Copilot Chat 0.42.0: mgt.clearMarks is not a function after each response and Claude Sonnet 4.6 is extremely slow/choppy

[DigitalMechanic]

Copilot Chat Extension Version: GitHub Copilot Chat 0.42.0

VS Code Version: 1.113.0

OS Version: Windows 10 Pro, version 2009, build 26200

Feature (e.g. agent/edit/ask mode): Agent mode in panel/editAgent

Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): Claude Sonnet 4.6

---

Logs:

Evidence from local Copilot session artifacts and VS Code logs.

Session artifact captured after responses:
{"errorDetails":{"message":"mgt.clearMarks is not a function","responseIsIncomplete":true,"isQuotaExceeded":false,"isRateLimited":false}}

Copilot request timing samples:
2026-03-31 10:38:09.238 [info] Copilot Chat: 0.42.0, VS Code: 1.113.0
2026-03-31 10:38:44.832 [info] ccreq:6581d036.copilotmd | success | claude-sonnet-4.6 -> claude-sonnet-4-6 | 24178ms | [panel/editAgent]
2026-03-31 10:39:10.169 [info] ccreq:b06d7f9f.copilotmd | success | claude-sonnet-4.6 -> claude-sonnet-4-6 | 19419ms | [panel/editAgent]
2026-03-31 10:40:00.670 [info] ccreq:5bb93384.copilotmd | success | claude-sonnet-4.6 -> claude-sonnet-4-6 | 50438ms | [panel/editAgent]
2026-03-31 10:45:26.967 [info] ccreq:aeed21ac.copilotmd | success | claude-sonnet-4.6 -> claude-sonnet-4-6 | 176989ms | [panel/editAgent]

Comparison requests in the same session:
2026-03-31 10:39:36.079 [info] ccreq:c09e6b5c.copilotmd | success | gpt-4o-mini -> gpt-4o-mini-2024-07-18 | 734ms | [copilotLanguageModelWrapper]
2026-03-31 10:44:15.379 [info] ccreq:5340a49c.copilotmd | success | gpt-4o-mini -> gpt-4o-mini-2024-07-18 | 389ms | [copilotLanguageModelWrapper]
2026-03-31 10:46:48.453 [info] ccreq:408b46df.copilotmd | success | gpt-5.4 -> gpt-5.4-2026-03-05 | 12819ms | [panel/editAgent]
2026-03-31 10:52:02.044 [info] ccreq:0ff1c798.copilotmd | success | gpt-5.4 -> gpt-5.4-2026-03-05 | 11726ms | [panel/editAgent]

Related renderer/runtime warnings in the same session:
2026-03-31 10:38:07.825 [error] chatParticipant must be declared in package.json: copilot-cloud-agent
2026-03-31 10:38:07.827 [error] chatParticipant must be declared in package.json: copilotcli
2026-03-31 10:38:14.137 [error] Extension 'github.copilot-chat' appears in product.json but enables LESS API proposals than the extension wants.

---

Notes:
The literal mgt.clearMarks string was not present in the standard renderer/exthost logs.
The mgt.clearMarks message was present in the Copilot chat session artifact after responses completed.
I did not see obvious 429 or 5xx network failures in the current VS Code log bundle.

---

Steps to Reproduce:

1. Open VS Code 1.113.0 on Windows with GitHub Copilot Chat 0.42.0 installed.
2. Open Copilot Chat in agent mode from the chat panel.
3. Select Claude Sonnet 4.6.
4. Send a normal prompt and wait for the response to complete.
5. Observe that the response streams slowly and feels choppy compared with GPT-5.4 or gpt-4o-mini in the same VS Code session.
6. After the response completes, observe the UI error message mgt.clearMarks is not a function.
7. Repeat with additional prompts and observe that the same UI error appears after each response.

[pwang347]

Regarding point 6, please see #306729

[DigitalMechanic]

Thanks, I couldn't find any references myself... appreciate the help!

-DM