diff --git a/packages/Bus/Sources/Bus/BusOutbound.swift b/packages/Bus/Sources/Bus/BusOutbound.swift
index 3bbbd2b..883e076 100644
--- a/packages/Bus/Sources/Bus/BusOutbound.swift
+++ b/packages/Bus/Sources/Bus/BusOutbound.swift
@@ -159,7 +159,20 @@ extension BusOutbound: Codable {
             try container.encode(text, forKey: .text)
         case .audioLevel(let rms):
             try container.encode(Discriminator.audioLevel, forKey: .type)
-            try container.encode(rms, forKey: .rms)
+            // Issue #47: JSON has no canonical encoding for `Float.nan` or
+            // `±Float.infinity`. Foundation's `JSONEncoder` defaults to
+            // throwing for non-finite floats (`JSONEncoder.NonConformingFloat-
+            // EncodingStrategy.throw`), which would crash the per-frame bus
+            // path the moment `AudioLevelEmitter` produces a NaN — easy to
+            // hit on a CoreAudio over-/underflow or a buffer of all-zero
+            // samples. Clamp non-finite to 0.0 inline at the encode site so
+            // the rms float in the wire JSON is always a finite, ring-shader-
+            // safe number. The HUD-side consumer (`webview/packages/hud/src/
+            // bus/client.ts`) currently no-ops on audioLevel; the clamp is
+            // defensive against the bug becoming active once the consumer is
+            // wired up.
+            let safe = rms.isFinite ? rms : 0.0
+            try container.encode(safe, forKey: .rms)
         case .toolCallStart(let id, let name, let argsPreview):
             try container.encode(Discriminator.toolCallStart, forKey: .type)
             try container.encode(id.uuidString.lowercased(), forKey: .id)
diff --git a/packages/Bus/Tests/BusTests/CodableRoundTripTests.swift b/packages/Bus/Tests/BusTests/CodableRoundTripTests.swift
index 66b3fdf..6a6fc3f 100644
--- a/packages/Bus/Tests/BusTests/CodableRoundTripTests.swift
+++ b/packages/Bus/Tests/BusTests/CodableRoundTripTests.swift
@@ -123,6 +123,44 @@ final class CodableRoundTripTests: XCTestCase {
         )
     }
 
+    /// Issue #47: `BusOutbound.audioLevel` encodes a `Float`, and Foundation's
+    /// default `JSONEncoder.NonConformingFloatEncodingStrategy` is `.throw`.
+    /// A non-finite `rms` (NaN from a CoreAudio over-/underflow, or ±∞ from a
+    /// pathological emitter buffer) would crash the per-frame bus path inside
+    /// `WebviewBridge.sendRaw`. The encode site in `BusOutbound.swift` clamps
+    /// non-finite floats to `0.0`; this test pins the contract so the clamp
+    /// can't silently regress.
+    func test_audioLevel_nanIsClampedToZero() throws {
+        let encoded = try BusCoder.makeEncoder().encode(BusOutbound.audioLevel(rms: .nan))
+        let decoded = try BusCoder.makeDecoder().decode(BusOutbound.self, from: encoded)
+        guard case .audioLevel(let rms) = decoded else {
+            XCTFail("expected .audioLevel after round-trip, got \(decoded)")
+            return
+        }
+        XCTAssertEqual(rms, 0.0, "NaN rms must be clamped to 0.0 at encode time (#47)")
+        XCTAssertTrue(rms.isFinite, "post-clamp rms must be finite")
+    }
+
+    /// Companion to `test_audioLevel_nanIsClampedToZero` — covers the ±∞
+    /// edge cases. CoreAudio drivers don't normally emit infinities, but the
+    /// encode-time clamp is sourced from `Float.isFinite`, which covers both
+    /// NaN and ±∞ via one check; locking the contract here prevents a future
+    /// refactor from narrowing the guard to NaN-only.
+    func test_audioLevel_infinityIsClampedToZero() throws {
+        let encoder = BusCoder.makeEncoder()
+        let decoder = BusCoder.makeDecoder()
+
+        for value in [Float.infinity, -Float.infinity] {
+            let encoded = try encoder.encode(BusOutbound.audioLevel(rms: value))
+            let decoded = try decoder.decode(BusOutbound.self, from: encoded)
+            guard case .audioLevel(let rms) = decoded else {
+                XCTFail("expected .audioLevel for \(value)")
+                continue
+            }
+            XCTAssertEqual(rms, 0.0, "±∞ rms (\(value)) must be clamped to 0.0 (#47)")
+        }
+    }
+
     func test_roundTrip_toolCallStart() throws {
         let id = UUID(uuidString: "550e8400-e29b-41d4-a716-446655440000")!
         try assertOutboundRoundTrips(
diff --git a/packages/Voice/Sources/Voice/STT/STTBackendSelector.swift b/packages/Voice/Sources/Voice/STT/STTBackendSelector.swift
index d17bb9a..e573fb9 100644
--- a/packages/Voice/Sources/Voice/STT/STTBackendSelector.swift
+++ b/packages/Voice/Sources/Voice/STT/STTBackendSelector.swift
@@ -28,6 +28,16 @@ public protocol STTWarningLogger: Sendable {
 public enum STTBackendSelector {
 
     // MARK: - Valid backend strings
+    //
+    // Canonical backend identifiers. Snake_case is the wire shape stored in
+    // `PerTurnSnapshot.stt.backend`. Call sites (`AppDelegate.sttBackend`)
+    // SHOULD reference these constants rather than the bare string literals;
+    // `STTBackendSwitchTests.testB5_canonicalConstantsRoundTripThroughMake`
+    // locks the round-trip contract by asserting both that the constants
+    // map to the right `STTProvider` AND that camelCase string literals
+    // (the pre-2026-05-12 anti-pattern from Issue #29 / #37) fall through
+    // to the default branch with a warning. Do not rename without updating
+    // `PerTurnSnapshot` and the regression test in lockstep.
 
     public static let backendSpeechAnalyzer = "speech_analyzer"
     public static let backendWhisperKit = "whisperkit"