BoundaryML/baml · error

unexpected type for bool type: %T

Error message

unexpected type for bool type: %T

What it means

typeBuilder.Bool() calls the FFI method "bool" and asserts the result implements the baml Type interface. When the bridge returns a value that is not a Type implementation, the assertion fails with this error. This is an internal decode/wrapping failure between the native BAML runtime and the Go bindings.

Solutions

1. Check %T in the error to identify what the bridge returned.
2. Update the baml Go module to match your native BAML runtime version.
3. Regenerate wrapper code if using a from-source build.
4. Escalate to boundaryml/baml with the printed type if versions are aligned.

Example fix

// before
boolTyp, err := tb.Bool()
if err != nil {
    return err
}

// after
boolTyp, err := tb.Bool()
if err != nil {
    return fmt.Errorf("bool type build failed (verify baml Go bindings match native runtime): %w", err)
}

if tb == nil {
    return errors.New("type builder is nil")
}

func isBamlType(v any) bool { _, ok := v.(baml.Type); return ok }

typ, err := tb.Bool()
if err != nil {
    log.Printf("Bool() decode failure: %v", err)
    return fmt.Errorf("bool type build failed: %w", err)
}

When it happens

Trigger: Calling TypeBuilder.Bool() when the underlying "bool" method returns an unwrapped raw object or nil instead of a Type.

Common situations: Go bindings out of sync with the native runtime; stale or custom-generated cffi wrapper registry; TypeBuilder created from an incompatible runtime instance.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• unexpected type for float type: %T
• unexpected type for int type: %T
• unexpected type for string type: %T
• encoding type builder
• unexpected type for as_base64: %T

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/a510960c871cbbc9. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/pkg/rawobjects_type_builder.go:75

	}

	typ, ok := result.(Type)
	if !ok {
		return nil, fmt.Errorf("unexpected type for float type: %T", result)
	}

	return typ, nil
}

func (tb *typeBuilder) Bool() (Type, error) {
	result, err := raw_objects.CallMethod(tb, "bool", nil)
	if err != nil {
		return nil, err
	}

	typ, ok := result.(Type)
	if !ok {
		return nil, fmt.Errorf("unexpected type for bool type: %T", result)
	}

	return typ, nil
}

func (tb *typeBuilder) Null() (Type, error) {
	result, err := raw_objects.CallMethod(tb, "null", nil)
	if err != nil {
		return nil, err
	}

	typ, ok := result.(Type)
	if !ok {
		return nil, fmt.Errorf("unexpected type for null type: %T", result)
	}

	return typ, nil
}

View on GitHub (pinned to bd85ce9dee)