🐛 Bug Report — Runtime APIs: console.log()/util.inspect() of a stream shows no state with typescript_implemented_streams #8048

Description

With typescript_implemented_streams, util.inspect() shows nothing about a stream's state, and neither does console.log(), which uses it. You get ReadableStream {}, WritableStream {} and TransformStream {}. The C++ implementation shows locked, [state], [supportsBYOB] and [length], plus readable/writable for a TransformStream. Logging response.body is a common way to debug a stuck or already-used body. Under the flag it shows nothing useful.

Reproduction

worker.js:

import { inspect } from "node:util";

export default {
  async test() {
    const stream = new ReadableStream({ start(controller) { controller.enqueue(new Uint8Array(3)); } });
    stream.getReader();
    const text = inspect(stream);
    console.log(text);
    console.log(inspect(new TransformStream()));
    console.log(inspect(new Response("body").body));
    if (!text.includes("locked")) throw new Error("inspect() shows none of the stream's state");
  },
};

config.capnp:

using Workerd = import "/workerd/workerd.capnp";

const config :Workerd.Config = (
  services = [(name = "main", worker = .main)],
  # typescript_implemented_streams only starts with this autogate.
  autogates = ["workerd-autogate-per-isolate-javascript-bootstrap"],
);

const main :Workerd.Worker = (
  modules = [(name = "worker", esModule = embed "worker.js")],
  compatibilityDate = "2026-10-27",
  compatibilityFlags = ["typescript_implemented_streams"],
);

workerd test --experimental config.capnp

With the flag (workerd 2026-10-27, @cloudflare/workerd-darwin-arm64@1.20261027.1):

ReadableStream {}
TransformStream {}
ReadableStream {}
[ FAIL ] main

With compatibilityFlags = [] (C++ streams):

ReadableStream {
  locked: true,
  [state]: 'readable',
  [supportsBYOB]: false,
  [length]: undefined
}
TransformStream {
  readable: ReadableStream {
    locked: false,
    [state]: 'readable',
    [supportsBYOB]: false,
    [length]: undefined
  },
  writable: WritableStream { locked: false, [state]: 'writable', [expectsBytes]: false }
}
ReadableStream {
  locked: false,
  [state]: 'readable',
  [supportsBYOB]: true,
  [length]: 4n
}
[ PASS ] main

Expected behavior

inspect()/console.log() of a stream shows the same state as with the C++ implementation, for example through a Symbol.for('nodejs.util.inspect.custom') method on the TypeScript classes.