[Bug]: Device Hub opens for stable Xcode when Xcode 27 beta is co-installed

Bug Description

When Xcode 26 stable and Xcode 27 beta are installed together, XcodeBuildMCP
2.7.0 opens the beta-only Device Hub even when DEVELOPER_DIR explicitly
selects stable Xcode and the target is an iOS 26.5 simulator.

This crosses the selected toolchain boundary. Stable Xcode 26 ships
Simulator.app; Device Hub is bundled only inside Xcode 27 beta on this host.

The cause appears to be the frontend candidate order in
focus-policy.ts:
the Device Hub candidate is attempted first for every non-headless simulator
launch—via devices:// when a simulator ID is known, or open -a DeviceHub
otherwise—without consulting the active Xcode selected by DEVELOPER_DIR.
Simulator.app is only a fallback if opening Device Hub fails. build_run_sim
invokes this policy automatically after boot.
The same behavior is still present on main as of 2026-08-10.

Debug Output

Platform: macOS 26.5.2 (25F84), arm64
Stable Xcode: 26.6 (17F113)
Beta Xcode: 27.0 (27A5237l)
Selected DEVELOPER_DIR: /Applications/Xcode.app/Contents/Developer
XcodeBuildMCP: 2.7.0
Codex CLI: 0.144.1

Stable frontend:
  /Applications/Xcode.app/Contents/Developer/Applications/Simulator.app
  bundle id: com.apple.iphonesimulator

Beta frontend:
  /Applications/Xcode-beta.app/Contents/Applications/DeviceHub.app
  bundle id: com.apple.dt.Devices

Target simulator:
  iPhone 17 Pro, iOS 26.5

Control result with XCODEBUILDMCP_HEADLESS_LAUNCH=1:
  build-and-run SUCCEEDED on the iOS 26.5 simulator
  app launched successfully
  no DevicesTrampoline process was created

Editor/Client

Codex desktop with Codex CLI 0.144.1

MCP Server Version

2.7.0

LLM

GPT-5 Codex

MCP Configuration

[mcp_servers.xcodebuildmcp]
command = "npx"
args = ["-y", "xcodebuildmcp@2.7.0", "mcp"]
env = {
  DEVELOPER_DIR = "/Applications/Xcode.app/Contents/Developer",
  XCODEBUILDMCP_ENABLED_WORKFLOWS = "simulator,ui-automation,debugging,logging",
}

Steps to Reproduce

1. Install Xcode 26 stable at /Applications/Xcode.app.
2. Install Xcode 27 beta at /Applications/Xcode-beta.app.
3. Set DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer.
4. Leave XCODEBUILDMCP_HEADLESS_LAUNCH unset.
5. Select an available iOS 26.5 simulator by UDID.
6. Run open_sim or build_run_sim.
7. Observe that Device Hub from Xcode 27 beta opens instead of Simulator.app
   from the selected stable Xcode.

Expected Behavior

In automatic GUI mode, the active Apple toolchain should own the simulator
frontend:

• Xcode 26 stable selected -> Simulator.app
• Xcode 27 selected -> Device Hub

An explicit auto | simulator | device-hub preference would also work, with
auto derived from the Xcode selected by DEVELOPER_DIR rather than from the
machine-wide devices:// URL handler.

Headless mode should continue to suppress both frontends.

Actual Behavior

For every non-headless launch, XcodeBuildMCP tries Device Hub first. Because the
Xcode 27 beta installation registers devices://, that succeeds even though
stable Xcode is selected, so Simulator.app is never attempted.

Error Messages

No command error is emitted; the wrong GUI frontend opens successfully.

Related

#498 covers the separate but complementary problem of selecting the newest
simulator runtime within a stable OS major when a next-major beta is installed.

從 src/utils/focus-policy.ts 中的前端候選順序開始，然後追蹤 src/mcp/tools/simulator/build_run_sim.ts 如何在啟動後呼叫 policy。安裝 stable 和 beta Xcode，並讓 DEVELOPER_DIR 選取 stable，以重現此問題；完成標準是自動 GUI 啟動由所選 Xcode 所擁有的前端，而 headless 模式仍然不會開啟任何前端。