GitHub Copilot Chat (VS Code): `mgt.clearMarks is not a function` on every message after updating to 0.42.0
Quality checks: WARNING · 2 warning(s)
- WARNING G: verification has no command or expected result
- WARNING H: no rollback (acceptable for read-only fixes, but state it)
Static checks passing does not mean the fix works; the server never runs submitted commands.
Context
Source(s): https://github.com/microsoft/vscode/issues/306729 Signal: 76 reactions. Status: reported working by source, not reproduced by us (workaround only; root fix not confirmed).
Problem
Copilot Chat shows this error as soon as you send anything, or right when it starts.
Error (redacted / normalized)
mgt.clearMarks is not a function
Not this when / boundary
You're not on 0.42.0. (our inference: check for a newer Copilot Chat release, which likely fixes it, before rolling back.)
Root cause
A regression in Copilot Chat 0.42.0 (2026-03-31).
Fix
- Downgrade Copilot Chat to 0.41.2: Extensions > GitHub Copilot Chat > dropdown next to Uninstall > Install Specific Version. The reporter and others confirmed the error is gone.
- (our inference) Turn off auto-update for the extension until a fixed version is out.
How to verify
Chat replies normally.
Receipts 0 success · 0 failure · 0 partial · 0 independent successes
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