diff --git a/native/computer-use-macos/Sources/OrcaComputerUseMacOS/main.swift b/native/computer-use-macos/Sources/OrcaComputerUseMacOS/main.swift
index e0c3609fa52..5f341d0c16a 100644
--- a/native/computer-use-macos/Sources/OrcaComputerUseMacOS/main.swift
+++ b/native/computer-use-macos/Sources/OrcaComputerUseMacOS/main.swift
@@ -159,6 +159,12 @@ struct Snapshot {
     let elements: [Int: ElementRecord]
     let truncated: Bool
     let maxDepthReached: Bool
+    /// On-screen windows of the app (menus and sheets included), so a new or closed window counts as a change.
+    let appWindowIds: [CGWindowID]
+
+    func showsSameState(as other: Snapshot) -> Bool {
+        treeText == other.treeText && windowBounds == other.windowBounds && appWindowIds == other.appWindowIds
+    }
 
     func withoutScreenshotPayload() -> Snapshot {
         Snapshot(
@@ -176,7 +182,8 @@ struct Snapshot {
             screenshotEngine: nil,
             elements: elements,
             truncated: truncated,
-            maxDepthReached: maxDepthReached
+            maxDepthReached: maxDepthReached,
+            appWindowIds: appWindowIds
         )
     }
 }
@@ -209,6 +216,11 @@ final class Provider {
     private var snapshots: [String: Snapshot] = [:]
     private var snapshotEntries: [CachedSnapshotEntry] = []
 
+    init() {
+        // Why: the system-wide element sets the wait for every AX call in this process, so a hung app costs one bounded timeout whatever the macOS default.
+        _ = AXUIElementSetMessagingTimeout(AXUIElementCreateSystemWide(), AccessibilityMessaging.timeoutSeconds)
+    }
+
     func handle(method: String, params: [String: JSONValue]) throws -> Any {
         switch method {
         case "handshake":
@@ -246,8 +258,21 @@ final class Provider {
         var action = try runAction()
         // Why: only --restore-window activates the app or posts to the HID tap; every other route leaves the user's desktop alone.
         action["delivery"] = isForegroundRequest(params) ? "foreground" : "background"
+        // Why: the action's currentSnapshot just cached this target's tree from before the action.
+        let before = action["path"] as? String == "accessibility" && action["verification"] == nil
+            ? try? cachedSnapshot(params: params)
+            : nil
         do {
-            return try renderActionResult(action: action, snapshot: observe(params: params))
+            let after = try observe(params: params)
+            guard let before, after.showsSameState(as: before) else {
+                return renderActionResult(action: action, snapshot: after)
+            }
+            if let changed = try observeLateChange(before: before, params: params) {
+                return renderActionResult(action: action, snapshot: changed)
+            }
+            // Why: an AX call can succeed and do nothing, as AXPress does on many web controls (#31758).
+            action["verification"] = unverifiedAction(reason: "suspected_noop")
+            return renderActionResult(action: action, snapshot: after)
         } catch let error as ProviderError where (error.code == "window_not_found" || error.code == "window_stale") && hasRequestedWindowSelector(params) {
             var fallbackParams = params
             fallbackParams.removeValue(forKey: "windowId")
@@ -259,6 +284,23 @@ final class Provider {
         }
     }
 
+    /// Re-reads the target for about a second, since apps often apply an action on a later run-loop turn.
+    /// Returns the observation to report once something changed, or nil if nothing did.
+    private func observeLateChange(before: Snapshot, params: [String: JSONValue]) throws -> Snapshot? {
+        let settleParams = params.merging(["restoreWindow": .bool(false)]) { _, replacement in replacement }
+        let quietParams = settleParams.merging(["noScreenshot": .bool(true)]) { _, replacement in replacement }
+        let deadline = Date().addingTimeInterval(1)
+        while Date() < deadline {
+            Thread.sleep(forTimeInterval: 0.2)
+            let polled = try observe(params: quietParams)
+            if !polled.showsSameState(as: before) {
+                if params["noScreenshot"]?.bool == true { return polled }
+                return try observe(params: settleParams)
+            }
+        }
+        return nil
+    }
+
     private func observe(params: [String: JSONValue]) throws -> Snapshot {
         let query = try requiredString(params, "app")
         let windowId = try requestedWindowId(params)
@@ -614,6 +656,12 @@ final class Provider {
             )
         }
         let appElement = AXUIElementCreateApplication(app.pid)
+        var role: CFTypeRef?
+        let probeStart = CFAbsoluteTimeGetCurrent()
+        // Why: a hung app costs one timeout per AX read; walking it anyway held the request lock for every agent until the 60 s RPC timeout.
+        if AccessibilityMessaging.timedOut(AXUIElementCopyAttributeValue(appElement, kAXRoleAttribute as CFString, &role), since: probeStart) {
+            throw AccessibilityMessaging.appNotResponding(app.name)
+        }
         enableManualAccessibilityIfNeeded(appElement, app: app)
         let windowCandidates = WindowCapture.candidates(pid: app.pid)
         let focused = try focusedWindow(
@@ -643,6 +691,9 @@ final class Provider {
             compactBrowserTabs: app.isKnownBrowser
         )
         renderer.render(window)
+        if renderer.appStoppedResponding {
+            throw AccessibilityMessaging.appNotResponding(app.name)
+        }
         let screenshot = includeScreenshot ? capture.screenshotPayload() : nil
         let screenshotStatus: ScreenshotStatus = if screenshot != nil {
             .captured
@@ -668,7 +719,8 @@ final class Provider {
             screenshotEngine: capture.image?.engine,
             elements: renderer.records,
             truncated: renderer.truncated,
-            maxDepthReached: renderer.maxDepthReached
+            maxDepthReached: renderer.maxDepthReached,
+            appWindowIds: windowCandidates.map(\.windowId).sorted()
         )
     }
 
