diff --git a/Packages/BitMatchEngine/Tests/BitMatchEngineTests/StreamingMemoryProbeTests.swift b/Packages/BitMatchEngine/Tests/BitMatchEngineTests/StreamingMemoryProbeTests.swift
new file mode 100644
index 00000000..58cf9908
--- /dev/null
+++ b/Packages/BitMatchEngine/Tests/BitMatchEngineTests/StreamingMemoryProbeTests.swift
@@ -0,0 +1,227 @@
+import Darwin
+import Foundation
+import XCTest
+@testable import BitMatchEngine
+
+#if os(macOS)
+final class StreamingMemoryProbeTests: XCTestCase {
+    /// Opt in and run this test alone: process-footprint measurements are
+    /// meaningless when another test is allocating in the same process.
+    /// The source is sparse, but the real fan-out writer copies every byte to
+    /// one and then two disposable destinations. No verification or report
+    /// pass runs, so the samples isolate copy-phase memory.
+    func testSparse512MiBCopyKeepsMemoryBoundedWithOneAndTwoDestinations() async throws {
+        guard ProcessInfo.processInfo.environment["BITMATCH_COPY_MEMORY_TEST"] == "1" else {
+            throw XCTSkip("Set BITMATCH_COPY_MEMORY_TEST=1 and run this test in isolation")
+        }
+        let size = Int64(512 * 1_024 * 1_024)
+        let available = (try FileManager.default.attributesOfFileSystem(
+            forPath: FileManager.default.temporaryDirectory.path)[.systemFreeSize] as? NSNumber)?.int64Value ?? 0
+        guard available > size * 2 + 2 * 1_024 * 1_024 * 1_024 else {
+            throw XCTSkip("Not enough free space for disposable copy outputs")
+        }
+
+        for destinationCount in [1, 2] {
+            try await measureCopy(destinationCount: destinationCount)
+        }
+    }
+
+    /// This separate opt-in probe exercises the byte-comparison loop used by
+    /// Paranoid folder Compare. The pinned Paranoid transfer verifier uses a
+    /// different loop. Sparse inputs make a full 4 MiB-at-a-time pass without
+    /// consuming 1 GiB of disk.
+    func testSparse512MiBByteComparisonKeepsMemoryBounded() async throws {
+        guard ProcessInfo.processInfo.environment["BITMATCH_COMPARE_MEMORY_TEST"] == "1" else {
+            throw XCTSkip("Set BITMATCH_COMPARE_MEMORY_TEST=1 and run this test in isolation")
+        }
+
+        let root = FileManager.default.temporaryDirectory
+            .appendingPathComponent("bitmatch-compare-memory-\(UUID())", isDirectory: true)
+        try FileManager.default.createDirectory(at: root, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: root) }
+        let size = UInt64(512 * 1_024 * 1_024)
+        let source = root.appendingPathComponent("source.bin")
+        let destination = root.appendingPathComponent("destination.bin")
+        for file in [source, destination] {
+            XCTAssertTrue(FileManager.default.createFile(atPath: file.path, contents: nil))
+            let handle = try FileHandle(forWritingTo: file)
+            try handle.truncate(atOffset: size)
+            try handle.close()
+        }
+
+        let samples = ComparisonFootprintSamples()
+        samples.started()
+        let match = try await ChecksumEngine.shared.performByteComparison(
+            sourceURL: source, destinationURL: destination,
+            progressCallback: { _, _ in samples.sample() }
+        )
+        let measured = samples.snapshot()
+        XCTAssertTrue(match)
+        XCTAssertGreaterThanOrEqual(measured.chunks, 128)
+        XCTAssertGreaterThan(measured.initialMiB, 0)
+        XCTAssertEqual(measured.missingSamples, 0, "Could not measure physical footprint")
+        print("compare memory initialMiB=\(measured.initialMiB) peakMiB=\(measured.peakMiB) finalMiB=\(measured.finalMiB) chunks=\(measured.chunks)")
+        XCTAssertLessThan(measured.peakMiB - measured.initialMiB, 128,
+                          "Byte comparison footprint grew with two 512 MiB files")
+    }
+
+    private func measureCopy(destinationCount: Int) async throws {
+        let root = FileManager.default.temporaryDirectory
+            .appendingPathComponent("bitmatch-copy-memory-\(UUID())", isDirectory: true)
+        try FileManager.default.createDirectory(at: root, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: root) }
+
+        let size = Int64(512 * 1_024 * 1_024)
+        let source = root.appendingPathComponent("source", isDirectory: true)
+        try FileManager.default.createDirectory(at: source, withIntermediateDirectories: true)
+        let clip = source.appendingPathComponent("clip.bin")
+        XCTAssertTrue(FileManager.default.createFile(atPath: clip.path, contents: nil))
+        let writer = try FileHandle(forWritingTo: clip)
+        try writer.truncate(atOffset: UInt64(size))
+        try writer.close()
+
+        let destinations = try (0..<destinationCount).map { index -> DestinationWriter.FanOutDestination in
+            let url = root.appendingPathComponent("backup-\(index)", isDirectory: true)
+            try FileManager.default.createDirectory(at: url, withIntermediateDirectories: true)
+            return .init(index: index, root: try PinnedDestinationDirectory.open(destination: url, rootComponents: []))
+        }
+        let samples = CopyFootprintSamples(expectedSize: size)
+        let hooks = DestinationWriter.FanOutHooks(sourceDidOpen: { _ in samples.opened() })
+
+        try await DestinationWriter.copyAllSafelyFanOut(
