feat(A.2 #13): replace SpeechBackend enum with Transcriber trait

New crates/transcription/src/transcriber.rs defines a Transcriber
trait (Send supertrait for spawn_blocking travel) with
TranscriberCapabilities (sample_rate, channels, supports_initial_prompt).
TranscriberCapabilities.sample_rate is load-bearing for the upcoming
progressive WAV writer (#19).

Concrete impls:
- SpeechModelAdapter wraps Box<dyn transcribe_rs::SpeechModel + Send>
  for Parakeet (and any future transcribe-rs-backed engine).
- WhisperRsBackend moves its transcribe_sync body into the impl,
  widening the signature from &self to &mut self so per-call
  WhisperState can be created cleanly through the trait object.

LocalEngine now holds Box<dyn Transcriber + Send>; dispatch in
transcribe_sync collapses from a match to a direct call. Adds
LocalEngine::capabilities() for the WAV-writer.

Cargo feature flag "whisper" (default on) makes whisper-rs, num_cpus,
and the whole whisper_rs_backend module optional. cargo check
--no-default-features -p kon-transcription now builds without pulling
whisper-rs-sys — the escape hatch brief item #6 / #13 called for on
Windows / non-AVX2 / cloud-only builds. load_whisper is cfg-gated
behind the same feature.

src-tauri/src/commands/models.rs load_model_from_disk returns
Box<dyn Transcriber + Send> instead of SpeechBackend; caller chain
(ensure_model_loaded, prewarm_default_model) is unchanged.

transcriber_trait_is_object_safe test lands alongside the trait as a
compile-time witness against future Self-returning / generic-method
additions.

crates/transcription/Cargo.toml

@@ -5,6 +5,15 @@ edition = "2021"
 description = "Speech-to-text engine wrappers, model management, and inference concurrency for Kon"
 build = "build.rs"
 
+[features]
+# Whisper backend (direct whisper-rs, vulkan-accelerated). Default on —
+# gating it exists so a future Windows non-AVX2 build, or a cloud-only
+# ASR configuration, can drop whisper-rs-sys entirely per brief item
+# #13. Disabling this feature also drops the WhisperRsBackend module
+# and the load_whisper entry point.
+default = ["whisper"]
+whisper = ["dep:whisper-rs", "dep:num_cpus"]
+
 [dependencies]
 kon-core = { path = "../core" }
 
@@ -21,10 +30,16 @@ futures-util = "0.3"
 # Download integrity verification
 sha2 = "0.10"
 
-whisper-rs = { version = "0.16", default-features = false, features = ["vulkan"] }
+# Gated behind the `whisper` feature (see [features] above).
+whisper-rs = { version = "0.16", default-features = false, features = ["vulkan"], optional = true }
 
-# Direct whisper-rs backend (WhisperRsBackend): thread pool sizing + typed errors.
+# Direct whisper-rs backend (WhisperRsBackend): thread pool sizing.
-num_cpus = "1"
+# Gated alongside whisper-rs since no other code in this crate needs it.
+num_cpus = { version = "1", optional = true }
+
+# Typed error enum used by WhisperRsBackend + elsewhere. Kept
+# unconditional because it is a derive-macro crate with negligible
+# build cost.
 thiserror = "2"
 
 # Structured logging at backend boundaries (observability for initial_prompt flow).

crates/transcription/src/lib.rs

@@ -1,9 +1,14 @@
 pub mod concurrency;
 pub mod local_engine;
 pub mod model_manager;
+pub mod transcriber;
+#[cfg(feature = "whisper")]
 pub mod whisper_rs_backend;
 
 pub use concurrency::run_inference;
-pub use local_engine::{load_parakeet, load_whisper, LocalEngine, SpeechBackend, TimedTranscript};
+pub use local_engine::{load_parakeet, LocalEngine, SpeechModelAdapter, TimedTranscript};
+#[cfg(feature = "whisper")]
+pub use local_engine::load_whisper;
 pub use model_manager::{download, is_downloaded, list_downloaded, model_dir, models_dir};
+pub use transcriber::{Transcriber, TranscriberCapabilities};
 pub use transcribe_rs::SpeechModel;

crates/transcription/src/local_engine.rs

@@ -9,6 +9,8 @@ use kon_core::types::{
     AudioSamples, EngineName, ModelId, Segment, Transcript, TranscriptionOptions,
 };
 
+use crate::transcriber::{Transcriber, TranscriberCapabilities};
+#[cfg(feature = "whisper")]
 use crate::whisper_rs_backend::WhisperRsBackend;
 
 /// Result of a timed transcription: transcript + inference duration.
@@ -17,22 +19,54 @@ pub struct TimedTranscript {
     pub inference_ms: u64,
 }
 
