BoundaryML/baml · error · ManifestError

{path}: `[package].prelude` is reserved for the standard lib

Error message

{path}: `[package].prelude` is reserved for the standard library's own manifests. Remove it.

What it means

The BAML manifest parser rejects `[package].prelude` tables in project manifests. This key is reserved exclusively for the standard library's own manifests, so its presence in a user project manifest is a hard parse error directing the author to remove it.

Source

Thrown at baml_language/crates/baml_db/src/manifest.rs:196

}

/// Why a manifest's `[package].name` could not be resolved.
#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
pub enum ManifestError {
    #[error(
        "{path}: missing `[package]` table.\nAdd:\n\n    [package]\n    name = \"<your-project-name>\"\n"
    )]
    MissingPackageTable { path: std::path::PathBuf },
    #[error("{path}: `[package]` is missing `name = \"<your-project-name>\"`.")]
    MissingPackageName { path: std::path::PathBuf },
    #[error("{path}: `[package].name` cannot be empty.")]
    EmptyPackageName { path: std::path::PathBuf },
    #[error(
        "{path}: `[dependencies]` is not supported in a project manifest yet: package imports are \
         pending a design pass. Remove the table; a project reaches the standard library implicitly."
    )]
    DependenciesUnsupported { path: std::path::PathBuf },
    #[error(
        "{path}: `[package].prelude` is reserved for the standard library's own manifests. Remove it."
    )]
    PreludeUnsupported { path: std::path::PathBuf },
}

/// Refuse the manifest tables only the standard library's own manifests may
/// carry. Package imports are not user surface area until they get a design
/// pass, so a project manifest with `[dependencies]` (or the stdlib-only
/// `[package].prelude`) is an error, not a silently ignored table.
pub fn reject_stdlib_only_tables(
    manifest: &BamlToml,
    toml_path: &std::path::Path,
) -> Result<(), ManifestError> {
    if !manifest.dependencies.is_empty() {
        return Err(ManifestError::DependenciesUnsupported {
            path: toml_path.to_path_buf(),
        });
    }

View on GitHub (pinned to bd85ce9dee)

Solutions

Remove the `[package].prelude` table (or just the `prelude` key) from the project manifest.
If prelude behavior is desired, rely on the standard library, which project manifests reach implicitly.
Check the manifest against current BAML manifest documentation for supported `[package]` keys.

Example fix

// before
[package]
name = "my-project"
prelude = ["std/prelude"]

// after
[package]
name = "my-project"

Defensive patterns

Strategy: validation

Validate before calling

let manifest: toml::Value = toml::from_str(&manifest_src)?;
if manifest.get("package").and_then(|p| p.get("prelude")).is_some() {
    return Err("[package].prelude is reserved; remove it from the project manifest".into());
}

Type guard

fn has_reserved_prelude(manifest: &toml::Value) -> bool {
    manifest.get("package").and_then(|p| p.get("prelude")).is_some()
}

Prevention

Only use documented `[package]` keys in project manifests
Never copy tables from stdlib manifests into project manifests
Lint manifests in CI against a schema that rejects `prelude`

When it happens

Trigger: Declaring a `prelude` key inside the `[package]` table of a project's manifest file; the manifest validator at baml_language/crates/baml_db/src/manifest.rs:196 rejects it while parsing.

Common situations: Copying manifest conventions from the standard library's own sources into a project manifest, guessing at manifest fields, or migrating an old manifest format that exposed prelude configuration.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/9e4a24409d60306b. Report an issue: GitHub.