@@ -775,10 +827,10 @@ final class Provider {
 
     private func performClickAction(record: ElementRecord, mouseButton: MouseButtonSelection) throws -> String? {
         if mouseButton == .right {
-            return performAction(record.element, "AXShowMenu") ? "AXShowMenu" : nil
+            return try performAction(record.element, "AXShowMenu") ? "AXShowMenu" : nil
         }
         for action in ["AXPress", "AXConfirm", "AXOpen"] {
-            if performAction(record.element, action) {
+            if try performAction(record.element, action) {
                 return action
             }
         }
@@ -793,7 +845,7 @@ final class Provider {
         guard let action else {
             throw ProviderError.coded("action_not_supported", "'\(requested)' is not a valid secondary action for element \(record.index)")
         }
-        guard performAction(record.element, action) else {
+        guard try performAction(record.element, action) else {
             throw ProviderError.coded("accessibility_error", "AXUIElementPerformAction(\(action)) failed")
         }
         return actionMetadata(path: "accessibility", actionName: action)
@@ -809,6 +861,7 @@ final class Provider {
         let current = rawAttributeValue(record.element, kAXValueAttribute as String)
         let coercion = AttributeValueCoercion(existingValue: current, requested: expected)
         let result: AXError
+        let writeStart = CFAbsoluteTimeGetCurrent()
         switch coercion.writeValue {
         case .string:
             result = AXUIElementSetAttributeValue(record.element, kAXValueAttribute as CFString, expected as CFString)
@@ -819,6 +872,9 @@ final class Provider {
         case let .boolean(value):
             result = AXUIElementSetAttributeValue(record.element, kAXValueAttribute as CFString, value ? kCFBooleanTrue : kCFBooleanFalse)
         }
+        if AccessibilityMessaging.timedOut(result, since: writeStart) {
+            throw AccessibilityMessaging.unanswered("the value write")
+        }
         guard result == .success else {
             throw ProviderError.coded("accessibility_error", "AXUIElementSetAttributeValue failed with \(result.rawValue)")
         }
@@ -837,7 +893,7 @@ final class Provider {
     private func typeText(params: [String: JSONValue]) throws -> [String: Any] {
         let snapshot = try currentKeyboardSnapshot(params: params)
         let text = try requiredString(params, "text")
-        if let focused = focusedRecord(snapshot), let verification = TextInput.replaceSelection(focused.element, with: text) {
+        if let focused = focusedRecord(snapshot), let verification = try TextInput.replaceSelection(focused.element, with: text) {
             return actionMetadata(path: "accessibility", actionName: "AXReplaceSelection", verification: verification)
         }
         try deliverKeys(try withTransportErrors { try BackgroundInputTransport.textEvents(text) }, snapshot: snapshot, params: params)
@@ -879,7 +935,7 @@ final class Provider {
     private func pasteText(params: [String: JSONValue]) throws -> [String: Any] {
         let snapshot = try currentKeyboardSnapshot(params: params)
         let text = try requiredString(params, "text")
-        if let focused = focusedRecord(snapshot), let verification = TextInput.replaceSelection(focused.element, with: text) {
+        if let focused = focusedRecord(snapshot), let verification = try TextInput.replaceSelection(focused.element, with: text) {
             return actionMetadata(path: "accessibility", actionName: "AXReplaceSelection", verification: verification)
         }
         let pasteKeys = try Input.keyEvents("cmd+v")
@@ -903,7 +959,7 @@ final class Provider {
             if pages.rounded() == pages, let pageCount = boundedInteger(pages, as: Int.self),
                record.actions.contains(action) {
                 for _ in 0..<max(1, pageCount) {
-                    _ = performAction(record.element, action)
+                    _ = try performAction(record.element, action)
                 }
                 return actionMetadata(path: "accessibility", actionName: action)
             }
@@ -1166,17 +1222,10 @@ private func focusedSystemWindow(systemWide: AXUIElement, app: AppDescriptor) ->
 }
 
