Blocked: `net::ERR_CONNECTION_CLOSED` for every plan write attempt on GitHub Copilot

Every time I try to write a plan document I am getting the following, both with Claude Sonnet 5 and Claude Opus 5, both with a full context window and a freshly-compacted context window. I am blocked from further progress.

> Sorry, there was a network error. Please try again later. Request id: …
>
> Reason: Please check your firewall rules and network connection then try again. Error Code: net::ERR_CONNECTION_CLOSED.

I've been working all day with no error. After much design discussion I used a custom prompt to ask Claude Sonnet 5 create an implementation plan. It considers, and then says it it creating a file. Then after a few minutes it produces an error. This happened three times in a row with Claude Sonnet 5 before I attempted workarounds (see below)

I seem to blocked. I have not yet found a way to write the implementation plan.

### Futile Workaround Attempts

* I've tried lowering the Claude Sonnet 5 thinking effort. That doesn't help.
* I've tried compacting the conversation. That doesn't help.
* I've tried switching to Claude Opus 5. That doesn't help.

```
Version: 1.130.0 (user setup)
Commit: 1b6a188127eeaf9194f945eb6eb89a657e93c54c
Date: 2026-07-22T14:55:04Z
Electron: 42.6.0
ElectronBuildId: 14623276
Chromium: 148.0.7778.280
Node.js: 24.18.0
V8: 14.8.178.38-electron.0
OS: Windows_NT x64 10.0.26200
```