What causes "Invalid weight format: '{}'. Expected 'name=weight'" in BoundaryML/baml?

Write each objective as `name=weight` with exactly one `=`, e.g. `--weights accuracy=0.6,latency=0.4`.

Invalid weight format

Error message

Invalid weight format: '{}'. Expected 'name=weight'

What it means

The Pareto optimizer's `parse_weight_args` parses objective weight arguments of the form `name=weight`. Each argument (and each comma-separated part) must contain exactly one `=`; anything else (no `=`, multiple `=`, or an empty string) is rejected with this message.

Solutions

Write each objective as `name=weight` with exactly one `=`, e.g. `--weights accuracy=0.6,latency=0.4`.
Remove extra `=` characters and empty comma segments; quote the argument so the shell preserves it.
Ensure the weight value itself is a parseable f64 (see the related 'Invalid weight value' error).

Example fix

// before
baml optimize --weights accuracy 0.5 latency
// after
baml optimize --weights "accuracy=0.5,latency=0.5"

Defensive patterns

Strategy: validation

Validate before calling

// Shell pre-check for each weight argument
for w in $WEIGHTS; do
  case "$w" in
    *=*) n="${w%%=*}"; v="${w##*=}";
      [[ "$n" == *"="* || -z "$n" || -z "$v" ]] && { echo "bad weight: $w"; exit 1; } ;;
    *) echo "bad weight (needs name=weight): $w"; exit 1 ;;
  esac
done

Prevention

Always quote --weights arguments so `=` and commas survive shell parsing.
Keep a canonical example (`accuracy=0.6,latency=0.4`) in your project docs/scripts.
Avoid trailing commas in the weights list.

When it happens

Trigger: Passing a weight argument to the optimizer CLI without exactly one `=`, e.g. `--weights accuracy` or `--weights accuracy=0.5=extra`, or an empty segment from a trailing comma like `accuracy=1,`.

Common situations: Typing `--weights "accuracy 0.5"` with a space instead of `=`; quoting mistakes in the shell dropping the `=`; copying objective names from docs without the weight portion.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/60acccf8adf0c37a. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-runtime/src/optimize/pareto.rs:401

/// Parse weight arguments from CLI (e.g., "accuracy=0.8,tokens=0.2")
pub fn parse_weight_args(weight_args: &[String]) -> Result<Vec<Objective>> {
    if weight_args.is_empty() {
        // Default: optimize accuracy only
        return Ok(vec![Objective {
            name: "accuracy".to_string(),
            weight: 1.0,
            direction: Direction::Maximize,
        }]);
    }

    let mut objectives = Vec::new();

    for arg in weight_args {
        for part in arg.split(',') {
            let parts: Vec<&str> = part.split('=').collect();
            if parts.len() != 2 {
                anyhow::bail!("Invalid weight format: '{}'. Expected 'name=weight'", part);
            }

            let name = parts[0].trim();
            let weight: f64 = parts[1]
                .trim()
                .parse()
                .with_context(|| format!("Invalid weight value: '{}'", parts[1]))?;

            let direction = match name {
                "accuracy" => Direction::Maximize,
                "tokens" | "prompt_tokens" | "completion_tokens" | "latency" => Direction::Minimize,
                name if name.starts_with("check:") => Direction::Maximize,
                _ => anyhow::bail!(
                    "Unknown objective: '{}'. Valid objectives: accuracy, tokens, prompt_tokens, completion_tokens, latency, check:<name>",
                    name
                ),
            };

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