GitHub MCP sub_issue_write: "404 Not Found" when adding a sub-issue
Error: failed to add sub-issue: POST https://api.github.com/repos/OWNER/REPO/issues/3/sub_issues: 404 Not Found
What causes it
sub_issue_write wants the sub-issue's numeric ID (for example 5925682839), not its #number. In our Oct 19, 2026 test, passing the child issue's number returned this bare 404, which reads like a permissions or missing-repo problem. Passing the id from the issue_write create response worked in about 1 s, and issue_read get_sub_issues listed the child right away.
How to fix it
- Pass the id field from the issue_write create response (or from issue_read with method get) as sub_issue_id.
- Keep issue_number as the parent's #number. Only sub_issue_id takes the ID.
- If you only know the child's #number, call issue_read method get first and read its id.
Where we hit it
- GitHub MCP test report (tested Oct 19, 2026). Setup: Claude, Claude Code, Cursor, VS Code, ChatGPT
Related fixes
- MCP error: "Incompatible auth server: does not support dynamic client registration"
- MCP Inspector keeps failing after a bad OAuth attempt
- GitHub MCP issue_write shows a form and creates nothing
- GitHub MCP: "403 Resource not accessible by personal access token" on create_pull_request
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