What causes "Cannot write to stdin" in apple/pkl?

Drop the `-w`/`--overwrite` flag when reading from stdin; capture the formatted output instead, e.g. `pkl format - in.pkl`.

apple/pkl · error · CliException

Cannot write to stdin

Error message

Cannot write to stdin

What it means

`pkl format` with `-w`/`--overwrite` and stdin (`-`) as a path cannot write the formatted result back to stdin, so handlePaths throws. Reading from stdin is only meaningful with diff/check modes where no in-place write is needed.

Solutions

Drop the `-w`/`--overwrite` flag when reading from stdin; capture the formatted output instead, e.g. `pkl format - < in.pkl > in.pkl`.
Or pass the real file path instead of `-` so in-place overwrite works.

Example fix

# before
cat foo.pkl | pkl format -w -
# after
cat foo.pkl | pkl format - > foo.pkl

Defensive patterns

Strategy: validation

Validate before calling

if (paths.contains("-") && overwrite) {
  throw IllegalArgumentException("cannot combine -w/--overwrite with stdin ('-')")
}

Try / catch

try {
  pklCli.runFormat(args)
} catch (e: CliException) {
  if (e.message == "Cannot write to stdin") args.remove("-w")
  else throw e
}

Prevention

Never pair `-w/--overwrite` with `-`.
When reading stdin, redirect stdout to the target file instead.
Document formatter pipelines to use `pkl format - < in > in`.

When it happens

Trigger: Running `pkl format -w -` or `echo 'x' | pkl format --overwrite -`.

Common situations: Piping editor buffers into the formatter while also passing `-w` out of habit from other tools.

Understand the failure class

Background: "mutually exclusive" flag errors: what "can't supply both nx and xx", "--raw is not compatible with -i" and "cannot be used with" mean, and how to fix them — this error's family across 29 libraries.

Related errors

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/8b62ce582c1e2131. Report an issue: GitHub.

Appendix: source

Thrown at pkl-cli/src/main/kotlin/org/pkl/cli/CliFormatterCommand.kt:109

        if (!silent) {
          writeErrLine("An error occurred during formatting.")
        }
        throw CliTestException("", status.status)
      }
    }
  }

  private fun handlePaths(status: Status) {
    for (path in allPaths()) {
      val pathStr = path.toString()
      try {
        val contents =
          when {
            pathStr == "-" -> IoUtils.readString(System.`in`)
            else -> Files.readString(path)
          }
        if (pathStr == "-" && overwrite) {
          throw CliException("Cannot write to stdin", ERROR)
        }

        val formatted = format(contents)
        if (contents != formatted) {
          if (diffNameOnly || overwrite) {
            // if `--diff-name-only` or `-w` is specified, only write file names
            writeLine(pathStr)
          }

          if (overwrite) {
            path.writeText(formatted, Charsets.UTF_8)
          } else {
            // only exit on violation for "check" operations, not when overwriting
            status.update(FORMATTING_VIOLATION)
          }
        }

        if (!diffNameOnly && !overwrite) {

View on GitHub (pinned to f3efcbfc9b)