 private func isTargetWindowFocused(_ snapshot: Snapshot) -> Bool {
-    guard let focusedWindow = focusedSystemWindow(systemWide: AXUIElementCreateSystemWide(), app: snapshot.app) else {
-        return false
-    }
-    if windowNumber(focusedWindow) == snapshot.windowId {
-        return true
-    }
-    guard let frame = absoluteFrame(focusedWindow) else {
-        return false
-    }
-    let intersection = frame.intersection(snapshot.windowBounds)
-    return !intersection.isNull && intersection.area >= min(frame.area, snapshot.windowBounds.area) * 0.75
+    focusedSyntheticClickRecipient(
+        targetPID: snapshot.app.pid,
+        targetCandidates: { WindowCapture.candidates(pid: snapshot.app.pid) }
+    ) == .focused(syntheticClickRecipient(pid: snapshot.app.pid, windowId: snapshot.windowId))
 }
 
 private enum AXElementProbe {
@@ -1260,19 +1309,8 @@ private func focusedSyntheticClickRecipient(
         return .unavailable
     }
 
-    if let windowId = windowNumber(focusedWindow) {
-        return .focused(syntheticClickRecipient(pid: ownerPID, windowId: windowId))
-    }
-    guard ownerPID == targetPID else { return .unavailable }
-    guard let frame = absoluteFrame(focusedWindow),
-          let candidate = SyntheticMouseClickDelivery.uniqueWindowCandidate(
-            from: targetCandidates(),
-            matching: {
-              windowFramesMatch($0.bounds, frame)
-            }
-          )
-    else { return .unavailable }
-    return .focused(syntheticClickRecipient(pid: ownerPID, windowId: candidate.windowId))
+    guard pidAttribute(focusedWindow) == ownerPID else { return .unavailable }
+    return pointerRecipient(of: focusedWindow, targetPID: targetPID, targetCandidates: targetCandidates)
 }
 
 private func hitTestSyntheticClickRecipient(
@@ -1308,13 +1346,13 @@ private func pointerRecipient(
     guard let ownerPID = pidAttribute(element), let window = containingWindow(element) else {
         return .unavailable
     }
-    var numberValue: CFTypeRef?
-    let numberStatus = AXUIElementCopyAttributeValue(window, "AXWindowNumber" as CFString, &numberValue)
-    if numberStatus == .success {
-        guard let number = numberValue as? NSNumber else { return .unavailable }
-        return .focused(syntheticClickRecipient(pid: ownerPID, windowId: CGWindowID(number.uint32Value)))
+    do {
+        if let windowID = try AccessibilityWindowIdentity.windowID(of: window) {
+            return .focused(syntheticClickRecipient(pid: ownerPID, windowId: windowID))
+        }
+    } catch {
+        return .unavailable
     }
-    guard numberStatus == .attributeUnsupported || numberStatus == .noValue else { return .unavailable }
     guard ownerPID == targetPID, let frame = absoluteFrame(window),
           let windows = copyArray(AXUIElementCreateApplication(ownerPID), kAXWindowsAttribute as String)
     else { return .unavailable }
@@ -1723,10 +1761,7 @@ private func numberAttribute(_ element: AXUIElement, _ attribute: String) -> NSN
 }
 
 private func windowNumber(_ element: AXUIElement) -> CGWindowID? {
-    guard let number = numberAttribute(element, "AXWindowNumber") else {
-        return nil
-    }
-    return CGWindowID(number.uint32Value)
+    try? AccessibilityWindowIdentity.windowID(of: element)
 }
 
 private func absoluteFrame(_ element: AXUIElement) -> CGRect? {
@@ -1763,9 +1798,44 @@ private func actions(_ element: AXUIElement) -> [String] {
     return value as? [String] ?? []
 }
 