+            from: source,
+            toPinnedRoots: destinations,
+            verificationMode: .standard,
+            workers: 1,
+            checksumService: ChecksumEngine.shared,
+            preEnumeratedFiles: [clip],
+            hooks: hooks,
+            onIOBytes: { index, phase, count, _, _ in
+                switch phase {
+                case .writing: samples.wrote(index: index, count: count)
+                case .readingBack: break
+                }
+            },
+            onProgress: { index, _, byteCount, wasReused in
+                samples.completed(index: index, byteCount: byteCount, wasReused: wasReused)
+            },
+            onError: { _, _, error in samples.failed(error) }
+        )
+
+        let measured = samples.snapshot()
+        XCTAssertTrue(measured.errors.isEmpty, "Copy failed: \(measured.errors)")
+        XCTAssertGreaterThan(measured.initialMiB, 0)
+        XCTAssertEqual(measured.missingSamples, 0, "Could not measure physical footprint")
+        XCTAssertEqual(measured.completedIndexes, Set(0..<destinationCount))
+        XCTAssertEqual(measured.writtenBytes.count, destinationCount)
+        for destination in destinations {
+            XCTAssertEqual(measured.writtenBytes[destination.index], size)
+            let copied = destination.root.logicalRootURL.appendingPathComponent("clip.bin")
+            let copiedSize = (try FileManager.default.attributesOfItem(atPath: copied.path)[.size] as? NSNumber)?.int64Value
+            XCTAssertEqual(copiedSize, size)
+        }
+        let sourceSize = (try FileManager.default.attributesOfItem(atPath: clip.path)[.size] as? NSNumber)?.int64Value
+        XCTAssertEqual(sourceSize, size)
+        print("copy memory destinations=\(destinationCount) initialMiB=\(measured.initialMiB) peakMiB=\(measured.peakMiB) finalMiB=\(measured.finalMiB) writtenBytes=\(measured.writtenBytes)")
+        XCTAssertLessThan(measured.peakMiB - measured.initialMiB, 128,
+                          "Copy footprint grew with a 512 MiB source")
+    }
+}
+
+private final class CopyFootprintSamples: @unchecked Sendable {
+    private let lock = NSLock()
+    private let expectedSize: Int64
+    private var initialMiB = 0
+    private var peakMiB = 0
+    private var writtenBytes: [Int: Int64] = [:]
+    private var completedIndexes: Set<Int> = []
+    private var errors: [String] = []
+    private var missingSamples = 0
+
+    init(expectedSize: Int64) { self.expectedSize = expectedSize }
+
+    func opened() {
+        lock.withLock {
+            if let footprint = TransferDiagnosticStore.physicalFootprintMB() {
+                initialMiB = footprint
+                peakMiB = footprint
+            } else {
+                missingSamples += 1
+            }
+        }
+    }
+
+    func wrote(index: Int, count: Int) {
+        lock.withLock {
+            writtenBytes[index, default: 0] += Int64(count)
+            if let footprint = TransferDiagnosticStore.physicalFootprintMB() {
+                peakMiB = max(peakMiB, footprint)
+            } else {
+                missingSamples += 1
+            }
+        }
+    }
+
+    func completed(index: Int, byteCount: Int64, wasReused: Bool) {
+        lock.withLock {
+            if wasReused { errors.append("Unexpected reuse at destination \(index)") }
+            if byteCount != expectedSize { errors.append("Unexpected progress size at destination \(index)") }
+            completedIndexes.insert(index)
+        }
+    }
+
+    func failed(_ error: Error) {
+        lock.withLock { errors.append(error.localizedDescription) }
+    }
+
+    func snapshot() -> (initialMiB: Int, peakMiB: Int, finalMiB: Int,
+                        writtenBytes: [Int: Int64], completedIndexes: Set<Int>,
+                        errors: [String], missingSamples: Int) {
+        lock.withLock {
+            let finalMiB = TransferDiagnosticStore.physicalFootprintMB()
+            if finalMiB == nil { missingSamples += 1 }
+            return (initialMiB, peakMiB, finalMiB ?? 0,
+                    writtenBytes, completedIndexes, errors, missingSamples)
+        }
+    }
+}
+
+private final class ComparisonFootprintSamples: @unchecked Sendable {
+    private let lock = NSLock()
+    private var initialMiB = 0
+    private var peakMiB = 0
+    private var chunks = 0
+    private var missingSamples = 0
+
+    func started() {
+        lock.withLock {
+            if let footprint = TransferDiagnosticStore.physicalFootprintMB() {
+                initialMiB = footprint
+                peakMiB = footprint
+            } else {
+                missingSamples += 1
+            }
+        }
+    }
+
+    func sample() {
+        lock.withLock {
+            chunks += 1
+            if let footprint = TransferDiagnosticStore.physicalFootprintMB() {
+                peakMiB = max(peakMiB, footprint)
+            } else {
+                missingSamples += 1
+            }
+        }
+    }
+
+    func snapshot() -> (initialMiB: Int, peakMiB: Int, finalMiB: Int,
+                        chunks: Int, missingSamples: Int) {
+        lock.withLock {
+            let finalMiB = TransferDiagnosticStore.physicalFootprintMB()
+            if finalMiB == nil { missingSamples += 1 }
+            return (initialMiB, peakMiB, finalMiB ?? 0, chunks, missingSamples)
+        }
+    }
+}
+#endif