diff --git a/apple/macos-helper/Sources/AgentDeviceMacOSHelper/AudioProbe.swift b/apple/macos-helper/Sources/AgentDeviceMacOSHelper/AudioProbe.swift
index dcf74b298e..10660a8cc8 100644
--- a/apple/macos-helper/Sources/AgentDeviceMacOSHelper/AudioProbe.swift
+++ b/apple/macos-helper/Sources/AgentDeviceMacOSHelper/AudioProbe.swift
@@ -31,7 +31,7 @@ private struct AudioProbeBucket {
   var peak: Double = 0
 }
 
-private final class AudioProbeStatusWriter {
+final class AudioProbeStatusWriter {
   private let outPath: String
   private let startedAt = Date()
   private let durationMs: Int
@@ -61,6 +61,9 @@ private final class AudioProbeStatusWriter {
     else {
       return
     }
+    // AVAudioPCMBuffer starts with frameLength 0, so every mDataByteSize is 0 until
+    // frameLength is set. Copying first leaves the destination empty (-12731).
+    pcmBuffer.frameLength = frameCount
     let status = CMSampleBufferCopyPCMDataIntoAudioBufferList(
       sampleBuffer,
       at: 0,
@@ -70,7 +73,6 @@ private final class AudioProbeStatusWriter {
     guard status == noErr else {
       return
     }
-    pcmBuffer.frameLength = frameCount
     add(audioBufferList: pcmBuffer.mutableAudioBufferList, format: format)
   }
 
diff --git a/apple/macos-helper/Tests/AgentDeviceMacOSHelperTests/AudioProbeTests.swift b/apple/macos-helper/Tests/AgentDeviceMacOSHelperTests/AudioProbeTests.swift
new file mode 100644
index 0000000000..054575a685
--- /dev/null
+++ b/apple/macos-helper/Tests/AgentDeviceMacOSHelperTests/AudioProbeTests.swift
@@ -0,0 +1,127 @@
+import AVFoundation
+import CoreMedia
+import XCTest
+
+@testable import AgentDeviceMacOSHelper
+
+/// AudioProbe must set AVAudioPCMBuffer.frameLength before the PCM copy.
+final class AudioProbeTests: XCTestCase {
+  func testAddSampleBufferRecordsANonSilentPeakFromRealPCM() throws {
+    let dir = FileManager.default.temporaryDirectory.appendingPathComponent(
+      "audio-probe-\(UUID().uuidString)", isDirectory: true)
+    try FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
+    defer { try? FileManager.default.removeItem(at: dir) }
+
+    let outPath = dir.appendingPathComponent("probe.json").path
+    let writer = AudioProbeStatusWriter(outPath: outPath, durationMs: 1_000, bucketMs: 100)
+    writer.add(sampleBuffer: try makeFloatPCMSampleBuffer(frames: 512, amplitude: 0.5))
+    try writer.finish(reason: "test")
+
+    let response = try JSONDecoder().decode(
+      AudioProbeResponse.self, from: Data(contentsOf: URL(fileURLWithPath: outPath)))
+    XCTAssertTrue(response.heard, "a 0.5 amplitude buffer must clear the silence floor")
+    XCTAssertFalse(response.peakDbfs.isEmpty)
+    XCTAssertGreaterThan(response.peakDbfs.max() ?? -90, -90)
+  }
+}
+
+private func makeFloatPCMSampleBuffer(frames: Int, amplitude: Float) throws -> CMSampleBuffer {
+  var asbd = AudioStreamBasicDescription(
+    mSampleRate: 48_000,
+    mFormatID: kAudioFormatLinearPCM,
+    mFormatFlags: kAudioFormatFlagIsFloat | kAudioFormatFlagIsPacked
+      | kAudioFormatFlagIsNonInterleaved,
+    mBytesPerPacket: UInt32(MemoryLayout<Float>.size),
+    mFramesPerPacket: 1,
+    mBytesPerFrame: UInt32(MemoryLayout<Float>.size),
+    mChannelsPerFrame: 1,
+    mBitsPerChannel: UInt32(MemoryLayout<Float>.size * 8),
+    mReserved: 0
+  )
+
+  var formatDescription: CMAudioFormatDescription?
