BoundaryML/baml · error

Unsupported {} `{}`: {}

Error message

Unsupported {} `{}`: {}

What it means

The error_unsupported! macro reports a construct the compiler/IR does not support, formatted 'Unsupported TYPE `NAME`: REASON'. Unlike not-found errors, the entity exists but some feature or shape of it is rejected (e.g. an unsupported type usage, feature, or expression).

Source

Thrown at engine/baml-lib/baml-core/src/ir/ir_helpers/error_utils.rs:84

                )
            }
            _ => {
                let suggestions = suggestions.join(", ");
                anyhow::bail!(
                    "{} `{}` not found. Did you mean one of: {}?",
                    $type,
                    $name,
                    suggestions
                )
            }
        }
    }};
}

#[macro_export]
macro_rules! error_unsupported {
    ($type:expr, $name:expr, $reason:expr) => {
        anyhow::bail!("Unsupported {} `{}`: {}", $type, $name, $reason)
    };
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Read the REASON in the message and replace the unsupported construct with a supported equivalent
2. Upgrade the BAML compiler/generator to a version supporting the feature
3. Simplify the type expression (e.g. replace exotic generics with a named class)
4. Check release notes for the feature's availability

Example fix

// before
function F() -> map<string, list<map<string, int>>>
// after
class Nested { counts map<string, int> }
function F() -> map<string, Nested>

match result {
  Err(e) if e.to_string().starts_with("Unsupported") => {
    // surface the reason to the user; do not retry
    report_unsupported_construct(&e);
  }
  other => other?,
}

When it happens

Trigger: Calling error_unsupported!("type", name, reason) during IR resolution when a declared construct uses an unsupported feature — e.g. unsupported generic usage or a type expression the backend cannot represent.

Common situations: Using a newer/experimental .baml language feature with an older compiler; attempting type constructs not implemented for a target language's client codegen; copying patterns from docs targeting a different BAML version.

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

• unsupported: {message}
• {} `{}` not found.
• Unsupported strategy provider: {}
• Unsupported strategy provider: {}. Available ones are: {}
• Completion prompts are not supported by this provider

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/f7fbf4619ea67b7c. Report an issue: GitHub.