BoundaryML/baml · error

cannot convert value

Error message

cannot convert value {value} to VM value: class '{name}' has no field '{field_name}'

What it means

When materializing a BamlValue::Class into a VM value, the runtime iterates the VM class's declared field names and requires each one to be present in the value's field map. This error means the value is missing a field that the class declares.

Solutions

1. Populate every declared field of the class in the value's field map (use null for optional fields)
2. Regenerate the client after changing the .baml class so field lists match
3. Compare the field map keys against the class definition in the .baml file

Example fix

// before (Rust)
BamlValue::Class("User".into(), vec![("name".into(), BamlValue::String("a".into()))].into_iter().collect())
// after (Rust)
BamlValue::Class("User".into(), vec![("name".into(), BamlValue::String("a".into())), ("age".into(), BamlValue::Int(0))].into_iter().collect())

// check all declared fields present before constructing value
for f in class.field_names { assert!(fields.contains_key(f), "missing field {f}") }

fn has_all_fields(fields: &HashMap<String, BamlValue>, required: &[&str]) -> bool { required.iter().all(|f| fields.contains_key(*f)) }

Err(e) if e.to_string().contains("has no field") => { /* add missing field or regenerate client */ }

When it happens

Trigger: Constructing a BamlValue::Class (e.g. from code that builds function arguments or results) with a field map that lacks one of the class's declared fields, then calling a function with it.

Common situations: Manually building class values in Rust/Python/TS integrations and forgetting a newly added field after editing the .baml class; optional/nullable handling changes; partial deserialization dropping fields.

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

• cannot convert value
• Class already exists
• Class not found
• Could not unify Class
• Unknown class

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/0d31b024bb9026f3. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-runtime/src/async_vm_runtime.rs:1292

            Ok(vm.alloc_map(vm_map))
        }

        BamlValue::Class(name, fields) => {
            let Some(class_index) = resolved_class_names.get(name) else {
                anyhow::bail!("cannot convert value {value} to VM value: class '{name}' not found");
            };

            let baml_vm::Object::Class(class) = &vm.objects[*class_index] else {
                anyhow::bail!(
                    "internal error: cannot convert value {value} to VM value: class '{name}' not found in VM objects"
                );
            };

            let mut ordered_field_values = Vec::new();
            for field_name in &class.field_names {
                let Some(value) = fields.get(field_name) else {
                    anyhow::bail!(
                        "cannot convert value {value} to VM value: class '{name}' has no field '{field_name}'"
                    );
                };

                ordered_field_values.push(value);
            }

            let mut vm_fields_layout = Vec::new();
            for v in ordered_field_values {
                vm_fields_layout.push(try_vm_value_from_baml_value(
                    vm,
                    resolved_class_names,
                    resolved_enums_names,
                    v,
                )?);
            }

            Ok(vm.alloc_instance(*class_index, vm_fields_layout))

View on GitHub (pinned to bd85ce9dee)