-/// Public discriminator selected by the loaders (`load_parakeet`, `load_whisper`)
+/// Adapts any `transcribe-rs` `SpeechModel` into the `Transcriber`
-/// and passed to `LocalEngine::load`. `src-tauri::commands::models` names this
+/// trait. Today this is only used for Parakeet (ONNX), but the adapter
-/// type as the return of `load_model_from_disk`, so it must be `pub`.
+/// is the path any future transcribe-rs-backed engine plugs through —
-pub enum SpeechBackend {
+/// Moonshine, fine-tuned Parakeet variants, etc.
-    /// transcribe-rs-owned model. Used for Parakeet ONNX (wrapped in
+pub struct SpeechModelAdapter(pub Box<dyn SpeechModel + Send>);
-    /// ParakeetWordGranularity for word-level timestamps).
+
-    Adapter(Box<dyn SpeechModel + Send>),
+impl Transcriber for SpeechModelAdapter {
-    /// Direct whisper-rs. The only path that actually forwards `initial_prompt`.
+    fn capabilities(&self) -> TranscriberCapabilities {
-    WhisperRs(WhisperRsBackend),
+        TranscriberCapabilities {
+            sample_rate: kon_core::constants::WHISPER_SAMPLE_RATE,
+            channels: 1,
+            supports_initial_prompt: false,
+        }
+    }
+
+    fn transcribe_sync(
+        &mut self,
+        samples: &[f32],
+        options: &TranscriptionOptions,
+    ) -> Result<Vec<Segment>> {
+        let opts = TranscribeOptions {
+            language: options.language.clone(),
+            translate: false,
+            leading_silence_ms: None,
+            trailing_silence_ms: None,
+        };
+        let result: TranscriptionResult = self
+            .0
+            .transcribe(samples, &opts)
+            .map_err(|e| KonError::TranscriptionFailed(e.to_string()))?;
+        Ok(result
+            .segments
+            .unwrap_or_default()
+            .into_iter()
+            .map(|s| Segment {
+                start: s.start as f64,
+                end: s.end as f64,
+                text: s.text,
+            })
+            .collect())
+    }
 }
 
-/// Wraps any transcribe-rs engine in Kon's SpeechToText trait.
+/// Owns the currently-loaded speech backend and serialises inference
-/// Encapsulates threading: inference always runs on a blocking thread.
+/// against model-swap operations via a `Mutex`. All transcription goes
-/// The rest of the app never imports transcribe-rs directly.
+/// through this struct; no caller ever holds a raw `Box<dyn Transcriber>`.
 pub struct LocalEngine {
-    engine: Mutex<Option<SpeechBackend>>,
+    engine: Mutex<Option<Box<dyn Transcriber + Send>>>,
     engine_name: EngineName,
     loaded_model_id: Mutex<Option<ModelId>>,
 }
@@ -46,7 +80,7 @@ impl LocalEngine {
         }
     }
 
-    pub fn load(&self, backend: SpeechBackend, model_id: ModelId) {
+    pub fn load(&self, backend: Box<dyn Transcriber + Send>, model_id: ModelId) {
         let mut guard = self.engine.lock().unwrap_or_else(|e| e.into_inner());
         *guard = Some(backend);
         let mut id_guard = self
@@ -90,6 +124,14 @@ impl LocalEngine {
         guard.is_some()
     }
 
+    /// Capabilities of the currently-loaded backend. Returns `None`
+    /// when nothing is loaded. Callers (live capture WAV writer, #19)
+    /// read sample_rate from here.
+    pub fn capabilities(&self) -> Option<TranscriberCapabilities> {
+        let guard = self.engine.lock().unwrap_or_else(|e| e.into_inner());
+        guard.as_ref().map(|b| b.capabilities())
+    }
+
     /// Run transcription synchronously with timing.
     /// Called from within spawn_blocking.
     pub fn transcribe_sync(
@@ -101,32 +143,7 @@ impl LocalEngine {
         let backend = guard.as_mut().ok_or(KonError::EngineNotLoaded)?;
 
         let start = Instant::now();
-        let segments: Vec<Segment> = match backend {
+        let segments = backend.transcribe_sync(audio.samples(), options)?;
-            SpeechBackend::Adapter(model) => {
-                let opts = TranscribeOptions {
-                    language: options.language.clone(),
-                    translate: false,
-                    leading_silence_ms: None,
-                    trailing_silence_ms: None,
-                };
-                let result: TranscriptionResult = model
-                    .transcribe(audio.samples(), &opts)
-                    .map_err(|e| KonError::TranscriptionFailed(e.to_string()))?;
-                result
-                    .segments
-                    .unwrap_or_default()
-                    .into_iter()
-                    .map(|s| Segment {
-                        start: s.start as f64,
-                        end: s.end as f64,
-                        text: s.text,
-                    })
-                    .collect()
-            }
-            SpeechBackend::WhisperRs(w) => w
-                .transcribe_sync(audio.samples(), options)
-                .map_err(|e| KonError::TranscriptionFailed(e.to_string()))?,
-        };
         let inference_ms = start.elapsed().as_millis() as u64;
 
