BoundaryML/baml · error

Expected a boolean

Error message

Expected a boolean

What it means

resolve_bool resolves an UnresolvedValue at runtime and requires the result to be ResolvedValue::Bool; any other resolved variant (or a resolution failure such as an unset env var) produces this error. It enforces that the field truly evaluates to a boolean rather than coercing truthy values.

Source

Thrown at engine/baml-lib/baml-types/src/value_expr.rs:548

            Self::Map(..) => anyhow::bail!("Expected a string, not a map"),
            Self::Null(..) => anyhow::bail!("Expected a string, not null"),
            Self::ClassConstructor(..) => {
                anyhow::bail!("Expected a string, not a class constructor")
            }
        }
    }

    pub fn resolve_string(&self, ctx: &impl GetEnvVar) -> Result<String> {
        match self.resolve(ctx) {
            Ok(ResolvedValue::String(s, ..)) => Ok(s),
            _ => Err(anyhow::anyhow!("Expected a string")),
        }
    }

    pub fn resolve_bool(&self, ctx: &impl GetEnvVar) -> Result<bool> {
        match self.resolve(ctx) {
            Ok(ResolvedValue::Bool(b, ..)) => Ok(b),
            _ => Err(anyhow::anyhow!("Expected a boolean")),
        }
    }

    pub fn resolve_array(&self, ctx: &impl GetEnvVar) -> Result<Vec<ResolvedValue>> {
        match self.resolve(ctx) {
            Ok(ResolvedValue::Array(a, ..)) => Ok(a),
            _ => Err(anyhow::anyhow!("Expected an array")),
        }
    }

    pub fn resolve_map(&self, ctx: &impl GetEnvVar) -> Result<IndexMap<String, ResolvedValue>> {
        match self.resolve(ctx) {
            Ok(ResolvedValue::Map(m, ..)) => Ok(m.into_iter().map(|(k, (_, v))| (k, v)).collect()),
            _ => Err(anyhow::anyhow!("Expected a map")),
        }
    }

    pub fn resolve_numeric(&self, ctx: &impl GetEnvVar) -> Result<String> {

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Author the value as a bare boolean literal (true/false) in .baml, not "true" or 1
2. If driven by an env var, ensure its value is exactly true or false, or convert in code after resolve_string
3. Use resolve_string and parse manually when the source cannot be changed to a boolean type

Example fix

// before (baml)
options {
  stream "true"
}
// after (baml)
options {
  stream true
}

// Rust
match value.resolve(&ctx) {
    Ok(ResolvedValue::Bool(_)) => {},
    Ok(ResolvedValue::String(s, _)) => {
        anyhow::ensure!(s == "true" || s == "false", "field must be boolean, got string {s}");
    }
    _ => anyhow::bail!("field must resolve to a boolean"),
}

fn resolves_to_bool(v: &UnresolvedValue<()>, ctx: &EvaluationContext) -> bool {
    matches!(v.resolve(ctx), Ok(ResolvedValue::Bool(..)))
}

// Rust
let b = value.resolve_bool(&ctx)
    .with_context(|| format!("field '{name}' must be true/false (unquoted), not a string or number"))?;

When it happens

Trigger: Calling resolve_bool(ctx) where resolve(ctx) returns a non-Bool ResolvedValue — e.g. the BAML field is the string "true", a number 1, or an env::VAR whose value is not a boolean literal.

Common situations: Setting boolean options via env vars (env::ENABLE_X) where the env value is "1"/"yes" instead of BAML boolean literals true/false, or quoting booleans as strings by mistake.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Could not unify Float with {:?}
• Could not unify Bool with {:?}
• Unification error
• Expected a string, not an array
• Expected a string, not a bool

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/d513a0c735bd8905. Report an issue: GitHub.