The ontology feature

The ontology surface is a per-project choice. A single flag gates the two ontology tools and the ontology resource. The default is enabled (ADR-0012).

The flag

Set features.ontology in .agent/config/rules.yml:

features:
  ontology: false

The runtime resolves the flag once at startup. An absent .agent/, an absent config/, or an absent rules.yml all resolve to enabled. A present file with no features block or no ontology key resolves to enabled. A present-but-broken file returns a typed error that names the path.

The enabled surface

When the feature is enabled, the runtime serves:

• truenorth_generate_ontology: seed .agent/ontology.yml from sources.
• truenorth_verify_ontology: scan changed code against the ontology and reject a prohibited alias or a constraint violation, citing the constraint id.
• The truenorth://ontology resource, backed by .agent/ontology.yml.

The scan baseline is a language-agnostic regex scan, with an optional tree-sitter AST plug-in for a supported language (ADR-2).

The disabled surface

When the feature is disabled:

• Neither ontology tool is registered, so neither is advertised or callable.
• truenorth://ontology is not listed, and a read of its URI is an unknown resource.
• No .agent/ontology.yml file is seeded.

Every other tool and resource is unchanged. To confirm the disabled surface, list the tools and the resources from your client: no tool name contains ontology, and the resource list omits truenorth://ontology.

The backing path

The ontology tools and the resource read and write one backing path, .agent/ontology.yml, through the write guard. A legacy specs/ontology.yaml is read as a fallback when the primary file is absent, but a write never mutates the legacy file (ADR-0013).