis of vitessio/vitess@01a25a7d17 (2026-09-01). Data as JSON: /api/errors/6dccb119e36c8594. Report an issue: GitHub.