Environment

• Node.js: 25.x (fails); 22.x and 24.x OK in downstream CI
• @omnigraph/json-schema: 0.109.40 (via @omnigraph/openapi 0.109.41)

Symptom

While building a GraphQL schema from the full Slack Web API OpenAPI 3 document (openapi.json from Slack’s public spec, ~170+ operations), schema construction throws:

TypeError: Cannot set property input of #<Object> which has only a getter
    at getUnionTypeComposers (.../@omnigraph/json-schema/esm/getUnionTypeComposers.js:52:41)

Suspected line

TypeScript source: packages/loaders/json-schema/src/getUnionTypeComposers.ts — assignment to subSchemaAndTypeComposers.input when Object.keys(unionInputFields).length === 1.

Repro

1. Depend on @omnigraph/openapi and call loadGraphQLSchemaFromOpenAPI with the full Slack Web API OpenAPI JSON.
2. Run on Node 25.

A minimal OpenAPI with only chat.postMessage does not reproduce in our tests — the failure appears tied to the large / union-heavy schema.

Downstream

• ClawQL — in-process OpenAPI→GraphQL for execute / notify; we document behavior and use a minimal Slack fixture in Vitest for the GraphQL path. Doc: graphql-mesh-node-compatibility.md.