BoundaryML/baml · error

instanceof requires a class name on the right side at

Error message

instanceof requires a class name on the right side at {:?}

What it means

For an `instanceof` expression, the interpreter expects the right-hand side to be a bare variable expression (Expr::Var) naming a class. If the RHS is any other expression form (string literal, call, field access, etc.) it throws "instanceof requires a class name on the right side at {:?}".

Solutions

1. Use the unquoted class identifier directly: `x instanceof MyClass`
2. Remove any dynamic expression from the RHS; instanceof does not evaluate computed class names
3. If you need dynamic type checking, compare the class name extracted from the value against a string yourself
4. Check syntax — parentheses or property access around the class name are not allowed

Example fix

// before
if (v instanceof "User") { ... }
// after
if (v instanceof User) { ... }

// Validate instanceof RHS is a bare identifier before evaluation
function validInstanceofRhs(exprText: string): boolean {
  return /^[A-Za-z_][A-Za-z0-9_]*$/.test(exprText.trim());
}

try {
  return evalBaml(instanceofExpr);
} catch (e) {
  if (String(e).includes("instanceof requires a class name")) {
    throw new Error("instanceof RHS must be an unquoted class identifier, e.g. x instanceof MyClass");
  }
  throw e;
}

When it happens

Trigger: Writing `x instanceof <non-var>` — e.g. `x instanceof "MyClass"` (quoted name) or `x instanceof get_class()` — during instanceof handling in evaluate_expr_with_context.

Common situations: Quoting the class name out of habit from string-based type checks; computing the class name dynamically; copy-pasting patterns from languages where the RHS can be an arbitrary expression.

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

• instanceof requires a class instance on the left side at
• instanceof requires class operands at
• method name must be an identifier at
• {0:?}
• An element at was a node when it should have been a token.

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/5244a6eb91a23e89. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2155

            } => {
                // Special handling for instanceof: right operand is a type name, not a value
                if matches!(operator, crate::hir::BinaryOperator::InstanceOf) {
                    let left_val = expect_value(
                        evaluate_expr(
                            left,
                            scopes,
                            thir,
                            run_llm_function,
                            watch_handler,
                            function_name,
                        )
                        .await?,
                    )?;

                    // Extract class name from right side (should be Expr::Var)
                    let class_name = match right.as_ref() {
                        Expr::Var(name, _) => name.clone(),
                        _ => bail!(
                            "instanceof requires a class name on the right side at {:?}",
                            meta.0
                        ),
                    };

                    // Check if left value is a class instance matching the class name
                    let result = match left_val {
                        BamlValueWithMeta::Class(ref left_class, ..) => {
                            BamlValueWithMeta::Bool(left_class == &class_name, meta.clone())
                        }
                        _ => bail!(
                            "instanceof requires a class instance on the left side at {:?}",
                            meta.0
                        ),
                    };

                    EvalValue::Value(result)
                } else {

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