Attach a sub-issue with the add_sub_issue tool

Goal: attach an existing child issue to an existing parent issue using GitHub’s native sub-issue relationship.

Prerequisites

• Both the parent and the child issue already exist.
• The parent has fewer than 100 existing sub-issues.
• Attaching the child would not place the resulting hierarchy at or past 8 nesting levels (the tool computes the parent’s current level by walking its own parent chain before deciding).
• The child may live in a different repository within the same organization — pass childOwner/childRepo if so.

Steps

1. Call add_sub_issue:

   {
     "owner": "your-org",
     "repo": "your-repo",
     "parentNumber": 101,
     "childNumber": 102
   }

2. If the child is in a different repo:

   {
     "owner": "your-org",
     "repo": "parent-repo",
     "parentNumber": 101,
     "childOwner": "your-org",
     "childRepo": "child-repo",
     "childNumber": 7
   }

3. On a concurrent re-parent (the child already has a different parent), the default replaceParent: true lets the call succeed by reassigning it. Pass replaceParent: false if you want it to fail instead when the child is already parented elsewhere.

Verify it worked

• Read the response: { parentNodeId, childNodeId, replacedParent }.
• Reload the parent issue on GitHub — the native “Sub-issues” panel lists the child.
• Call list_sub_issues against the parent and confirm the child appears in items.
• If the parent was already at the sub-issue or nesting limit, the call fails before reaching GitHub with { error: "limit_exceeded", limit: "max_sub_issues_per_parent" | "max_nesting_levels", ... }.

See also: tool reference.