Copilot Chat fails on macOS arm64 with Electron fetcher: “Cannot convert argument to a ByteString … char 65279 (U+FEFF)” (regression between 1.110.1 and 1.115.0)

[EsWees]

Summary

On macOS arm64, GitHub Copilot Chat crashes/fails to fetch models/sessions with:

TypeError: Cannot convert argument to a ByteString because the character at index 0 has a value of 65279 which is greater than 255.

This occurs when VS Code uses the Electron fetcher (log shows: “Using the Electron fetcher.”). Downgrading VS Code to 1.110.1 resolves the problem with the same Copilot Chat extension version, indicating a regression in newer VS Code/Electron/undici/WebIDL header handling.

Environment (WORKING)

• VS Code: 1.110.1
  • Commit: 61b3d0a
  • Date: 2026-03-06T23:03:27.520Z
• Electron: 39.6.0
• Chromium: 142.0.7444.265
• Node.js: 22.22.0
• V8: 14.2.231.22-electron.0
• OS: Darwin arm64 25.3.0
• Extension: GitHub Copilot Chat 0.38.2

Result: Copilot Chat works.

Environment (FAILING)

• VS Code: 1.115.0 (from Copilot Chat logs before downgrade)
• OS: macOS arm64
• Extension: GitHub Copilot Chat 0.43.0

Result: Copilot Chat fails with ByteString error repeatedly when fetching sessions/models.

Steps to Reproduce

1. On macOS arm64, install VS Code 1.115.0.
2. Install GitHub Copilot + GitHub Copilot Chat (tested with Copilot Chat 0.43.0).
3. Sign in to GitHub/Copilot.
4. Open Copilot Chat or let it activate (it attempts to fetch models/sessions).

Actual Behavior

Copilot Chat fails to fetch sessions/models. Output log repeatedly contains:

• “Using the Electron fetcher.”
• Then crash while constructing a Request / headers via Electron fetch (undici WebIDL converters)

Example stack (trimmed):

TypeError: Cannot convert argument to a ByteString because the character at index 0 has a value of 65279 which is greater than 255.
at webidl.converters.ByteString (node:internal/deps/undici/undici:4005:17)
at Object.record<ByteString, ByteString> (node:internal/deps/undici/undici:3912:32)
at webidl.converters.HeadersInit (node:internal/deps/undici/undici:9210:67)
at Object.RequestInit (node:internal/deps/undici/undici:3968:21)
at new Request (node:internal/deps/undici/undici:9767:34)
at fetchWithSession (node:electron/js2c/utility_init:2:358)
...

The error suggests a request header value begins with U+FEFF (BOM) and Electron/undici rejects it as ByteString.

Expected Behavior

Copilot Chat should not crash/fail due to a non-ASCII/ByteString character in headers; VS Code/Electron fetch should either:

• sanitize/reject with a clearer error that identifies the offending header, and/or
• be consistent with previous versions where Copilot Chat worked.

Notes / Diagnostics

• No proxy variables were set in the environment (http_proxy, https_proxy, etc. empty).
• VS Code settings.json does not contain BOM.
• git config --global -l showed no UTF-8 BOM bytes (checked via xxd).
• Downgrading VS Code to 1.110.1 makes Copilot Chat work again without other changes.

Request

Please investigate regression in VS Code 1.115.0 around Electron fetch / undici headers / WebIDL ByteString conversion that breaks Copilot Chat network requests on macOS arm64.