Start Migration to Swift 6

AGENTS.md

@@ -37,6 +37,102 @@ swift format --in-place --recursive --configuration .swift-format Sources/ Tests
 - Thread safety: Use actors, `@MainActor`, or proper locking - never `@unchecked Sendable`
 - Control flow: Prefer flattened if statements with early returns/continues over nested if statements. Use guard statements and inverted conditions to exit early. Nested if statements should be absolutely avoided.
 
+## Swift 6 Concurrency Migration
+
+### Common Warning Patterns & Fixes
+
+#### 1. Non-Sendable Struct Types
+When a struct is used across actor boundaries or in concurrent contexts, add `Sendable` conformance:
+
+```swift
+// Before
+struct MyConfig {
+    let timeout: Int
+}
+
+// After
+struct MyConfig: Sendable {
+    let timeout: Int
+}
+```
+
+**Examples from codebase:**
+- `TdtDecoderState` (ASR/TDT/TdtDecoderState.swift) - LSTM state management
+- `AssignmentConfig` (Diarizer/Clustering/SpeakerOperations.swift) - speaker assignment config
+- `DownloadConfig` (DownloadUtils.swift) - download timeout settings
+
+#### 2. Function Type Aliases
+Closure types that cross actor boundaries must be marked `@Sendable`:
+
+```swift
+// Before
+public typealias DataWriter = (Data, URL) throws -> Void
+
+// After
+public typealias DataWriter = @Sendable (Data, URL) throws -> Void
+```
+
+**Applied to:** AssetDownloader.swift - `DataWriter` and `FileMover` typealias
+
+#### 3. Singleton/Shared Static Properties
+Global actor isolation for non-Sendable classes:
+
+```swift
+// Before
+static let shared = EspeakG2P()
+
+// After
+@MainActor static let shared = EspeakG2P()
+```
+
+**Applied to:** EspeakG2P.swift - eSpeak NG wrapper singleton
+
+#### 4. Mutable Global State (#MutableGlobalVariable)
+Mutable static properties require proper synchronization or actor isolation. Options:
+
+**Option A: Use @MainActor for entire type**
+```swift
+@MainActor
+class MyService {
+    static var cache: [String: Data] = [:]
+    static let cacheLock = NSLock()
+}
+```
+
+**Option B: Use actor for concurrent access**
+```swift
+actor CacheManager {
+    private var cache: [String: Data] = [:]
+
+    func set(_ key: String, _ value: Data) {
+        cache[key] = value
+    }
+
+    func get(_ key: String) -> Data? {
+        cache[key]
+    }
+}
+```
+
+**Current cases:** KokoroSynthesizer.swift - voiceEmbeddingPayloads, voiceEmbeddingVectors
+
+#### 5. Non-Sendable Framework Types (MLMultiArray)
+CoreML's `MLMultiArray` doesn't conform to Sendable. When passing across actor boundaries:
+- Wrap in a Sendable struct/class
+- Use `@MainActor` for related processing
+- Create separate Sendable representations for cross-actor data
+
+### Migration Checklist
+
+When fixing concurrency warnings:
+1. Run `swift build` and capture full warning output
+2. Identify warning categories (Sendable, @MainActor, #MutableGlobalVariable, etc.)
+3. Start with "low-hanging fruit": simple struct/typealias additions
+4. Address mutable state with proper synchronization
+5. Handle framework non-Sendable types last (most complex)
+6. Run `swift build` again to verify warning reduction
+7. Never use `@unchecked Sendable` as a shortcut
+
 ## Clean code
 
 - When adding new interfaces, make sure that the API is consistent with the other model managers

CLAUDE.md

@@ -232,6 +232,9 @@ FluidAudio/
 - **Persistent States**: Decoder states maintained across chunks for streaming
 - **Memory Management**: Automatic cleanup and ANE optimization
 - **Parallel Processing**: Multi-stream support for batch operations
+- **Swift 6 Strict Concurrency Migration** (in progress):
+  - **Completed**: TdtDecoderState, AssignmentConfig, DownloadConfig, AssetDownloader, EspeakG2P
+  - **Remaining**: KokoroSynthesizer mutable state, MLMultiArray non-Sendable issues
 