         Ok(TimedTranscript {
@@ -177,20 +194,21 @@ impl transcribe_rs::SpeechModel for ParakeetWordGranularity {
 }
 
 /// Load a Parakeet model from a directory path.
-pub fn load_parakeet(model_dir: &Path) -> Result<SpeechBackend> {
+pub fn load_parakeet(model_dir: &Path) -> Result<Box<dyn Transcriber + Send>> {
     use transcribe_rs::onnx::Quantization;
     let model = transcribe_rs::onnx::parakeet::ParakeetModel::load(model_dir, &Quantization::Int8)
         .map_err(|e| KonError::TranscriptionFailed(format!("Failed to load Parakeet: {e}")))?;
-    Ok(SpeechBackend::Adapter(Box::new(ParakeetWordGranularity(
+    Ok(Box::new(SpeechModelAdapter(Box::new(
-        model,
+        ParakeetWordGranularity(model),
     ))))
 }
 
 /// Load a Whisper model from a GGML file path via whisper-rs.
-pub fn load_whisper(model_path: &Path) -> Result<SpeechBackend> {
+#[cfg(feature = "whisper")]
+pub fn load_whisper(model_path: &Path) -> Result<Box<dyn Transcriber + Send>> {
     let backend = WhisperRsBackend::load(model_path)
         .map_err(|e| KonError::TranscriptionFailed(format!("Failed to load Whisper: {e}")))?;
-    Ok(SpeechBackend::WhisperRs(backend))
+    Ok(Box::new(backend))
 }
 
 #[cfg(test)]
@@ -202,5 +220,6 @@ mod tests {
         let engine = LocalEngine::new(EngineName::new("test"));
         assert!(!engine.is_loaded());
         assert!(engine.loaded_model_id().is_none());
+        assert!(engine.capabilities().is_none());
     }
 }

crates/transcription/src/transcriber.rs

@@ -0,0 +1,61 @@
+//! Engine-abstraction trait for speech-to-text backends.
+//!
+//! Replaces the previous `SpeechBackend` enum so new backends
+//! (Moonshine, whisper-rs forks, cloud ASR shims, Windows non-AVX2
+//! fallbacks) can drop in without adding a match arm in `LocalEngine`.
+//!
+//! Concrete implementers today: `SpeechModelAdapter` (wraps any
+//! `transcribe-rs` model, currently used for Parakeet) and — behind the
+//! `whisper` feature — `WhisperRsBackend` (direct whisper-rs, the only
+//! path that pipes `initial_prompt`).
+
+use kon_core::error::Result;
+use kon_core::types::{Segment, TranscriptionOptions};
+
+/// Static capabilities a `Transcriber` advertises to callers.
+///
+/// `sample_rate` is load-bearing for the progressive WAV writer (#19)
+/// which writes live capture samples to disk at the transcriber's
+/// native rate. `supports_initial_prompt` lets the Settings surface
+/// hide the initial-prompt field for backends that ignore it (Parakeet
+/// today).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct TranscriberCapabilities {
+    pub sample_rate: u32,
+    pub channels: u16,
+    pub supports_initial_prompt: bool,
+}
+
+/// Unified interface for speech-to-text backends.
+///
+/// `Send` is a supertrait so `Box<dyn Transcriber + Send>` travels
+/// across `spawn_blocking` boundaries without a per-site bound. All
+/// inference is synchronous — async callers wrap a `tokio::spawn_blocking`
+/// around `transcribe_sync`.
+pub trait Transcriber: Send {
+    fn capabilities(&self) -> TranscriberCapabilities;
+
+    /// Synchronously transcribe 16 kHz mono f32 PCM (or whatever the
+    /// backend's `capabilities().sample_rate` declares). `&mut self` so
+    /// backends that keep per-call scratch state (whisper-rs's
+    /// `WhisperState`, Parakeet's decoder buffers) can mutate them
+    /// without interior-mutability gymnastics.
+    fn transcribe_sync(
+        &mut self,
+        samples: &[f32],
+        options: &TranscriptionOptions,
+    ) -> Result<Vec<Segment>>;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn transcriber_trait_is_object_safe() {
+        // Compile-time witness: if the trait stops being object-safe
+        // (e.g. someone adds a generic method or a Self-returning
+        // method) this declaration fails to build. No runtime work.
+        let _: Option<Box<dyn Transcriber + Send>> = None;
+    }
+}

crates/transcription/src/whisper_rs_backend.rs

@@ -10,8 +10,11 @@ use std::path::Path;
 
 use whisper_rs::{FullParams, SamplingStrategy, WhisperContext, WhisperContextParameters};
 
