BoundaryML/baml · error · RenderError

Type '{0}' is not supported in outputs

Error message

Type '{0}' is not supported in outputs

What it means

RenderError::UnsupportedType is raised when the output_format renderer encounters a type it cannot represent in rendered outputs. The '{0}' placeholder is the type's display string. The renderer supports a fixed set of output types; anything else (e.g. certain generics, images, or internal-only types) is rejected with this error.

Source

Thrown at baml_language/crates/sys_ops/src/output_format.rs:15

use std::fmt::Write as _;

use ::sys_types::SapTy;
use baml_base::Literal as LiteralValue;
use indexmap::IndexMap;
use thiserror::Error;

/// Error type for output format rendering.
#[derive(Clone, Debug, Error)]
pub enum RenderError {
    #[error("Enum '{0}' not found")]
    EnumNotFound(String),
    #[error("Class '{0}' not found")]
    ClassNotFound(String),
    #[error("Type '{0}' is not supported in outputs")]
    UnsupportedType(String),
    #[error(
        "Non-regular recursive generic class '{class}' expands from '{ancestor}' to '{instantiation}'"
    )]
    NonRegularRecursiveGeneric {
        class: String,
        ancestor: String,
        instantiation: String,
    },
    #[error(
        "Output definitions '{first}' and '{second}' both render as '{rendered_name}' in the output schema"
    )]
    RenderedClassNameCollision {
        rendered_name: String,
        first: String,
        second: String,
    },
    #[error(

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Change the output schema field to a supported type (primitive, supported class/enum, list/map)
2. Upgrade the library if the type should be supported in a newer version
3. Strip or transform unsupported fields before rendering
4. Check release notes for the supported output type matrix

Example fix

// before
class Result { image Image }
// after
class Result { image_url string }

const SUPPORTED = new Set(['string','int','float','bool','list','map','class','enum']);
function allFieldsSupported(schema) {
  return schema.fields.every(f => SUPPORTED.has(f.type.kind));
}
// call before render: allFieldsSupported(outputSchema)

function isUnsupportedTypeError(e) {
  return e instanceof Error && /Type '.*' is not supported in outputs/.test(e.message);
}

try {
  rendered = render(value, registry);
} catch (e) {
  if (isUnsupportedTypeError(e)) {
    rendered = stripUnsupported(value); // drop/serialize unrepresentable fields
  } else throw e;
}

When it happens

Trigger: Declaring an output schema field whose type is outside the renderer's supported set, or piping runtime values of unsupported types into output rendering.

Common situations: Using exotic BAML types (unsupported generics, media/internal types) as class fields for output, or newer type kinds rendered by an older renderer version that predates support.

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

• Enum '{0}' not found
• Class '{0}' not found
• Tuple types are not supported in CFFI
• Arrow types are not supported in CFFI
• `{}` is not a BAML source file. Use a `.baml` file with `--f

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/c93db23595836569. Report an issue: GitHub.