Let agents see sibling threads that work on the same repository

[t3dotgg]

Problem

Users often run several threads on one repository at the same time. Each agent works alone, and that causes real mistakes:

• An agent merged its PR on a stale base while two sibling threads had open PRs that changed the same files. Nothing warned it.
• An agent tried to message a sibling by the provider's own session name. That name changed when the sibling restarted, so the message failed.

T3 Code already knows the missing facts: the other threads in the project, their branch or worktree, their linked PRs, and whether they are running.

Proposal

Smallest useful scope: one read-only tool on the existing t3-code MCP server.

sibling_threads() returns a bounded list of the other active threads in the calling thread's project:

• stable T3 thread id and title
• state: running, waiting for input, or idle
• branch and worktree
• linked PRs: number, state, base, and head
• changed files against the base branch, capped (for example 50 paths)

The tool description tells agents to call it before they merge or rebase.

Agents address threads by the T3 thread id. It does not change on restart or rename. If messaging lands (#7001), it uses the same id.

Decisions needed

1. Scope. Same project only, or every project with the same repository (other worktrees, other environments)?
2. Pull or push. Only the tool, or also a short note into a thread when a sibling's PR merges into its base branch?
3. Changed files. From the linked PR (needs the forge), or from a local git diff of each worktree (works offline, one git call per sibling)?
4. Messaging. Keep this read-only and leave sending to Let agents message existing sibling threads #7001, or ship both together?

Related

• Let agents message existing sibling threads #7001: thread_list and thread_send between sibling threads. Its implementation, feat(server): let sibling threads exchange messages #6573, was closed.
• feat(mcp): expose thread settlement in t3_thread_list and t3_thread_read #13489: settlement state in t3_thread_list for Orchestrator V2.
• list_thread_pull_requests already gives the calling thread its own linked PRs.

Written with Claude Opus 5.5 in Claude Code.