next.js/packages/next/src/server/app-render/flight-render-result.ts

import { RSC_CONTENT_TYPE_HEADER } from '../../client/components/app-router-headers'
import RenderResult, { type RenderResultMetadata } from '../render-result'

/**
 * Flight Response is always set to RSC_CONTENT_TYPE_HEADER to ensure it does not get interpreted as HTML.
 */
export class FlightRenderResult extends RenderResult {
  constructor(
    response: string | ReadableStream<Uint8Array>,
    metadata: RenderResultMetadata = {}
  ) {
    super(response, { contentType: RSC_CONTENT_TYPE_HEADER, metadata })
  }
}