What causes "%v" in FiloSottile/age?

Inspect the stderr output from the plugin — fatalInteractf prints the real underlying message there

%v

Error message

%v

What it means

fatalInteractf formats the underlying interaction error, writes it to the plugin's stderr, sets the plugin's broken flag so subsequent Wrap/Unwrap calls exit with an error, and returns the formatted error. The message text is simply the wrapped interaction protocol error, so the real cause is the format arguments (e.g. a malformed stanza or read failure).

Source

Thrown at plugin/plugin.go:580

	s, err = readOkOrFail(p.sr)
	if err != nil {
		return false, p.fatalInteractf("%v", err)
	}
	if s.Type == "fail" {
		return false, fmt.Errorf("client failed to request confirmation")
	}
	if err := expectStanzaWithNoBody(s, 1); err != nil {
		return false, p.fatalInteractf("%v", err)
	}
	return s.Args[0] == "yes", nil
}

// fatalInteractf prints the error to stderr and sets the broken flag, so the
// Wrap/Unwrap caller can exit with an error.
func (p *Plugin) fatalInteractf(format string, args ...any) error {
	p.broken = true
	fmt.Fprintf(p.stderr, format, args...)
	return fmt.Errorf(format, args...)
}

func (p *Plugin) fatalf(format string, args ...any) int {
	fmt.Fprintf(p.stderr, format, args...)
	return 1
}

func expectStanzaWithNoBody(s *format.Stanza, wantArgs int) error {
	if len(s.Args) != wantArgs {
		return fmt.Errorf("%s stanza has %d arguments, want %d", s.Type, len(s.Args), wantArgs)
	}
	if len(s.Body) != 0 {
		return fmt.Errorf("%s stanza has %d bytes of body, want 0", s.Type, len(s.Body))
	}
	return nil
}

func expectStanzaWithBody(s *format.Stanza, wantArgs int) error {

View on GitHub (pinned to b74dce4cdb)

Solutions

Inspect the stderr output from the plugin — fatalInteractf prints the real underlying message there
Verify the age client and plugin versions both implement the same plugin protocol version
Run without output redirection to ensure the plugin's stdin/stdout protocol channel is intact

Defensive patterns

Strategy: try-catch

Validate before calling

// Before interaction, ensure the protocol pipes are unmodified:
// (in plugin main) if os.Stdout has been redirected away from the client, abort early

Try / catch

if err != nil {
    // fatalInteractf already printed the cause to the plugin's stderr;
    // log err and mark the plugin broken so Wrap/Unwrap exits cleanly
    return err
}

Prevention

Never redirect the plugin's stdin/stdout while it talks to the age client
Keep age client and plugin protocol versions aligned
Check plugin stderr output first when interactions fail

When it happens

Trigger: DisplayMessage, RequestValue, or Confirm reads a malformed/unexpected stanza from the client and calls fatalInteractf; readOkOrFail fails on a protocol violation; the client connection breaks mid-interaction.

Common situations: A buggy or too-old age client speaking a broken plugin protocol; the plugin's stdout/stderr being redirected causing protocol desync; corrupted interaction stream.

Related errors

AI-assisted analysis of FiloSottile/age@b74dce4cdb (2026-08-31). Data as JSON: /api/errors/c66733ec6720e5af. Report an issue: GitHub.