Copilot model fetch fails when telemetry.devDeviceId starts with UTF-8 BOM #315612

• Copilot Chat Extension Version: 0.48.2026051101 in VS Code Insiders; 0.47.0 in Stable
• VS Code Version: 1.120.0-insider, commit 05ca786, date 2026-05-11T03:48:51Z. Also observed in Stable 1.119.0.
• OS Version: macOS 26.4.1 (25E253), arm64
• Feature (e.g. agent/edit/ask mode): Chat/model picker/model fetch
• Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): none; models fail to load
• Logs:

GitHub Copilot Chat output repeatedly shows:

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 async US._fetchModels (.../extensions/copilot/dist/extension.js:7036:4553): Failed to fetch models

The UI shows Language model unavailable, and the Language Models view shows no Copilot auto mode endpoints.

Root-cause evidence from local VS Code storage:

• ~/Library/Application Support/Code/User/globalStorage/storage.json contained a UTF-8 BOM (U+FEFF, decimal 65279) at the start of telemetry.devDeviceId.
• ~/Library/Application Support/Code - Insiders/User/globalStorage/storage.json also contained the same BOM at the start of telemetry.devDeviceId.
• Removing only that BOM while VS Code was closed fixed model fetching.
• If VS Code Insiders remains running, it rewrites the bad in-memory value back to disk, causing the issue to recur.
• Stable stayed clean after the repair, while Insiders reintroduced the BOM in telemetry.devDeviceId after restart/runtime.

No GitHub auth or network change was required; the failure appears to happen before/while constructing fetch headers because the BOM-containing devDeviceId is passed into request metadata/header construction.

Steps to Reproduce:

1. Have telemetry.devDeviceId in VS Code global storage begin with U+FEFF / UTF-8 BOM.
2. Start VS Code / VS Code Insiders with GitHub Copilot Chat enabled.
3. Open Copilot Chat or the Language Models picker.
4. Observe Language model unavailable and repeated Cannot convert argument to a ByteString... value of 65279 errors while fetching models.

Expected:

Copilot should either sanitize/strip invalid characters from devDeviceId before using it in request headers/metadata, or regenerate the value if it is malformed.

Actual:

Model fetching fails completely until the hidden BOM is manually removed from telemetry.devDeviceId while VS Code is closed.

Related: #314693 reports the same 65279 ByteString failure but does not identify telemetry.devDeviceId as the corrupt value.