output ` ` contains newlines, consider serializing with…

Error message

output `{tag}` contains newlines, consider serializing with json and deserializing in gha with fromJSON()

What it means

gha_output writes a `tag=content` line to the GITHUB_OUTPUT environment file so later GitHub Actions steps can read the value. GitHub Actions environment files cannot contain raw multiline values (actions/toolkit#403), so this function proactively rejects any content containing a newline ('\n') instead of silently producing a corrupted workflow output. The caller is expected to serialize the value as JSON and read it back with fromJSON() in the workflow.

Solutions

Serialize the multi-line value with serde_json::to_string (or json!([...])) before passing it to gha_output, then in the workflow read it with fromJSON(steps.<step>.outputs.<tag>).
Join lines with a single-line delimiter (e.g. space or comma) if JSON round-tripping is overkill.
Split the content into multiple scalar outputs (one gha_output call per line item) if the consumer only needs individual values.

Example fix

// before
let targets = targets.join("\n");
gha_output("targets", &targets)?;

// after
let targets = serde_json::to_string(&targets)?;
gha_output("targets", &targets)?;
// workflow: ${{ fromJSON(steps.build.outputs.targets)[0] }}

Defensive patterns

Strategy: validation

Validate before calling

fn gha_output_safe(tag: &str, content: &str) -> cross::Result<()> {
    if content.contains('\n') {
        eyre::bail!("refusing to write multi-line output `{tag}`; serialize as JSON first");
    }
    Ok(())
}
// call gha_output_safe(tag, &content)?; before gha_output

Type guard

fn is_single_line(s: &str) -> bool { !s.contains('\n') }

Prevention

Always JSON-serialize list/multi-line values before passing to gha_output.
Add a unit test asserting outputs passed to gha_output contain no '\n'.
Prefer building outputs from typed structs serialized with serde_json rather than hand-joined strings.

When it happens

Trigger: Calling gha_output(tag, content) with a content string that contains a '\n' character, e.g. multi-line tool output, a multi-line version list, or text built with format! over several lines.

Common situations: A maintainer adds a CI task (via the ci, build_docker_image, or run xtask commands) that emits multi-line output such as a list of matrix entries, test names, or docker tags; locally or in a runner the collected content ends up multiline and the xtask step fails.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of cross-rs/cross@8c1a8aa4b6 (2026-09-13). Data as JSON: /api/errors/1ee7694ce171bb4a. Report an issue: GitHub.

Appendix: source

Thrown at xtask/src/util.rs:322

        eprintln!($fmt $(,$args)*);
    };
}

// note: for GHA actions we need to output these tags no matter the verbosity level
pub fn gha_print(content: &str) {
    gha_output!("{}", content);
}

// note: for GHA actions we need to output these tags no matter the verbosity level
pub fn gha_error(content: &str) {
    gha_output!("::error {}", content);
}

#[track_caller]
pub fn gha_output(tag: &str, content: &str) -> cross::Result<()> {
    if content.contains('\n') {
        // https://github.com/actions/toolkit/issues/403
        eyre::bail!(
            "output `{tag}` contains newlines, consider serializing with json and deserializing in gha with fromJSON()"
        );
    }
    write_to_gha_env_file("GITHUB_OUTPUT", &format!("{tag}={content}"))?;
    Ok(())
}

pub fn read_dockerfiles(msg_info: &mut MessageInfo) -> cross::Result<Vec<(PathBuf, String)>> {
    let root = project_dir(msg_info)?;
    let docker = root.join("docker");
    let mut dockerfiles = vec![];
    for entry in fs::read_dir(docker)? {
        let entry = entry?;
        let file_type = entry.file_type()?;
        let file_name = entry.file_name();
        if file_type.is_file() && file_name.to_utf8()?.starts_with("Dockerfile") {
            let contents = fs::read_to_string(entry.path())?;
            dockerfiles.push((entry.path().to_path_buf(), contents));

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