BoundaryML/baml · error · RenderError

Class '{0}' not found

Error message

Class '{0}' not found

What it means

RenderError::ClassNotFound is raised by the sys_ops output_format renderer when a class referenced during output rendering cannot be found in the renderer's type registry. The '{0}' placeholder is the class name. Like EnumNotFound, it indicates a schema lookup failure: the class definition is missing from the loaded types.

Source

Thrown at baml_language/crates/sys_ops/src/output_format.rs:13

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,

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Ensure the .baml file defining the class is loaded into the renderer registry
2. Regenerate client code after class renames or deletions
3. Correct the class name in the referencing code/template
4. Verify the renderer is initialized with the same project sources as the runtime

Example fix

// before
render(result, registryMissing(User))  // ClassNotFound('User')
// after
const registry = build_registry([user_baml, ...other_sources]);
render(result, registry)

function classIsRenderable(registry, name) {
  return registry.classes.some(c => c.name === name);
}
// call before render: classIsRenderable(registry, 'User')

function isClassNotFoundError(e) {
  return e instanceof Error && /Class '.*' not found/.test(e.message);
}

try {
  rendered = render(value, registry);
} catch (e) {
  if (isClassNotFoundError(e)) {
    rendered = { __missing_class__: e.message.match(/Class '(.*)' not found/)[1] };
  } else throw e;
}

When it happens

Trigger: Rendering output whose type references a class absent from the loaded BAML sources, or generated code that references a class deleted/renamed since generation.

Common situations: Schema files not fully loaded into the renderer, drift between generated client code and current .baml sources, or misspelled class names in dynamic output schemas.

Related errors

• Enum '{0}' not found
• Type '{0}' is not supported in outputs
• Class {name} not found
• Class {} does not exist
• Class {} not found

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/23b2effefae793f3. Report an issue: GitHub.