+  let formatStatus = CMAudioFormatDescriptionCreate(
+    allocator: kCFAllocatorDefault,
+    asbd: &asbd,
+    layoutSize: 0,
+    layout: nil,
+    magicCookieSize: 0,
+    magicCookie: nil,
+    extensions: nil,
+    formatDescriptionOut: &formatDescription
+  )
+  guard formatStatus == noErr, let formatDescription else {
+    throw NSError(
+      domain: "AudioProbeTests", code: Int(formatStatus),
+      userInfo: [NSLocalizedDescriptionKey: "CMAudioFormatDescriptionCreate failed"])
+  }
+
+  let byteCount = frames * MemoryLayout<Float>.size
+  var blockBuffer: CMBlockBuffer?
+  let blockStatus = CMBlockBufferCreateWithMemoryBlock(
+    allocator: kCFAllocatorDefault,
+    memoryBlock: nil,
+    blockLength: byteCount,
+    blockAllocator: nil,
+    customBlockSource: nil,
+    offsetToData: 0,
+    dataLength: byteCount,
+    flags: 0,
+    blockBufferOut: &blockBuffer
+  )
+  guard blockStatus == noErr, let blockBuffer else {
+    throw NSError(
+      domain: "AudioProbeTests", code: Int(blockStatus),
+      userInfo: [NSLocalizedDescriptionKey: "CMBlockBufferCreateWithMemoryBlock failed"])
+  }
+  let assureStatus = CMBlockBufferAssureBlockMemory(blockBuffer)
+  guard assureStatus == noErr else {
+    throw NSError(
+      domain: "AudioProbeTests", code: Int(assureStatus),
+      userInfo: [NSLocalizedDescriptionKey: "CMBlockBufferAssureBlockMemory failed"])
+  }
+
+  var lengthAtOffset = 0
+  var totalLength = 0
+  var dataPointer: UnsafeMutablePointer<Int8>?
+  let pointerStatus = CMBlockBufferGetDataPointer(
+    blockBuffer,
+    atOffset: 0,
+    lengthAtOffsetOut: &lengthAtOffset,
+    totalLengthOut: &totalLength,
+    dataPointerOut: &dataPointer
+  )
+  guard pointerStatus == noErr, let dataPointer else {
+    throw NSError(
+      domain: "AudioProbeTests", code: Int(pointerStatus),
+      userInfo: [NSLocalizedDescriptionKey: "CMBlockBufferGetDataPointer failed"])
+  }
+  let floatPointer = UnsafeMutableRawPointer(dataPointer).bindMemory(to: Float.self, capacity: frames)
+  for index in 0..<frames {
+    floatPointer[index] = amplitude
+  }
+
+  var timing = CMSampleTimingInfo(
+    duration: CMTime(value: 1, timescale: 48_000),
+    presentationTimeStamp: .zero,
+    decodeTimeStamp: .invalid
+  )
+  var sampleBuffer: CMSampleBuffer?
+  let sampleStatus = CMSampleBufferCreateReady(
+    allocator: kCFAllocatorDefault,
+    dataBuffer: blockBuffer,
+    formatDescription: formatDescription,
+    sampleCount: frames,
+    sampleTimingEntryCount: 1,
+    sampleTimingArray: &timing,
+    sampleSizeEntryCount: 0,
+    sampleSizeArray: nil,
+    sampleBufferOut: &sampleBuffer
+  )
+  guard sampleStatus == noErr, let sampleBuffer else {
+    throw NSError(
+      domain: "AudioProbeTests", code: Int(sampleStatus),
+      userInfo: [NSLocalizedDescriptionKey: "CMSampleBufferCreateReady failed"])
+  }
+  return sampleBuffer
+}