How to compare two EnergyPlus versions

Answering "what changed between 9.4 and 26.1" is a manifest comparison here, not a deep diff of two 10 MB schema documents. Because definitions are content-addressed, a type either has the same hash in both versions or it does not.

The diff

import { localBundle } from '@idfkit/schemas/node';

const bundle = localBundle();
const older = await bundle.load('9.4.0');
const newer = await bundle.load('26.1.0');

const delta = newer.changedFrom(older);

delta.added; // type names introduced since 9.4
delta.removed; // type names that no longer exist
delta.changed; // type names whose definition differs

changedFrom reads as "what this version changed, relative to that one", so call it on the newer schema.

In a browser, swap localBundle() for new SchemaBundle(httpSource('/schemas/')); everything else is identical.

Looking at what actually changed

changed gives you names. For the detail, compare the two definitions:

for (const typeName of delta.changed) {
  const before = older.get(typeName);
  const after = newer.get(typeName);

  const gained = after.f.filter((field) => !before.f.includes(field));
  const lost = before.f.filter((field) => !after.f.includes(field));

  if (gained.length || lost.length) {
    console.log(typeName, { gained, lost });
  }
}

A type can appear in changed without gaining or losing fields — a default, a range, or a choice list may have moved instead. f is the positional field order, so a change there is the one that matters most for reading old files.

Checking whether a model would survive a version

const target = await bundle.load('26.1.0');
const unsupported = doc.types().filter((typeName) => target.resolve(typeName) === undefined);

That catches types removed between releases. It does not catch fields that moved or changed meaning, and it is not a migration: this library has no version-migration API, and the transition binaries that do it properly ship with EnergyPlus itself. The Python idfkit wraps them.

Why this is cheap

Loading the second version only pays for definitions the two versions do not share, and 87% of them are identical. changed falls out of comparing hashes, with no structural walk at all.