Duo Chat Classic throws error for /explain

Summary

From VSCode IDE, when I select code section and use GitLab Duo tools from the right click, I see error in the chat window. I am selecting GitLab Duo -> Explain tool which send '/ecplain' command to the chat window and it shows error code G3001. Sceenshot below

Note - This was working before 18.8 and I started seeing this issue after our 18.8 release on .com.

Initial investigation pointed to the duo membership or duo licenses not assigned which has not changed and this error is seen even after I validated that membership and licenses are properly assigned. Also, Duo Agentic Chat works fine.

Error Message in Language Server

2026-01-14T08:35:44:156 [debug]: [SecretRedactor] redacting secret in file:///Users/manjeet/mywork/demo-delphi/customer-onboarding/scripts/backfill-coverage-history.js 2026-01-14T08:35:44:160 [debug]: [OpenTabsService] uri file:///Users/manjeet/mywork/demo-delphi/customer-onboarding/scripts/backfill-coverage-history.js was updated in the LRU cache with onDidSetActive 2026-01-14T08:35:52:983 [debug]: [WebviewInstanceMessageBus:duo-chat-v2:a2200090-e82e-488e-b7e1-077e49b30dd4] Sending notification: focusChat 2026-01-14T08:35:52:985 [debug]: [SecretContextTransformer] Transforming context item "editor_selection:scripts/backfill-coverage-history.js_e8f36548-70d4-47b2-a9a7-d6acb8f894fa" 2026-01-14T08:35:52:985 [debug]: [SecretRedactor] redacting secret in editor_selection:scripts/backfill-coverage-history.js_e8f36548-70d4-47b2-a9a7-d6acb8f894fa 2026-01-14T08:35:52:985 [debug]: [WebviewInstanceMessageBus:duo-chat-v2:a2200090-e82e-488e-b7e1-077e49b30dd4] Sending notification: focusChat 2026-01-14T08:35:52:985 [info]: ChatContextManager clearing all selected context items 2026-01-14T08:35:52:985 [info]: ChatContextManager returning 11 available context provider categories: "file, file, dependency, directory, snippet, issue, file, local_git, merge_request, repository, snippet" 2026-01-14T08:35:52:985 [info]: ContextItems: refreshing current context item selections 2026-01-14T08:35:52:985 [debug]: GitLabChatApi: subscribeToUpdates, additionalContextEnabled: true 2026-01-14T08:35:52:987 [info]: ChatContextManager returning 0 current context items 2026-01-14T08:35:52:987 [debug]: [WebviewInstanceMessageBus:duo-chat-v2:a2200090-e82e-488e-b7e1-077e49b30dd4] Sending notification: contextCurrentItemsResult 2026-01-14T08:35:53:235 [info]: Duo Chat: successfully subscribed to updates 2026-01-14T08:35:53:235 [info]: ChatContextManager returning 11 available context provider categories: "file, file, dependency, directory, snippet, issue, file, local_git, merge_request, repository, snippet" 2026-01-14T08:35:53:235 [debug]: [SimpleApiClient] Making GraphQL request: mutation: chat 2026-01-14T08:35:53:236 [debug]: fetch: request for https://gitlab.com/api/graphql made with https agent. 2026-01-14T08:35:53:648 [debug]: fetch: request to https://gitlab.com/api/graphql returned HTTP 200 after 411 ms 2026-01-14T08:35:53:651 [debug]: [WebviewInstanceMessageBus:duo-chat-v2:a2200090-e82e-488e-b7e1-077e49b30dd4] Sending notification: newRecord 2026-01-14T08:35:53:653 [debug]: [WebviewInstanceMessageBus:duo-chat-v2:a2200090-e82e-488e-b7e1-077e49b30dd4] Sending notification: newRecord 2026-01-14T08:35:54:251 [debug]: [WebviewInstanceMessageBus:duo-chat-v2:a2200090-e82e-488e-b7e1-077e49b30dd4] Sending notification: updateRecord

Steps to reproduce

Select the code, right click to launch the menu, select GitLab Duo -> explain

Example Project

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

 (For installations with omnibus-gitlab package run and paste the output of: \`sudo gitlab-rake gitlab:env:info\`)  (For installations from source run and paste the output of: \`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production\`)  

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of: `sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of: `sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

Possible fixes

Patch release information for backports

If the bug fix needs to be backported in a patch release to a version under the maintenance policy, please follow the steps on the patch release runbook for GitLab engineers.

Refer to the internal "Release Information" dashboard for information about the next patch release, including the targeted versions, expected release date, and current status.

High-severity bug remediation

To remediate high-severity issues requiring an internal release for single-tenant SaaS instances, refer to the internal release process for engineers.