kovidgoyal/kitty · error

Failed to send message to kitty with I/O error: %w

Error message

Failed to send message to kitty with I/O error: %w

What it means

Writing the JSON message body to the backend pipe returned an I/O error — typically EPIPE because the kitty subprocess died, or the pipe was closed concurrently. Raised inside the send goroutine.

Source

Thrown at kittens/choose_fonts/backend.go:78

		k.wait_for_exit <- k.cmd.Wait()
	}()
	return
}

var kitty_font_backend kitty_font_backend_type

func (k *kitty_font_backend_type) send(v any) error {
	if k.to == nil {
		return fmt.Errorf("Trying to send data when to pipe is nil")
	}
	data, err := json.Marshal(v)
	if err != nil {
		return fmt.Errorf("Could not encode message to kitty with error: %w", err)
	}
	c := make(chan error)
	go func() {
		if _, err = k.to.Write(data); err != nil {
			c <- fmt.Errorf("Failed to send message to kitty with I/O error: %w", err)
			return
		}
		if _, err = k.to.Write([]byte{'\n'}); err != nil {
			c <- fmt.Errorf("Failed to send message to kitty with I/O error: %w", err)
			return
		}
		c <- nil
	}()
	select {
	case err := <-c:
		return err
	case <-time.After(k.timeout):
		return fmt.Errorf("Timed out waiting to write to kitty font backend after %v", k.timeout)
	case err := <-k.wait_for_exit:
		k.exited = true
		if err == nil {
			err = fmt.Errorf("kitty font backend exited with no error while waiting for a response from it")
		} else {

View on GitHub (pinned to 6d5d0c4406)

Solutions

1. Check k.stderr and process exit state to find why the backend died
2. Restart the backend (start()) and retry the query once
3. Verify the kitty executable and its kittens package are intact/consistent versions

Example fix

// before
err := backend.send(msg)
// after
if err := backend.send(msg); err != nil {
    if strings.Contains(err.Error(), "I/O error") {
        backend.restart(); err = backend.send(msg)
    }
}

if kitty_font_backend.cmd.ProcessState != nil && kitty_font_backend.cmd.ProcessState.Exited() { /* restart backend before sending */ }

On write error, capture stderr, restart the backend via start(), and resend the message once; abort on second failure.

When it happens

Trigger: k.to.Write(data) fails after the '+runpy' kitty process exited (crash, kill, or normal shutdown racing a query).

Common situations: Backend python process crashed (import error in kittens.choose_fonts.backend); kitty was closed while a font query was in flight; pipe buffer closed early.

Related errors

• Failed to find the kitty executable, this kitten requires th
• Trying to send data when to pipe is nil
• Could not encode message to kitty with error: %w
• The font specification %s is invalid as %s does not contain
• Terminal does not support querying the: %s

AI-assisted analysis of kovidgoyal/kitty@6d5d0c4406 (2026-08-27). Data as JSON: /api/errors/5c2d3a8dc07b89c3. Report an issue: GitHub.