Upstream advance to 0.1.4: what to handle #772

Things the advance to upstream's 0.1.4 line has to handle. Add to this as items come up.

Linked environments merged upstream

Upstream merged nine layers of Julius Marminge's linked-environment stack into main on 2026-10-27: pingdotgg#16653 through pingdotgg#17130. They landed as nine squash commits, 1b1ad0466d to 5e9d56416f. The first nightly that contains them is v0.1.4-nightly.20261027.3796. The v0.1.3 stable release does not. The handoff layers (pingdotgg#16731 → pingdotgg#16734 → pingdotgg#16751 → pingdotgg#16758) are still open upstream.

It overlaps J5's own peering (docs/j5/product/cross-device.md), so the advance needs a deliberate merge, not just conflict resolution.

What it brings

• Off by default. Linked environments is a Labs beta: the server setting linkedEnvironmentsEnabled in Settings → Labs, turned on separately on each machine. While it is off, new links, forwarded calls and remote delegate_task are refused, and the link tools are hidden.
• Settings → Connections → Linked environments on web and mobile. It sits beside J5's peering in Connections. Decide how the two coexist there.
• Forwarding in McpToolAccess. A tool that declares it can reach a linked environment forwards a call that names another machine's environmentId. Upstream's thread, project and queue tools now accept one. J5 hides t3_thread_send, t3_thread_wait, t3_thread_interrupt, t3_thread_launch and the queue mutators. Check they stay hidden, and that J5's own tools, which go through McpToolAccess, don't pick up forwarding by accident.
• The link fence. At every access level below full access, work a link starts may change only that link's own work. A full-access link is not fenced.
• delegate_task to a linked environment, with remote tasks followed from the parent's machine.
• Agent-initiated links. Link cards and the t3_environment_links, t3_environment_link and t3_environment_unlink tools.
• ThreadLaunchService gains linkOrigin and delegatedFrom, and agent launches now dispatch under the caller's mode ceiling (DispatchModeLimit). reuseExistingThread is unchanged, but re-test j5_spawn_agent and Crew launches on it.
• /mcp narrows a client's modes from the T3-Mode-Limit header, and t3_thread_launch takes a clientRequestId.
• Migrations 070–072: PeerLinks, PeerLinkFence and WaitingLinkRequestIndex. J5's list ends at 062 and J5 claims no numbers in that range, so they don't collide.
• Follow-ups already on upstream's main: feat(web,mobile): the @ picker lists threads on linked machines pingdotgg/t3code#26201 (the @ picker lists threads on linked machines), feat(web,mobile): Copy thread copies a chip that works on linked machines pingdotgg/t3code#26202 (Copy thread works across machines), feat(server,web,mobile): thread links name their environment and never a title pingdotgg/t3code#26239 (thread links name their environment) and fix(web): remote delegates show their harness icon, a parent row, and new links in settings pingdotgg/t3code#26434 (remote delegates show their harness icon and parent row).

Likely breakage to check during the merge

• The link fence may refuse J5's writes tools for work a lower-access link started. Full-access links are exempt.
• Remote delegation calls the launch, wait and interrupt tools on the other machine, and J5 hides them.
• Children created with delegatedFrom may register as J5 participants.

Decisions this unparks

• Reconcile J5's thread tools with upstream's once linked environments settle #695: the "upstream owns spawning and control, J5 owns agent-to-agent messaging" tool boundary, including bringing back j5 thread.
• Show J5 command relationships in upstream's Lineage panel #283: how Peer Agents and Crew seats show in Lineage.

What upstream said about J5's points. Julius replied to Jackson's comment on pingdotgg#16655 (comment). Links work over any route that reaches the machine, such as Tailscale, a tunnel or https. A call to a sleeping machine fails retryably, and clientRequestId makes the retry safe. A poll mode isn't planned for this first version. That bears on #399.