Expected a statically defined string, not expression

Error message

Expected a statically defined string, not expression

What it means

as_static_str only returns values that are literal strings (or numerics) written directly in BAML source. A Jinja expression (e.g. "{{ something }}") needs evaluation at runtime, so the static accessor rejects it. The library throws this because a static string is required where the value is used (e.g. names, enum tags, provider fields) and an expression cannot be known at parse time.

Solutions

Replace the Jinja expression with a static quoted string literal in the .baml file
Move the dynamic interpolation to a place where it is allowed (e.g. prompt bodies, not static identifiers)
In consuming Rust code, use resolve_string(&ctx) (with a context that can render the expression) instead of as_static_str()

Example fix

// before (baml)
enum Sentiment {
  "{{ Positive }}"
}
// after (baml)
enum Sentiment {
  Positive
}

Defensive patterns

Strategy: validation

Validate before calling

// Rust
if matches!(v, UnresolvedValue::String(StringOr::JinjaExpression(_), _)) {
    anyhow::bail!("Jinja expressions are not allowed in this static field");
}
let s = v.as_static_str()?;

Type guard

fn is_static_string<Meta>(v: &UnresolvedValue<Meta>) -> bool {
    matches!(v, UnresolvedValue::String(StringOr::Value(_), _))
}

Prevention

Use Jinja only inside prompt bodies, never in static identifiers/options
Validate baml_src early with a linter that flags expressions in scalar fields
Resolve expressions at runtime with resolve_string when dynamism is required

When it happens

Trigger: Calling as_static_str() on UnresolvedValue::String(StringOr::JinjaExpression(..)) — a BAML field whose value is a Jinja template expression like {{ ctx.field }} or {{ _.name }} instead of a quoted literal.

Common situations: Developers interpolate dynamic values into BAML fields that must be static (client names, enum variant values, metadata strings), or copy template-style config between fields, then a static validation/registration pass fails.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/b0adba2e1ffe4061. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-lib/baml-types/src/value_expr.rs:522

impl Default for EvaluationContext<'_> {
    fn default() -> Self {
        Self {
            env_vars: None,
            fill_missing_env_vars: true,
        }
    }
}

impl<Meta> UnresolvedValue<Meta> {
    pub fn as_static_str(&self) -> Result<&str> {
        match self {
            Self::String(StringOr::Value(v), ..) => Ok(v.as_str()),
            Self::String(StringOr::EnvVar(..), ..) => {
                anyhow::bail!("Expected a statically defined string, not env variable")
            }
            Self::String(StringOr::JinjaExpression(..), ..) => {
                anyhow::bail!("Expected a statically defined string, not expression")
            }
            Self::String(StringOr::TemplateStringCall { .. }, ..) => {
                anyhow::bail!("Expected a statically defined string, not a template_string call")
            }
            Self::Numeric(num, ..) => Ok(num.as_str()),
            Self::Array(..) => anyhow::bail!("Expected a string, not an array"),
            Self::Bool(..) => anyhow::bail!("Expected a string, not a bool"),
            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),

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