 ### Model Management
 - **Automatic Downloads**: Models fetched from HuggingFace on first use
@@ -297,11 +300,31 @@ The project uses GitHub Actions with the following workflows:
 9. **Git Operations**: NEVER run `git push` unless explicitly requested by the user. Only commit when asked.
 10. **Code Formatting**: All code must pass swift-format checks before merge
 
+## Swift 6 Concurrency Migration Status
+
+### Completed Fixes
+- **TdtDecoderState** (ASR/TDT/TdtDecoderState.swift): Added `Sendable` conformance for LSTM state across actor boundaries
+- **AssignmentConfig** (Diarizer/Clustering/SpeakerOperations.swift): Added `Sendable` conformance for clustering configuration
+- **DownloadConfig** (DownloadUtils.swift): Added `Sendable` conformance for download settings
+- **AssetDownloader** (Shared/AssetDownloader.swift): Marked `DataWriter` and `FileMover` typealias as `@Sendable`
+- **EspeakG2P** (TextToSpeech/Kokoro/Assets/Lexicon/EspeakG2P.swift): Added `@MainActor` to shared singleton
+
+### Remaining Work
+- **KokoroSynthesizer** (TextToSpeech/Kokoro/Pipeline/Synthesize/KokoroSynthesizer.swift):
+  - Mutable static state: `voiceEmbeddingPayloads`, `voiceEmbeddingVectors` (lines 58-59)
+  - Requires `@MainActor` annotation or actor-based refactoring
+  - Multiple MLMultiArray sending warnings (framework limitation)
+
+### Framework Limitations
+- **MLMultiArray** (CoreML): Does not conform to Sendable - wrap in Sendable types for cross-actor use
+- **Non-Sendable Types**: Create Sendable wrapper structs when passing non-Sendable data across actor boundaries
+
 ## Next Steps
 
-1. **Multi-file validation**: Test optimal config on all AMI files
-2. **Real-world testing**: Validate on non-AMI audio
-3. **Documentation**: Update API documentation
+1. **Complete Swift 6 Migration**: Finish KokoroSynthesizer mutable state handling
+2. **Multi-file validation**: Test optimal config on all AMI files
+3. **Real-world testing**: Validate on non-AMI audio
+4. **Documentation**: Update API documentation
 
 ## Testing Strategy
 

Package.swift

@@ -30,7 +30,10 @@ let package = Package(
                 "FastClusterWrapper",
             ],
             path: "Sources/FluidAudio",
-            exclude: ["Frameworks"]
+            exclude: ["Frameworks"],
+            swiftSettings: [
+                .enableUpcomingFeature("StrictConcurrency"),
+            ]
         ),
         .target(
             name: "FastClusterWrapper",
@@ -48,7 +51,10 @@ let package = Package(
         ),
         .testTarget(
             name: "FluidAudioTests",
-            dependencies: ["FluidAudio"]
+            dependencies: ["FluidAudio"],
+            swiftSettings: [
+                .enableUpcomingFeature("StrictConcurrency"),
+            ]
         ),
     ],
     cxxLanguageStandard: .cxx17

Sources/FluidAudio/ASR/AsrManager.swift

@@ -1,4 +1,4 @@
-import AVFoundation
+@preconcurrency import AVFoundation
 import CoreML
 import Foundation
 import OSLog

Sources/FluidAudio/ASR/Streaming/StreamingAsrManager.swift

@@ -1,4 +1,4 @@
-import AVFoundation
+@preconcurrency import AVFoundation
 import Foundation
 import OSLog
 

Sources/FluidAudio/ASR/TDT/TdtDecoderState.swift

@@ -3,7 +3,7 @@ import CoreML
 import Foundation
 
 /// Manages LSTM hidden and cell states for the Parakeet decoder
-struct TdtDecoderState {
+struct TdtDecoderState: Sendable {
     var hiddenState: MLMultiArray
     var cellState: MLMultiArray
     /// Stores the last decoded token from the previous audio chunk.

