What causes "Final result error writing" in gleam-lang/gleam?

Ensure stderr is writable and the consuming pipe/process is still alive when running gleam commands

Final result error writing

Error message

Final result error writing

What it means

When the gleam CLI main function finishes with an Err, it pretty-prints the error to stderr and uses .expect("Final result error writing") on the print operation. If writing to stderr fails, this panics. It only triggers while already reporting a failure, so it compounds an existing error.

Solutions

Ensure stderr is writable and the consuming pipe/process is still alive when running gleam commands
Fix the underlying command failure first — this panic only occurs while reporting another error
Avoid closing/redirecting stderr in CI wrappers that run gleam
Report a broken-pipe scenario upstream if it happens in normal terminal use

Example fix

// before
$ gleam build 2>&1 | head -1   # downstream closes the pipe early
// after
$ gleam build 2>&1 | tee build.log   # keep the consumer alive while gleam writes

Defensive patterns

Strategy: try-catch

Validate before calling

fn stderr_is_writable() -> bool {
    use std::io::Write;
    std::io::stderr().flush().is_ok()
}

Try / catch

if let Err(e) = write_stderr(&buffer) {
    eprintln!("could not report error: {}", e);
    std::process::exit(1);
}

Prevention

Do not close or early-exit consumers of gleam's stderr pipe
Avoid 2>/dev/null variants in restricted sandboxes
Fix the root command failure first
Use tee instead of short-lived pipes when capturing output

When it happens

Trigger: The command failed (main returned Err), and stderr.print(&buffer) fails — typically because stderr is closed, redirected to a file/pipe that is full or unwritable, or the terminal detached.

Common situations: Running `gleam build 2>/dev/null` in environments where /dev/null write fails, CI runners with closed stderr descriptors, piping gleam output to a process that exited early (broken pipe), or a full disk when redirecting output to a file.

Related errors

AI-assisted analysis of gleam-lang/gleam@49f8762da5 (2026-09-14). Data as JSON: /api/errors/35aaec8a4f9e58ae. Report an issue: GitHub.

Appendix: source

Thrown at compiler-cli/src/lib.rs:968

        version: String,
    },
}

pub fn main() {
    initialise_logger();
    panic::add_handler();
    let stderr = cli::stderr_buffer_writer();
    let result = get_current_directory()
        .and_then(|working_directory| Command::parse().run(working_directory));
    match result {
        Ok(_) => {
            tracing::info!("Successfully completed");
        }
        Err(error) => {
            tracing::error!(error = ?error, "Failed");
            let mut buffer = stderr.buffer();
            error.pretty(&mut buffer);
            stderr.print(&buffer).expect("Final result error writing");
            std::process::exit(1);
        }
    }
}

fn command_check(paths: &ProjectPaths, target: Option<Target>) -> Result<()> {
    let _ = build::main(
        paths,
        Options {
            root_target_support: TargetSupport::Enforced,
            warnings_as_errors: false,
            codegen: Codegen::DepsOnly,
            compile: Compile::All,
            mode: Mode::Dev,
            target,
            no_print_progress: false,
            erlang_output: ErlangOutput::Binary,
        },

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