+use kon_core::error::{KonError, Result};
 use kon_core::types::{Segment, TranscriptionOptions};
 
+use crate::transcriber::{Transcriber, TranscriberCapabilities};
+
 #[derive(Debug, thiserror::Error)]
 pub enum WhisperBackendError {
     #[error("whisper-rs load failed: {0}")]
@@ -27,20 +30,32 @@ pub struct WhisperRsBackend {
 }
 
 impl WhisperRsBackend {
-    pub fn load(model_path: &Path) -> Result<Self, WhisperBackendError> {
+    pub fn load(model_path: &Path) -> std::result::Result<Self, WhisperBackendError> {
         let ctx = WhisperContext::new_with_params(model_path, WhisperContextParameters::default())
             .map_err(|e| WhisperBackendError::Load(e.to_string()))?;
         Ok(Self { ctx })
     }
+}
+
+impl Transcriber for WhisperRsBackend {
+    fn capabilities(&self) -> TranscriberCapabilities {
+        TranscriberCapabilities {
+            sample_rate: kon_core::constants::WHISPER_SAMPLE_RATE,
+            channels: 1,
+            supports_initial_prompt: true,
+        }
+    }
 
     /// Synchronously transcribe 16 kHz mono f32 PCM.
     ///
-    /// `options.initial_prompt` is piped directly to whisper-rs.
+    /// `options.initial_prompt` is piped directly to whisper-rs — this
-    pub fn transcribe_sync(
+    /// is the only backend path that honours it; `SpeechModelAdapter`
-        &self,
+    /// discards it (Parakeet has no equivalent).
+    fn transcribe_sync(
+        &mut self,
         samples: &[f32],
         options: &TranscriptionOptions,
-    ) -> Result<Vec<Segment>, WhisperBackendError> {
+    ) -> Result<Vec<Segment>> {
         tracing::info!(
             language = ?options.language,
             has_initial_prompt = options.initial_prompt.as_deref().map(|p| !p.is_empty()).unwrap_or(false),
@@ -50,7 +65,7 @@ impl WhisperRsBackend {
         let mut state = self
             .ctx
             .create_state()
-            .map_err(|e| WhisperBackendError::State(e.to_string()))?;
+            .map_err(|e| KonError::TranscriptionFailed(WhisperBackendError::State(e.to_string()).to_string()))?;
 
         let mut params = FullParams::new(SamplingStrategy::Greedy { best_of: 1 });
         if let Some(lang) = options.language.as_deref() {
@@ -70,7 +85,7 @@ impl WhisperRsBackend {
 
         state
             .full(params, samples)
-            .map_err(|e| WhisperBackendError::Transcribe(e.to_string()))?;
+            .map_err(|e| KonError::TranscriptionFailed(WhisperBackendError::Transcribe(e.to_string()).to_string()))?;
 
         let n = state.full_n_segments();
 
@@ -81,7 +96,7 @@ impl WhisperRsBackend {
             };
             let text = seg
                 .to_str()
-                .map_err(|e| WhisperBackendError::Transcribe(e.to_string()))?
+                .map_err(|e| KonError::TranscriptionFailed(WhisperBackendError::Transcribe(e.to_string()).to_string()))?
                 .to_string();
             // whisper-rs timestamps are centiseconds (10ms units). Convert to seconds (f64).
             let start = seg.start_timestamp() as f64 * 0.01;

src-tauri/src/commands/models.rs

@@ -9,7 +9,7 @@ use kon_core::hardware::{self, CpuFeatures};
 use kon_core::model_registry::{self, Engine, LanguageSupport, ModelEntry};
 use kon_core::types::{AudioSamples, ModelId, TranscriptionOptions};
 use kon_transcription::model_manager;
-use kon_transcription::{load_parakeet, load_whisper, LocalEngine};
+use kon_transcription::{load_parakeet, load_whisper, LocalEngine, Transcriber};
 
 /// Map legacy size strings to ModelId.
 fn whisper_model_id(size: &str) -> ModelId {
@@ -73,7 +73,7 @@ fn model_capability(
 
 pub fn load_model_from_disk(
     model_id: &ModelId,
-) -> Result<kon_transcription::SpeechBackend, String> {
+) -> Result<Box<dyn Transcriber + Send>, String> {
     let entry =
         model_registry::find_model(model_id).ok_or_else(|| format!("Unknown model: {model_id}"))?;