BoundaryML/baml · error

cannot derive an output name from `{}`; pass `-o <PATH>` to

Error message

cannot derive an output name from `{}`; pass `-o <PATH>` to name the output.

What it means

resolve_output_basename derives the output file name for the packed binary. For single-file `--file` packs it uses the file stem; if the path has no usable UTF-8 stem (e.g. a nameless/odd path), and no `-o <PATH>` was supplied, the CLI cannot name the output and bails, telling you to pass `-o` explicitly.

Source

Thrown at baml_language/crates/baml_cli/src/pack_command.rs:433

    ///   `foo`). `baml.toml` isn't consulted — single-file packs are
    ///   intentionally hermetic.
    /// - Project mode: `[package].name` from `<from>/baml.toml` when a
    ///   manifest is present, else the project directory name for a
    ///   manifest-less `baml_src/` project (see
    ///   [`crate::project_load::resolve_project_name`]).
    fn resolve_output_basename(&self) -> Result<String> {
        if let Some(file) = self.file.as_deref() {
            // Keep `--file` hermetic: derive the name from the file path
            // alone, never from `--from`/project markers. A path with no
            // usable file-name component (no stem, e.g. `..`, or a non-UTF-8
            // name) can't be named automatically — bail rather than leak the
            // cwd's project context into a single-file pack.
            return file
                .file_stem()
                .and_then(|s| s.to_str())
                .map(str::to_string)
                .ok_or_else(|| {
                    anyhow::anyhow!(
                        "cannot derive an output name from `{}`; pass `-o <PATH>` to name the output.",
                        file.display()
                    )
                });
        }
        crate::project_load::resolve_project_name(self.from.as_deref())
    }
}

/// Resolve a single function-name string against the engine; returns
/// canonical qualified/display/subcommand-name triple.
fn resolve_one(engine: &BexEngine, func: &str) -> Result<ResolvedPackTarget> {
    if !engine.function_exists(func) {
        let suggestions = function_suggestions(engine, func);
        if suggestions.is_empty() {
            anyhow::bail!(
                "function `{func}` not found. Use `baml run --list` to see \
                 available targets."

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Pass `-o <PATH>` to name the output explicitly, e.g. `baml pack --file ./weird\ name.baml -o out`.
2. Rename the source file so it has a normal UTF-8 stem, then re-run without `-o`.
3. For project packs, ensure the project root resolves to a valid project name (resolve_project_name) or supply `-o`.

Example fix

// before
baml pack --file "$(printf 'weird\xff.baml')"

// after
baml pack --file ./my_func.baml -o my_func_bin

# always supply -o when the input path is not a simple UTF-8 name
case "$FILE" in
  *[![:print:]]*|"") echo "use -o to name output" >&2; OUT=(-o packed_bin) ;;
  *) OUT=() ;;
esac
baml pack --file "$FILE" "${OUT[@]}"

When it happens

Trigger: Running `baml pack --file <path>` where `<path>` yields no valid file_stem (non-UTF-8 or empty stem) without specifying `-o`.

Common situations: Programmatically constructed paths with unusual characters or non-UTF-8 bytes; piping a path from another tool on systems with odd filenames; forgetting `-o` on scripted pack invocations.

Understand the failure class

Background: "--flag is required" and "must specify" CLI errors: how missing-required-flag validation works and how to fix it — this error's family across 20 libraries.

Related errors

• positional `<TARGET>` and `-f/--function` are mutually exclu
• no target specified. Pass a positional `<TARGET>` to pack on
• compilation failed: {e:?}
• no `.baml` files found in {}
• two targets share subcommand name `{}` (`{}` and `{}`). Subc

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/506745b195e8440a. Report an issue: GitHub.