diff --git a/Sources/SstCore/AppGlue.swift b/Sources/SstCore/AppGlue.swift
new file mode 100644
index 0000000..51c3cdb
--- /dev/null
+++ b/Sources/SstCore/AppGlue.swift
@@ -0,0 +1,68 @@
+import Foundation
+
+/// Inserts text only while `shouldInsert()` says so (not when Sst itself is in front, nor for window recordings).
+public struct GatedInserter: TextInserting {
+    private let inner: TextInserting
+    private let shouldInsert: () -> Bool
+
+    public init(inner: TextInserting, shouldInsert: @escaping () -> Bool) {
+        self.inner = inner
+        self.shouldInsert = shouldInsert
+    }
+
+    public func insert(_ text: String) async throws {
+        guard shouldInsert() else { return }
+        try await inner.insert(text)
+    }
+}
+
+/// Lets the app replace the speech model (for example after a model switch) without rebuilding the controller.
+public final class SwappableTranscriber: Transcriber, @unchecked Sendable {
+    public enum SwapError: Error, LocalizedError {
+        case noTranscriber
+
+        public var errorDescription: String? { "Spraakmodel is nog niet geladen." }
+    }
+
+    private let lock = NSLock()
+    private var current: Transcriber?
+
+    public init(_ initial: Transcriber? = nil) {
+        current = initial
+    }
+
+    public func swap(to transcriber: Transcriber?) {
+        lock.withLock { current = transcriber }
+    }
+
+    public var hasTranscriber: Bool {
+        lock.withLock { current != nil }
+    }
+
+    public func transcribe(_ samples: [Float]) async throws -> String {
+        try await transcribe(samples, vocabulary: [])
+    }
+
+    public func transcribe(_ samples: [Float], vocabulary: [String]) async throws -> String {
+        guard let transcriber = lock.withLock({ current }) else { throw SwapError.noTranscriber }
+        return try await transcriber.transcribe(samples, vocabulary: vocabulary)
+    }
+}
+
+/// The latest input level, written from the audio thread and read by the meter once per frame.
+public final class LevelSource: @unchecked Sendable {
+    private let lock = NSLock()
+    private var value = AudioLevel.silenceFloorDbfs
+
+    public init() {}
+
+    public var dbfs: Float { lock.withLock { value } }
+
+    public func update(_ dbfs: Float) {
+        lock.withLock { value = dbfs }
+    }
+
+    public func reset() {
+        update(AudioLevel.silenceFloorDbfs)
+    }
+}
diff --git a/Tests/SstCoreTests/AppGlueTests.swift b/Tests/SstCoreTests/AppGlueTests.swift
new file mode 100644
index 0000000..ca67bc7
--- /dev/null
+++ b/Tests/SstCoreTests/AppGlueTests.swift
@@ -0,0 +1,69 @@
+import Foundation
+import Testing
+@testable import SstCore
+
+struct GatedInserterTests {
+    @Test func insertsWhenAllowed() async throws {
+        let inner = FakeInserter()
+        try await GatedInserter(inner: inner, shouldInsert: { true }).insert("hallo")
+        #expect(inner.inserted == ["hallo"])
+    }
+
+    @Test func staysSilentWhenNotAllowed() async throws {
+        let inner = FakeInserter()
+        try await GatedInserter(inner: inner, shouldInsert: { false }).insert("hallo")
+        #expect(inner.inserted.isEmpty)
+    }
+
+    @Test func askedAtInsertionTimeNotAtCreation() async throws {
+        let inner = FakeInserter()
+        var allowed = false
+        let gated = GatedInserter(inner: inner, shouldInsert: { allowed })
+        allowed = true
+        try await gated.insert("nu wel")
+        #expect(inner.inserted == ["nu wel"])
+    }
+}
+
+struct SwappableTranscriberTests {
+    @Test func throwsWithoutATranscriber() async {
+        let swappable = SwappableTranscriber()
+        #expect(!swappable.hasTranscriber)
+        await #expect(throws: SwappableTranscriber.SwapError.self) {
+            _ = try await swappable.transcribe([0])
+        }
+    }
+
+    @Test func delegatesIncludingTheVocabulary() async throws {
+        let inner = FakeVocabularyTranscriber()
+        inner.result = "klaar"
+        let swappable = SwappableTranscriber(inner)
+        #expect(try await swappable.transcribe([0], vocabulary: ["Anthropic"]) == "klaar")
+        #expect(inner.vocabularies == [["Anthropic"]])
+        #expect(try await swappable.transcribe([0]) == "klaar")
+        #expect(inner.vocabularies == [["Anthropic"], []])
+    }
+
+    @Test func swapReplacesTheTranscriber() async throws {
+        let first = FakeVocabularyTranscriber(), second = FakeVocabularyTranscriber()
+        first.result = "een"; second.result = "twee"
+        let swappable = SwappableTranscriber(first)
+        #expect(try await swappable.transcribe([0]) == "een")
+        swappable.swap(to: second)
+        #expect(try await swappable.transcribe([0]) == "twee")
+        swappable.swap(to: nil)
+        #expect(!swappable.hasTranscriber)
+    }
+}
+
+struct LevelSourceTests {
+    @Test func startsSilentAndKeepsTheLatestValue() {
+        let source = LevelSource()
+        #expect(source.dbfs == AudioLevel.silenceFloorDbfs)
+        source.update(-23)
+        source.update(-18)
+        #expect(source.dbfs == -18)
+        source.reset()
+        #expect(source.dbfs == AudioLevel.silenceFloorDbfs)
+    }
+}