diff --git a/apps/macos/Sources/Clawdis/CanvasFileWatcher.swift b/apps/macos/Sources/Clawdis/CanvasFileWatcher.swift
index 1b4bc82d9..6f1279a44 100644
--- a/apps/macos/Sources/Clawdis/CanvasFileWatcher.swift
+++ b/apps/macos/Sources/Clawdis/CanvasFileWatcher.swift
@@ -1,11 +1,10 @@
 import Foundation
-import Darwin
+import CoreServices
 
 final class CanvasFileWatcher: @unchecked Sendable {
     private let url: URL
     private let queue: DispatchQueue
-    private var source: DispatchSourceFileSystemObject?
-    private var fd: Int32 = -1
+    private var stream: FSEventStreamRef?
     private var pending = false
     private let onChange: () -> Void
 
@@ -20,42 +19,76 @@ final class CanvasFileWatcher: @unchecked Sendable {
     }
 
     func start() {
-        guard self.source == nil else { return }
-        let path = (self.url as NSURL).fileSystemRepresentation
-        let fd = open(path, O_EVTONLY)
-        guard fd >= 0 else { return }
-        self.fd = fd
+        guard self.stream == nil else { return }
 
-        let source = DispatchSource.makeFileSystemObjectSource(
-            fileDescriptor: fd,
-            eventMask: [.write, .delete, .rename, .attrib, .extend, .link, .revoke],
-            queue: self.queue)
+        let retainedSelf = Unmanaged.passRetained(self)
+        var context = FSEventStreamContext(
+            version: 0,
+            info: retainedSelf.toOpaque(),
+            retain: nil,
+            release: { pointer in
+                guard let pointer else { return }
+                Unmanaged<CanvasFileWatcher>.fromOpaque(pointer).release()
+            },
+            copyDescription: nil)
 
-        source.setEventHandler { [weak self] in
-            guard let self else { return }
-            if self.pending { return }
-            self.pending = true
-            self.queue.asyncAfter(deadline: .now() + 0.12) { [weak self] in
-                guard let self else { return }
-                self.pending = false
-                self.onChange()
-            }
+        let paths = [self.url.path] as CFArray
+        let flags = FSEventStreamCreateFlags(
+            kFSEventStreamCreateFlagFileEvents |
+                kFSEventStreamCreateFlagUseCFTypes |
+                kFSEventStreamCreateFlagNoDefer)
+
+        guard let stream = FSEventStreamCreate(
+            kCFAllocatorDefault,
+            Self.callback,
+            &context,
+            paths,
+            FSEventStreamEventId(kFSEventStreamEventIdSinceNow),
+            0.05,
+            flags)
+        else {
+            retainedSelf.release()
+            return
         }
 
-        source.setCancelHandler { [weak self] in
-            guard let self else { return }
-            if self.fd >= 0 {
-                close(self.fd)
-                self.fd = -1
-            }
+        self.stream = stream
+        FSEventStreamSetDispatchQueue(stream, self.queue)
+        if FSEventStreamStart(stream) == false {
+            self.stream = nil
+            FSEventStreamSetDispatchQueue(stream, nil)
+            FSEventStreamInvalidate(stream)
+            FSEventStreamRelease(stream)
         }
-
-        self.source = source
-        source.resume()
     }
 
     func stop() {
-        self.source?.cancel()
-        self.source = nil
+        guard let stream = self.stream else { return }
+        self.stream = nil
+        FSEventStreamStop(stream)
+        FSEventStreamSetDispatchQueue(stream, nil)
+        FSEventStreamInvalidate(stream)
+        FSEventStreamRelease(stream)
+    }
+}
+
+extension CanvasFileWatcher {
+    private static let callback: FSEventStreamCallback = { _, info, numEvents, _, eventFlags, _ in
+        guard let info else { return }
+        let watcher = Unmanaged<CanvasFileWatcher>.fromOpaque(info).takeUnretainedValue()
+        watcher.handleEvents(numEvents: numEvents, eventFlags: eventFlags)
+    }
+
+    private func handleEvents(numEvents: Int, eventFlags: UnsafePointer<FSEventStreamEventFlags>?) {
+        guard numEvents > 0 else { return }
+        guard eventFlags != nil else { return }
+
+        // Coalesce rapid changes (common during builds/atomic saves).
+        if self.pending { return }
+        self.pending = true
+        self.queue.asyncAfter(deadline: .now() + 0.12) { [weak self] in
+            guard let self else { return }
+            self.pending = false
+            self.onChange()
+        }
     }
 }
diff --git a/apps/macos/Tests/ClawdisIPCTests/CanvasFileWatcherTests.swift b/apps/macos/Tests/ClawdisIPCTests/CanvasFileWatcherTests.swift
new file mode 100644
index 000000000..5b3c90a45
--- /dev/null
+++ b/apps/macos/Tests/ClawdisIPCTests/CanvasFileWatcherTests.swift
@@ -0,0 +1,78 @@
+import Foundation
+import os
+import Testing
+@testable import Clawdis
+
+@Suite(.serialized) struct CanvasFileWatcherTests {
+    private func makeTempDir() throws -> URL {
+        let base = URL(fileURLWithPath: NSTemporaryDirectory(), isDirectory: true)
+        let dir = base.appendingPathComponent("clawdis-canvaswatch-\(UUID().uuidString)", isDirectory: true)
+        try FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
+        return dir
+    }
+
+    @Test func detectsInPlaceFileWrites() async throws {
+        let dir = try self.makeTempDir()
+        defer { try? FileManager.default.removeItem(at: dir) }
+
+        let file = dir.appendingPathComponent("index.html")
+        try "hello".write(to: file, atomically: false, encoding: .utf8)
+
+        let fired = OSAllocatedUnfairLock(initialState: false)
+        let waitState = OSAllocatedUnfairLock<(fired: Bool, cont: CheckedContinuation<Void, Never>?)>(
+            initialState: (false, nil))
+
+        func waitForFire(timeoutNs: UInt64) async -> Bool {
+            await withTaskGroup(of: Bool.self) { group in
+                group.addTask {
+                    await withCheckedContinuation { cont in
+                        let resumeImmediately = waitState.withLock { state in
+                            if state.fired { return true }
+                            state.cont = cont
+                            return false
+                        }
+                        if resumeImmediately {
+                            cont.resume()
+                        }
+                    }
+                    return true
+                }
+
+                group.addTask {
+                    try? await Task.sleep(nanoseconds: timeoutNs)
+                    return false
+                }
+
+                let result = await group.next() ?? false
+                group.cancelAll()
+                return result
+            }
+        }
+
+        let watcher = CanvasFileWatcher(url: dir) {
+            fired.withLock { $0 = true }
+            let cont = waitState.withLock { state in
+                state.fired = true
+                let cont = state.cont
+                state.cont = nil
+                return cont
+            }
+            cont?.resume()
+        }
+        watcher.start()
+        defer { watcher.stop() }
+
+        // Give the stream a moment to start.
+        try await Task.sleep(nanoseconds: 150 * 1_000_000)
+
+        // Modify the file in-place (no rename). This used to be missed when only watching the directory vnode.
+        let handle = try FileHandle(forUpdating: file)
+        try handle.seekToEnd()
+        try handle.write(contentsOf: Data(" world".utf8))
+        try handle.close()
+
+        let ok = await waitForFire(timeoutNs: 2_000_000_000)
+        #expect(ok == true)
+        #expect(fired.withLock { $0 } == true)
+    }
+}