cannot convert value

Error message

cannot convert value {value} to VM value: enum '{enm}' has no variant '{variant}'

What it means

Runtime conversion error while marshaling a BamlValue into a VM object: the value is an Enum whose variant name does not exist in the resolved enum definition known to the VM. This happens when the enum's variants changed between compile time and call time (stale bytecode vs. fresh types), or the variant name was never declared — the conversion refuses to invent a variant.

Solutions

Update the variant name to one declared in the .baml enum (exact match)
Regenerate the client after enum changes
Validate LLM-parsed output against the enum's variant list before constructing values

Example fix

// before (Rust)
BamlValue::Enum("Status".into(), "Completed".into())
// after (Rust)
BamlValue::Enum("Status".into(), "Complete".into()) // matches enum Status { Complete | Failed }

Defensive patterns

Strategy: validation

Validate before calling

assert enum_variants("Status").contains(&variant)

Type guard

fn is_valid_variant(enm: &BamlEnumDef, v: &str) -> bool { enm.variant_names.iter().any(|n| n == v) }

Try / catch

Err(e) if e.to_string().contains("has no variant") => { /* use a declared variant or update schema */ }

Prevention

Regenerate clients after variant changes
Validate LLM output variants against the enum before constructing values
Use exhaustive matching when mapping external strings to variants

When it happens

Trigger: Constructing a BamlValue::Enum with a variant name that is misspelled or no longer exists in the .baml enum definition, then passing it to a function.

Common situations: Removing/renaming an enum variant in .baml without regenerating clients; LLM output producing an invalid variant that was constructed into a value instead of rejected; programmatic value building with typos.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/830752250381397a. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-runtime/src/async_vm_runtime.rs:1325

                )?);
            }

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

        BamlValue::Enum(enm, variant) => {
            let Some(enum_index) = resolved_enums_names.get(enm) else {
                anyhow::bail!("cannot convert value {value} to VM value: enum '{enm}' not found");
            };

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

            let Some(variant_index) = enm.variant_names.iter().position(|v| v == variant) else {
                anyhow::bail!(
                    "cannot convert value {value} to VM value: enum '{enm}' has no variant '{variant}'"
                );
            };

            Ok(vm.alloc_variant(*enum_index, variant_index))
        }

        BamlValue::Media(media) => Ok(vm.alloc_media(media.clone())),
    }
}

fn try_vm_value_from_function_result(
    vm: &mut Vm,
    resolved_class_names: &HashMap<String, ObjectIndex>,
    resolved_enums_names: &HashMap<String, ObjectIndex>,
    result: anyhow::Result<FunctionResult>,
) -> anyhow::Result<baml_vm::Value> {
    let fn_result = result.context("failed to get function result")?;

View on GitHub (pinned to bd85ce9dee)