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0030. Extensibility and safety: declared types, data plugins, limits

Status: accepted (extensibility-and-safety change, 11 October 2026)

Context

Section titled “Context”

The proposal asks new diagram types to register their schema, validation, layout, editing tools and conversion to primitives; diagrams never run code by being opened; executable plugins are explicitly installed extensions with permissions and identifiable versions; plugins and resource packs keep provenance and version; extensions declare compatibility with the versions of the intermediate model and the renderer. Imported SVG is sanitised, paths are confined to the project, network references are off by default, there are size and complexity limits for images, fonts, imports and renders, and errors never leak secrets — protecting both the local editor and automated jobs that process external repositories.

Decision

Section titled “Decision”
  • A plugin is data. A folder with meltemi-plugin.json declares diagram types and an icon pack. Nothing a project installs is executed; fields that would name code are ignored with a warning. Executable extension is the business of applications that embed the libraries: they register types with the same format (readDiagramType, compile(text, { types })) and decide what code they import themselves.
  • Explicit installation. project.json plugins names each plugin’s folder (inside the project), exact version and the permissions granted (diagram-types, icons; network does not exist). A plugin contributes what it asks for and is granted, nothing else. The lock records version, folder and the manifest’s SHA-256; changes are reported until the lock is written.
  • Declared compatibility. Manifests declare semver ranges for the model API (1.0.0) and the renderer API (1.0.0); others are refused.
  • Declared types on the free profile. A type’s kinds compile as boxes carrying typeKind and data, with the kind’s style applied first (before rules, classes and styles), rules by kind, required properties and allowed relations checked, its layout defaults (mode, layered or tree algorithm, direction), and its palette in the canvas. Every free feature keeps working, so stories, exports, catalogs, reviews and assistance need nothing new.
  • Limits. Sources (2 Mi characters), diagrams (5,000 elements, 10,000 relations or events), projects (2,000 sources), manifests (1 Mi characters), on top of the asset limits of decision 0014, as limit.* diagnostics.
  • Errors without secrets. The command line redacts secret-looking environment values from unexpected errors and prints no stack unless asked; assistance redacts keys (decision 0029).
  • One page states it. docs/security.md lists every guarantee and limit.

Alternatives considered

Section titled “Alternatives considered”
  • Executable plugins in a sandbox. Node’s permission model limits files and processes but not the network in the supported versions, and in-process sandboxes are not security boundaries; executing project-provided code would break “diagrams never run code”.
  • A type per profile in code. Each new type would need a release; declared types cover the common cases (kinds, shapes, properties, rules, layouts) as data.
  • Silent caps. Truncating without a diagnostic hides data; every limit is reported.

Consequences

Section titled “Consequences”
  • Types beyond kinds-and-relations (new layouts, new primitives) need a change to the product.
  • The model JSON gains typeKind, data and declaredType.
  • Untrusted repositories can be validated and exported in CI without running anything from them.