Design

ALTO Front Matter favors predictable document metadata over complete YAML compatibility.

Guarantees

• A single-pass byte cursor performs parsing without a token stream or AST.
• Unsupported YAML is rejected rather than silently reinterpreted.
• Scalar typing is deterministic and independent of locale or configuration.
• The first grammar violation reports a line and column.
• Source offsets refer to the original input bytes.
• The package has no runtime Composer dependencies.

Rejecting aliases also prevents alias-expansion attacks by construction. The decoder never resolves anchors, aliases, or merge keys.

Limitations

• TOML front matter is detected but not decoded.
• The decoder intentionally implements a subset rather than the complete YAML specification.
• Metadata access is limited to top-level keys and explicit nested arrays.
• The body remains owned by the caller and is not stored in Metadata.
• Parsing stops at the first error and has no lenient mode.

Read Decoding for the exact syntax boundary and Integration for the replaceable contracts.