BoundaryML/baml · error

undefined field: {class_name}.{field}

Error message

undefined field: {class_name}.{field}

What it means

This is a Rust panic raised during bytecode generation in the BAML compiler's codegen pass. When compiling an assignment to an instance field (obj.field = value), the compiler resolved the base's class type but the field name was not found in the class's resolved field map. It means the field name in your BAML source does not exist on the declared class.

Source

Thrown at engine/baml-compiler/src/codegen.rs:604

            thir::Statement::Assign { left, value, .. } => {
                match left {
                    thir::Expr::Var(name, _) => {
                        self.compile_expression_with_block_behavior(value, true);
                        self.emit(Instruction::StoreVar(self.locals[name]));
                    }
                    thir::Expr::FieldAccess { base, field, meta: _ } => {
                        // Get class name from type metadata
                        let class_name = match base.meta().1.as_ref() {
                            Some(TypeIR::Class { name, .. }) => name,
                            _ => panic!("Field access on non-class type"),
                        };

                        // Resolve field index
                        let Some(resolved_fields) = self.classes.get(class_name) else {
                            panic!("undefined class: {class_name}");
                        };
                        let Some(&field_index) = resolved_fields.get(field) else {
                            panic!("undefined field: {class_name}.{field}");
                        };

                        // Generate bytecode: load base, load value, store field
                        self.compile_expression(base);
                        self.compile_expression_with_block_behavior(value, true);
                        self.emit(Instruction::StoreField(field_index));
                    }
                    thir::Expr::ArrayAccess {base, index, meta: _} => {

                        self.compile_expression(base);
                        self.compile_expression(index);
                        self.compile_expression_with_block_behavior(value, true);

                        self.emit(match base.meta().1.as_ref().expect("must have a resolved type") {
                            TypeIR::List(_, _) => Instruction::StoreArrayElement,
                            TypeIR::Map(_, _, _) => Instruction::StoreMapElement,
                            _ => panic!("array access should be either map or array.")
                        });

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Fix the field name in the BAML source to match the class definition
2. Check the class declaration for the exact field spelling/casing
3. Regenerate the IR if the class was recently edited so type metadata is fresh

Example fix

// before
obj.nmae = "x"
// after
obj.name = "x"

fn validate_field(class_fields: &[&str], class_name: &str, field: &str) -> Result<(), String> {
    if !class_fields.contains(&field) {
        return Err(format!("field '{}' does not exist on class '{}'", field, class_name));
    }
    Ok(())
}

When it happens

Trigger: Compiling a BAML statement like `obj.field = value` where `field` is not a member of the class of `obj`; misspelled field name; assigning a field on the wrong class instance.

Common situations: Typos in BAML source; renaming a class field without updating assignments; generated code drift after class schema edits.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• array access should be either map or array.
• undefined function: {name}
• watch codegen error: undefined variable: {variable}
• sys_op callee must resolve to a statically-known global func
• expected jump instruction at index {instruction_idx}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/c937da922a3ea076. Report an issue: GitHub.