What causes "moon build failed for AI task '{}' with status {}" in nikivdev/code?

Inspect moon build output above the error for the compiler diagnostics

moon build failed for AI task '{}' with status {}

Error message

moon build failed for AI task '{}' with status {}

What it means

Thrown by build_task_cached in src/ai_tasks.rs when the `moon build` invocation for an AI task exits with a non-zero status. The cached binary could not be produced, so the cached-run path aborts.

Source

Thrown at src/ai_tasks.rs:336

    let mut cmd = Command::new("moon");
    cmd.arg("build")
        .arg("--target")
        .arg("native")
        .arg("--release");
    if std::env::var("FLOW_AI_TASK_NO_FROZEN").is_err() {
        cmd.arg("--frozen");
    }
    cmd.arg(&run_path).current_dir(&workspace_dir);
    let status = cmd.status().with_context(|| {
        format!(
            "failed to build AI task '{}' with moon build (workspace: {})",
            task.id,
            workspace_dir.display()
        )
    })?;
    if !status.success() {
        bail!(
            "moon build failed for AI task '{}' with status {}",
            task.id,
            status
        );
    }

    let built_binary = find_built_binary(&workspace_dir)?;
    fs::copy(&built_binary, &binary_path).with_context(|| {
        format!(
            "failed to copy built binary {} -> {}",
            built_binary.display(),
            binary_path.display()
        )
    })?;
    #[cfg(unix)]
    {
        let mut perms = fs::metadata(&binary_path)
            .with_context(|| format!("failed to stat {}", binary_path.display()))?

View on GitHub (pinned to a747e741ae)

Solutions

Inspect moon build output above the error for the compiler diagnostics
Run `moon build` manually in the workspace and fix the reported errors
Update/refresh dependencies (moon update) or the moon toolchain, clean _build, and retry

Defensive patterns

Strategy: try-catch

Try / catch

match build_task_cached(task, root, force) {
    Ok(a) => Ok(a),
    Err(e) if e.to_string().contains("moon build failed") => {
        eprintln!("Fix compiler errors shown by moon build output, then retry.");
        Err(e)
    }
    Err(e) => Err(e),
}

Prevention

Run `moon build` in CI to catch build breakage early
Keep moon dependencies updated and the toolchain version pinned
Clean _build when the moon version changes

When it happens

Trigger: build_task_cached (via run_task_cached/run_task_cached_output) shells out to moon build in the workspace and the compiler/build fails — syntax errors, missing deps, or target config problems.

Common situations: Broken MoonBit source after refactors; unresolved moon module dependencies; mismatched moon toolchain version; stale _build directory interfering.

Related errors

AI-assisted analysis of nikivdev/code@a747e741ae (2026-09-01). Data as JSON: /api/errors/9d8c1c2f539b41c0. Report an issue: GitHub.