What causes "Tuple types are not supported in CFFI" in BoundaryML/baml?

Replace the tuple type in your BAML schema with a class or a list of a single type

Tuple types are not supported in CFFI

Error message

Tuple types are not supported in CFFI

What it means

The CFFI layer encodes BAML types for cross-language FFI. Tuple types (TypeGeneric::Tuple) cannot be represented in the C FFI type encoding, so encoding panics with this message. It is a hard unsupported-feature abort, not a recoverable error.

Source

Thrown at engine/language_client_cffi/src/ctypes/baml_type_encode.rs:82

            cType::CheckedType(Box::new(CffiFieldTypeChecked {
                value: Some(Box::new(
                    WithIr {
                        value,
                        lookup,
                        mode,
                        curr_type,
                    }
                    .encode(),
                )),
                checks,
            }))
        } else {
            match curr_type {
                TypeGeneric::Top(_) => panic!(
                    "TypeGeneric::Top should have been resolved by the compiler before code generation. \
                    This indicates a bug in the type resolution phase."
                ),
                TypeGeneric::Tuple(_, _) => panic!("Tuple types are not supported in CFFI"),
                TypeGeneric::Arrow(_, _) => panic!("Arrow types are not supported in CFFI"),
                TypeGeneric::Primitive(type_value, _) => type_value.encode(),
                TypeGeneric::Literal(literal_value, _) => cType::LiteralType(literal_value.encode()),
                TypeGeneric::Enum {
                    name,
                    dynamic: _,
                    meta: _,
                } => cType::EnumType(CffiFieldTypeEnum { name }),
                TypeGeneric::Class {
                    name,
                    mode,
                    dynamic: _,
                    meta: _,
                } => {
                    cType::ClassType(CffiFieldTypeClass {
                        name: Some(create_cffi_type_name(name, match mode {
                            baml_types::StreamingMode::NonStreaming => CffiTypeNamespace::Types,
                            baml_types::StreamingMode::Streaming => CffiTypeNamespace::StreamTypes,

View on GitHub (pinned to bd85ce9dee)

Solutions

Replace the tuple type in your BAML schema with a class or a list of a single type
Use a generic list (e.g. `string[]`) or a dedicated class with named fields instead of a tuple
Upgrade BAML — check whether newer versions added CFFI tuple support

Example fix

// before (schema.baml)
class Result { coords (int, int) }
// after
class Coords { x int
  y int }
class Result { coords Coords }

Defensive patterns

Strategy: validation

Validate before calling

# scan schema for tuple types before loading via CFFI
import re
schema = open('schema.baml').read()
if re.search(r'\([^)]*\bint|string|bool|float\b[^)]*\)', schema):
    raise ValueError('possible tuple type in BAML schema — unsupported in CFFI')

Prevention

Model structured data as classes, not tuples, in BAML schemas
Keep schema types limited to primitives, lists, maps, enums, classes
Upgrade BAML to check for new type support

When it happens

Trigger: A BAML schema (class field, function argument/return type) uses a tuple type (e.g. `(int, string)`) and is compiled/loaded through the language_client_cffi runtime.

Common situations: Defining tuple types in a BAML file for a client language bound via CFFI (e.g. Python ctypes build); copying schemas that used tuple types allowed in other backends.

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/acf79eda257cccdc. Report an issue: GitHub.