Hey API migration to the shared runtime

This is the 3.0.0-alpha.2 major-version migration, published on npm's next channel. Existing v2 packages and the latest tag are unchanged. Newly generated builders import @mimlet/core; install @mimlet/core@0.1.0-alpha.2 alongside hey-api-builders@3.0.0-alpha.2 and regenerate clients together.

The plugin still discovers Hey API model/request/response factories, applies existing naming settings, resolves symbol collisions, and exposes the same named builder classes and with-property helpers. It no longer emits an independent merge implementation. Every class delegates to the core class facade/runtime. Factory options and generated model names remain unchanged.

Behavioral changes

Object-union transitions require a complete replace() rather than a partial patch of a discriminant. Record/non-record transitions also require explicit replacement. Dates, maps and similar native values are never object-spread. Lists have the core's default allocation budget of 10,000 values. Split very large fixture collections into explicitly bounded batches or use a separately configured core builder.

The familiar with, transform, constructor patches, build and buildList remain available for ordinary object models. Generated methods now retain the polymorphic subclass type. Core withFactory, replaceFactory, omit, async transforms/builds and operation descriptions are also available. All patches run before transforms. Async chains retain generated helpers but not sync build capabilities in TypeScript.

runtimeModule overrides the import specifier for controlled packaging or a self-contained runtime module produced from the canonical core. It is not a switch back to the legacy implementation. No file is fetched or executed from this configuration during emission.

Verification

The original real-generator fixtures still compile and execute for Swagger 2, OpenAPI 3.0, and OpenAPI 3.1. Additional generated declaration checks reject incorrect field types, incomplete union changes, and synchronous calls after an async transition. The packed consumer acceptance test installs actual plugin and core tarballs, generates a client, compiles it in NodeNext mode, and imports the emitted ESM without the integration test's module loader.

Keep the generated-code diff in a migration PR and regenerate all clients when upgrading the runtime/plugin together. These behavior changes must not be silently released under the existing v2 version.