# RB-07 MAJOR: get_runtime_capabilities advertises wrong accelerators

**Severity:** MAJOR
**Path:** `src-tauri/src/commands/models.rs:435-489`
**Source:** [2026-04-22 code review](../code-review-2026-04-22.md)
**Labels:** release-blocker, major, ui-integrity
**Status:** RESOLVED (2026-04-22)

## Resolution

Added a pure `compose_accelerators(whisper_enabled, loader_available,
target) -> Vec<String>` helper. It always emits `"cpu"` first and
appends `"metal"` (macOS) or `"vulkan"` (Linux/Windows) only when
whisper is compiled in *and* `vulkan_loader_available()` resolves the
platform's loader shim.

`supported_accelerators()` reads `cfg!(feature = "whisper")`, runs the
live loader probe, and delegates to the helper.
`get_runtime_capabilities` calls `supported_accelerators()` in place
of the hard-coded `vec!["cpu", "vulkan"]`. Whisper's `supports_gpu`
is now `cfg!(feature = "whisper")` — false on whisper-disabled
builds.

Five regression tests in `commands::models::tests` cover:

- `cpu_only_when_whisper_disabled` (both targets)
- `cpu_only_when_loader_missing` (both targets)
- `macos_with_loader_advertises_metal`
- `non_macos_with_loader_advertises_vulkan`
- `cpu_is_always_first_entry` (contract the frontend relies on)

Both `cargo test -p lumotia --lib` and `cargo test -p lumotia --lib
--no-default-features` pass the new suite; both `cargo build -p lumotia`
and `cargo build -p lumotia --no-default-features` compile clean.

Runtime GUI verification on a real macOS box is still on the ship-gate
checklist — the detection logic is correct in code; Metal-loader
resolution on hardware is not something we can unit-test from a Linux
CI.

## Problem

The IPC response hard-codes `accelerators = ["cpu", "vulkan"]` and `supports_gpu = true` for Whisper, even when:

- `detect_active_compute_device` would report `metal` on macOS (via MoltenVK).
- The binary was compiled without the `whisper` feature — in which case Whisper `supports_gpu` is meaningless because there is no Whisper backend at all.

The frontend uses this response to render Settings toggles (GPU selection, active-device badge, feature availability). Wrong values mean wrong UI states on exactly the builds this function is meant to describe.

## Acceptance

- `accelerators` is derived from actual build configuration and runtime probe, not hard-coded.
- On macOS, `accelerators` includes `"metal"` when the Metal loader resolves.
- On whisper-disabled builds, Whisper entries advertise `supports_gpu = false` (or the engine is omitted from the response entirely).
- Regression tests cover each platform variant via cfg-gated test cases.

## Fix scope

Medium. The detection helpers already exist (`detect_active_compute_device`, `vulkan_loader_available`); this is about wiring their output into the RuntimeCapabilities struct honestly.

## Dependencies

- None — standalone fix.