What causes "`baml pack` failed for fixture {}" in BoundaryML/baml?

Read the baml CLI's stderr output above this message for the root cause

`baml pack` failed for fixture {}

Error message

`baml pack` failed for fixture {}

What it means

After spawning `<cli> pack -i <fixture> -o <output>`, a non-zero exit status bails with this message naming the fixture. The actual pack errors are printed by the baml CLI on stderr before this bail is raised.

Source

Thrown at baml_language/crates/tools_size_gate/src/measure.rs:148

    let output = pack_output_path(workspace_root, name);
    if let Some(parent) = output.parent() {
        std::fs::create_dir_all(parent)
            .with_context(|| format!("failed to create {}", parent.display()))?;
    }

    eprintln!("  packing {} -> {}", pack.function, output.display());
    let status = Command::new(&cli_path)
        .arg("pack")
        .arg("--file")
        .arg(&fixture)
        .arg(&pack.function)
        .arg("-o")
        .arg(&output)
        .current_dir(workspace_root)
        .status()
        .with_context(|| format!("failed to run {} pack", cli_path.display()))?;
    if !status.success() {
        bail!("`baml pack` failed for fixture {}", fixture.display());
    }

    if !output.exists() {
        bail!("pack produced no output at {}", output.display());
    }
    Ok(output)
}

/// Deterministic output path for a packed artifact.
fn pack_output_path(workspace_root: &Path, name: &str) -> PathBuf {
    workspace_root
        .join("target/size-gate")
        .join(exe_filename(name))
}

/// Find the built artifact in the target directory (public for --no-build mode).
pub(crate) fn locate_artifact_public(
    workspace_root: &Path,

View on GitHub (pinned to bd85ce9dee)

Solutions

Read the baml CLI's stderr output above this message for the root cause
Run the same pack command manually: <cli> pack -i <fixture> -o <output>
Fix the fixture's BAML source or any missing generator configuration
Ensure required env vars/network for codegen are available in CI

Example fix

// before: fixture references missing env
// runtime "python" env { OPENAI_API_KEY }
// after: provide it in the measure environment
OPENAI_API_KEY=sk-... size-gate measure ...

Defensive patterns

Strategy: try-catch

Validate before calling

// dry-run the pack command before measuring
let status = std::process::Command::new(&cli_path)
    .args(["pack", "-i", &fixture, "-o", "/tmp/probe-output"])
    .envs(required_codegen_env())
    .status()?;
if !status.success() { eprintln!("pack fails on fixture; see CLI stderr"); std::process::exit(1); }

Try / catch

if let Err(e) = run_baml_pack(ws, name, pack) {
    if e.to_string().contains("baml pack` failed") {
        eprintln!("inspect the baml CLI stderr above; often missing env vars or invalid BAML in the fixture");
    }
    return Err(e);
}

Prevention

Provide all env vars the fixture's generators need in the measure environment
Validate fixture BAML files (baml CLI check) before the size run
Pin the baml CLI version used by the gate
Test pack locally on the same fixture after BAML schema changes

When it happens

Trigger: The baml CLI's pack command fails on the fixture: invalid BAML source in the fixture, missing referenced generators/clients, or CLI runtime panics during packing.

Common situations: Fixture references a generator requiring env vars not set; BAML schema errors after a language change; incompatible fixture format after a CLI version bump; pack requires network for codegen and CI blocks it.

Understand the failure class

Background: "API request failed": what wrapped HTTP errors from external APIs mean and how to find the real cause — this error's family across 29 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/3d81fe728b1c1e61. Report an issue: GitHub.