What causes "Trying to send data when to pipe is nil" in kovidgoyal/kitty?

Ensure start() is called and its error checked before any query()

Trying to send data when to pipe is nil

Error message

Trying to send data when to pipe is nil

What it means

send() was called on the kitty font backend before its output pipe was established (k.to == nil), i.e. before start() successfully ran or after the backend died and pipes were torn down. It is a programming-order/lifecycle error inside the choose-fonts kitten.

Source

Thrown at kittens/choose_fonts/backend.go:69

	k.cmd.Stdout = k.w
	k.json_decoder = json.NewDecoder(k.from)
	if err = k.cmd.Start(); err != nil {
		return err
	}
	k.started = true
	k.timeout = 60 * time.Second
	k.wait_for_exit = make(chan error)
	go func() {
		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 {

View on GitHub (pinned to 6d5d0c4406)

Solutions

Ensure start() is called and its error checked before any query()
If the backend crashed, inspect k.stderr for the subprocess error output
Verify the kitty executable works: kitty +runpy 'print(1)'

Example fix

// before
kitty_font_backend.send(msg)
// after
if err := kitty_font_backend.start(); err != nil { return err }
kitty_font_backend.send(msg)

Defensive patterns

Strategy: type-guard

Validate before calling

if kitty_font_backend.to == nil { if err := kitty_font_backend.start(); err != nil { return err } }

Type guard

func backendReady(k *kitty_font_backend_type) bool { return k != nil && k.to != nil }

Try / catch

Check readiness via guard; on send error mentioning nil pipe, call start() and retry once.

Prevention

Always start and error-check the backend before issuing queries
Monitor the subprocess and surface its stderr on crash

When it happens

Trigger: Calling query() -> send() before kitty_font_backend.start() completed os.Pipe(), or after the subprocess exited and pipes were closed.

Common situations: Kitty's backend subprocess crashed at startup; race where a query is issued before init finishes; broken kitty installation causing immediate subprocess exit.

Related errors

AI-assisted analysis of kovidgoyal/kitty@6d5d0c4406 (2026-08-27). Data as JSON: /api/errors/0c39658351b1b652. Report an issue: GitHub.