`device_id` in `.zed/debug.json` is silently ignored — Flutter device selection never works

[FrantisekGazo]

Summary

The device_id field documented in the debug adapter schema is read from user config and forwarded to Flutter's DAP as deviceId, but Flutter's DAP never reads this field. As a result, flutter run is invoked without -d <id> and falls back to its auto-pick logic — so users either hit "More than one device connected" errors or silently launch on the wrong device.

Reproduction

.zed/debug.json:

{
  "label": "UI Demo: Debug on macOS",
  "adapter": "Dart",
  "type": "flutter",
  "request": "launch",
  "program": "lib/main.dart",
  "cwd": "$ZED_WORKTREE_ROOT/path/to/flutter-project",
  "device_id": "macos",
  "useFvm": true
}

With both macOS desktop and Chrome web available, launching this task fails with:

More than one device connected; please specify a device with the '-d <deviceId>' flag, or use '-d all' to act on all devices.

macOS (desktop) • macos  • darwin-arm64   • macOS 26.1 25B78 darwin-arm64
Chrome (web)    • chrome • web-javascript • Google Chrome 147.0.7727.138

Exited (1).

With an Android emulator additionally connected, the macOS task instead launches on the emulator (auto-pick of the only ephemeral device) — device_id: "macos" is still ignored.

Root cause (verified end-to-end)

1. Extension side (src/dart.rs get_dap_binary) reads device_id and emits it as deviceId in the launch JSON sent to flutter debug_adapter. ✅
2. Flutter side (flutter_tools/lib/src/debug_adapters/flutter_adapter.dart, launchImpl) builds the flutter run --machine ... argv from toolArgs and args.toolArgs (i.e. user-provided toolArgs). It never reads the deviceId field — grep -r deviceId packages/flutter_tools/lib/src/debug_adapters/ returns a single hit, and it's a comment.

Verified by sending hand-built DAP launch requests to flutter debug_adapter:

Launch JSON	Result
{"deviceId": "macos", ...}	"More than one device connected", exits
{"toolArgs": ["-d", "macos"], ...}	macOS app launches successfully

Tested on Flutter 3.41.6 stable, Zed 1.1.6, dart extension 0.3.6.

Proposed fix

In src/dart.rs get_dap_binary, translate device_id into toolArgs: ["-d", device_id] in the launch JSON. Two viable shapes:

• Implicit: keep the device_id config field; in the extension, append ["-d", device_id] to a toolArgs array sent to the DAP.
• Explicit: expose a toolArgs field in the schema (which Fix debug adapter config bugs and add missing config options #66 already proposes) and let users write toolArgs: ["-d", "macos"] themselves. Less convenient, but more flexible.

Ideally both — auto-translate device_id and support toolArgs for other flags (--flavor, --dart-define, etc.).

Related work

• Add hot reload and hot restart custom actions #72 explicitly notes "Fix device selection by passing -d <device_id> via toolArgs" and verifies device_id: "macos" in its test plan.
• Fix debug adapter config bugs and add missing config options #66 adds toolArgs config support, which would unblock users as a manual workaround.

Either approach (or both combined) would close this issue.