Let Copilot CLI host composite loops, and run a composite's workers on its backend

[scgopi]

Copilot CLI can now host composite loops, and a composite's workers run on the composite's backend.

Why Copilot couldn't host a composite

canHost(.composite) is capabilities.supportsSubAgents, and Copilot's row said false: it was read off Copilot CLI 0.0.410 / 1.0.75, whose help listed no sub-agent fan-out, and the capability table's rule is that anything unverified stays false rather than guessed.

There was a second, silent gap behind that one. A composite never runs a session of its own; its workers do. A worker created inside a composite with no backend named fell to NodeDraft.effectiveBackend, which is Claude Code. Flipping the row alone would have produced a composite labelled Copilot whose every worker ran Claude.

What changes

1. Capability row flipped with evidence. Copilot CLI 1.0.80's copilot help commands lists /fleet ("enable fleet mode for parallel subagent execution"), /tasks ("view and manage tasks (subagents and shell commands)") and /subagents, and --agent <agent> is on the launch line. supportsSubAgents is now true, with the same older-Copilot caveat the time-based row carries.
2. Workers inherit the composite's backend in the daemon. runInSubGraph fills an unnamed backend from the composite before the child store creates the node. A creator the tree can find still wins, and an explicitly named backend wins over both, the same precedence createNode already applies to loops fanning out from inside a session.
3. The app form seeds the picker the same way. Adding a loop while a composite is open defaults the backend picker to that composite's backend, then the human's default.
4. Tests. CopilotBackendTests pins the pairing being allowed; new CompositeBackendTests pins inheritance and the named-backend override (its own file because CompositeAndGlobalGraphTests sits at the lint type-body budget).

Verification

• xcodebuild test in a worktree: full suite 1508 tests, only failures were the three tests that pinned composite+Copilot as refused (repointed at composite+Codex, which stays refused); the five affected suites then re-run green (74 tests, including the new CompositeBackendTests).
• swiftlint 0 errors, swift format lint --strict clean on the changed files.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Pxwzz1AZnVE3peZ8qMejLa