HUGR Core Metadata Specification

HUGR nodes can have associated metadata. This is described in the Extensible Metadata section of the specification.

This document describes metadata keys reserved by reference HUGR tooling, which are all prefixed with “core.”.

Model

The HUGR “model” serialization format has its own notion of node metadata, which it uses to encode some data that is considered core to the HUGR specification. It uses a special key to store HUGR metadata, so in some sense it is nested.

Here we list the model-level metadata keys reserved by HUGR tooling; the types refer to model format types.

Key	Model Type	Description
core.meta.description	string	Human-readable descriptions.
core.entrypoint	-	Marks the module entrypoint.
core.order_hint.key	nat	Unique per-node key within a dataflow region used for ordering.
core.order_hint.input_key	nat	Order hint key for the region’s input node.
core.order_hint.output_key	nat	Order hint key for the region’s output node.
core.order_hint.order	before: nat, after: nat	Specify ordering of node keys as (before, after).
core.title	string	Human-readable symbol title preserved across serialization.
compat.meta_json	name:string, json:string	JSON-encoded metadata for compatibility with hugr-core.

The remaining reserved keys are stored under the compat.meta_json key, for which the value types will be given as JSON types below.

Core

The following reserved HUGR metadata keys use JSON values.

Key	JSON Type	Description
core.generator	object { name: string, version: string, … }	Identifies the tooling that generated the module. Optional extra fields permitted.
core.used_extensions	array of object { name: string, version: string }	Names and versions of all HUGR extensions used in the module.