Sources/FluidAudio/Diarizer/Clustering/SpeakerOperations.swift

@@ -13,7 +13,7 @@ public enum SpeakerUtilities {
     // MARK: - Configuration
 
     /// Platform-specific configuration for speaker assignment
-    public struct AssignmentConfig {
+    public struct AssignmentConfig: Sendable {
         public let maxDistanceForAssignment: Float
         public let maxDistanceForUpdate: Float
         public let minSpeakerDuration: Float
@@ -128,7 +128,7 @@ public enum SpeakerUtilities {
     // MARK: - Speaker Assignment Decision
 
     /// Decision result for speaker assignment
-    public struct AssignmentDecision {
+    public struct AssignmentDecision: Sendable {
         public let shouldAssign: Bool
         public let shouldUpdate: Bool
         public let confidence: Float
@@ -199,7 +199,7 @@ public enum SpeakerUtilities {
     // MARK: - Speaker Creation
 
     /// Validated speaker creation parameters
-    public struct SpeakerCreationParams {
+    public struct SpeakerCreationParams: Sendable {
         public let id: String
         public let name: String
         public let duration: Float
@@ -379,7 +379,7 @@ public enum SpeakerUtilities {
     // MARK: - Complete Speaker Update Operations
 
     /// Complete speaker update operation including raw tracking
-    public struct SpeakerUpdateResult {
+    public struct SpeakerUpdateResult: Sendable {
         public let updatedMainEmbedding: [Float]?
         public let updatedRawEmbeddings: [RawEmbedding]
         public let updatedDuration: Float

Sources/FluidAudio/DownloadUtils.swift

@@ -248,7 +248,7 @@ public class DownloadUtils {
     public typealias ProgressHandler = (Double) -> Void
 
     /// Download configuration
-    public struct DownloadConfig {
+    public struct DownloadConfig: Sendable {
         public let timeout: TimeInterval
 
         public init(timeout: TimeInterval = 1800) {  // 30 minutes for large models

Sources/FluidAudio/FluidAudioSwift.swift

@@ -26,6 +26,6 @@ public typealias SpeakerDiarizationError = DiarizerError
 // Types like RawEmbedding and SendableSpeaker are already public in their respective files
 // and will be available when importing FluidAudio module
 
-public struct FluidAudio {
+public struct FluidAudio: Sendable {
     // Empty struct for namespace - all functionality is in the module's types
 }

Sources/FluidAudio/Shared/AppLogger.swift

@@ -3,12 +3,12 @@ import OSLog
 
 /// Lightweight logger that writes to Unified Logging and, optionally, to console.
 /// Use this instead of `OSLog.Logger` so CLI runs can surface logs without `print`.
-public struct AppLogger {
+public struct AppLogger: Sendable {
     /// Default subsystem for all loggers in FluidAudio.
     /// Keep this consistent; categories should vary per component.
-    public static var defaultSubsystem: String = "com.fluidinference"
+    public static let defaultSubsystem: String = "com.fluidinference"
 
-    public enum Level: Int {
+    public enum Level: Int, Sendable {
         case debug = 0
         case info
         case notice
@@ -82,6 +82,9 @@ public struct AppLogger {
     }
 
     private func logToConsole(_ level: Level, _ message: String) {
+        let level = level
+        let category = category
+        let message = message
         Task.detached(priority: .utility) {
             await LogConsole.shared.write(level: level, category: category, message: message)
         }

Sources/FluidAudio/Shared/AssetDownloader.swift

@@ -5,8 +5,8 @@ import OSLog
 /// Provides reusable logic for simple file/data transfers used across FluidAudio modules.
 public enum AssetDownloader {
 
-    public typealias DataWriter = (Data, URL) throws -> Void
-    public typealias FileMover = (URL, URL) throws -> Void
+    public typealias DataWriter = @Sendable (Data, URL) throws -> Void
+    public typealias FileMover = @Sendable (URL, URL) throws -> Void
 
     public static let defaultDataWriter: DataWriter = { data, destination in
         try data.write(to: destination, options: [.atomic])
@@ -19,12 +19,12 @@ public enum AssetDownloader {
         try FileManager.default.moveItem(at: tempURL, to: destination)
     }
 
-    public enum TransferMode {
+    public enum TransferMode: Sendable {
         case data(DataWriter = AssetDownloader.defaultDataWriter)
         case file(FileMover = AssetDownloader.defaultFileMover)
     }
 
-    public struct Descriptor {
+    public struct Descriptor: Sendable {
         public let description: String
         public let remoteURL: URL
         public let destinationURL: URL