-private func performAction(_ element: AXUIElement, _ action: String) -> Bool {
-    actions(element).contains(where: { $0.caseInsensitiveCompare(action) == .orderedSame }) &&
-        AXUIElementPerformAction(element, action as CFString) == .success
+private enum AccessibilityMessaging {
+    static let timeoutSeconds: Float = 3
+
+    /// Why: AX also returns cannotComplete at once for some attributes; only a call that waited out the timeout means the app is not answering.
+    static func timedOut(_ result: AXError, since start: CFAbsoluteTime) -> Bool {
+        result == .cannotComplete && CFAbsoluteTimeGetCurrent() - start >= Double(timeoutSeconds) * 0.9
+    }
+
+    static func appNotResponding(_ appName: String?) -> ProviderError {
+        .coded("action_timeout", "\(appName.map { "'\($0)'" } ?? "The app") is not answering accessibility requests (no reply within \(timeoutSeconds) s); it may be busy or hung. Wait until it responds, then retry.")
+    }
+
+    /// The request reached the app, which went quiet while handling it, so the effect may have happened.
+    static func unanswered(_ request: String) -> ProviderError {
+        .coded("action_timeout", "the app did not answer \(request) within \(timeoutSeconds) s; it may be busy or showing a dialog, and the action may already have taken effect. Run get-app-state to check before retrying.")
+    }
+}
+
+private func performAction(_ element: AXUIElement, _ action: String) throws -> Bool {
+    var names: CFArray?
+    let start = CFAbsoluteTimeGetCurrent()
+    let namesResult = AXUIElementCopyActionNames(element, &names)
+    if AccessibilityMessaging.timedOut(namesResult, since: start) {
+        throw AccessibilityMessaging.appNotResponding(nil)
+    }
+    guard namesResult == .success,
+          let names = names as? [String],
+          names.contains(where: { $0.caseInsensitiveCompare(action) == .orderedSame })
+    else {
+        return false
+    }
+    let pressStart = CFAbsoluteTimeGetCurrent()
+    let result = AXUIElementPerformAction(element, action as CFString)
+    // Why: falling through to another route after a delivered but unanswered press ran the action twice.
+    if AccessibilityMessaging.timedOut(result, since: pressStart) {
+        throw AccessibilityMessaging.unanswered(action)
+    }
+    return result == .success
 }
 
 private func isSettable(_ element: AXUIElement, _ attribute: String) -> Bool {
@@ -1889,6 +1959,8 @@ private final class TreeRenderer {
     static let maxNodes = 1200
     static let maxDepth = 64
 
+    var appStoppedResponding: Bool { reader.appStoppedResponding }
+
     init(windowBounds: CGRect, focused: AXUIElement?, compactBrowserTabs: Bool) {
         self.windowBounds = windowBounds
         self.focused = focused
@@ -1896,6 +1968,7 @@ private final class TreeRenderer {
     }
 
     func render(_ element: AXUIElement, depth: Int = 0, ancestors: [AXUIElement] = []) {
+        guard !reader.appStoppedResponding else { return }
         guard nextIndex < Self.maxNodes else {
             truncated = true
             return
@@ -2096,7 +2169,7 @@ private final class AXSnapshotReader {
             return actions
         }
         var value: CFArray?
-        let actions = AXUIElementCopyActionNames(element, &value) == .success ? value as? [String] ?? [] : []
+        let actions = answered { AXUIElementCopyActionNames(element, &value) } == .success ? value as? [String] ?? [] : []
         cache.actions = actions
         return actions
     }
@@ -2107,11 +2180,24 @@ private final class AXSnapshotReader {
             return cached
         }
         var settable = DarwinBoolean(false)
-        let value = AXUIElementIsAttributeSettable(element, attribute as CFString, &settable) == .success && settable.boolValue
+        let value = answered { AXUIElementIsAttributeSettable(element, attribute as CFString, &settable) } == .success && settable.boolValue
         cache.settable[attribute] = value
         return value
     }
 
+    /// Set once the app waits out a messaging timeout; later reads return nothing instead of each waiting it out again.
+    private(set) var appStoppedResponding = false
+
+    private func answered(_ call: () -> AXError) -> AXError? {
+        guard !appStoppedResponding else { return nil }
+        let start = CFAbsoluteTimeGetCurrent()
+        let result = call()
+        if AccessibilityMessaging.timedOut(result, since: start) {
+            appStoppedResponding = true
+        }
+        return result
+    }
+
     func frame(_ element: AXUIElement, windowBounds: CGRect) -> CGRect? {
         guard let absolute = absoluteFrame(element) else { return nil }
         return CGRect(
@@ -2238,7 +2324,7 @@ private final class AXSnapshotReader {
             return nil
         }
         var value: CFTypeRef?
-        guard AXUIElementCopyAttributeValue(element, attribute as CFString, &value) == .success, let value else {
+        guard answered({ AXUIElementCopyAttributeValue(element, attribute as CFString, &value) }) == .success, let value else {
             cache.attributes[attribute] = .missing
             return nil
         }
@@ -2252,7 +2338,7 @@ private final class AXSnapshotReader {
         }
         cache.loadedAttributeNames = true
         var value: CFArray?
-        guard AXUIElementCopyAttributeNames(cache.element, &value) == .success, let attributes = value as? [String] else {
+        guard answered({ AXUIElementCopyAttributeNames(cache.element, &value) }) == .success, let attributes = value as? [String] else {
             return nil
         }
         cache.advertisedAttributes = Set(attributes)
@@ -2813,7 +2899,7 @@ private enum Input {
 }
 
 private enum TextInput {
-    static func replaceSelection(_ element: AXUIElement, with text: String) -> [String: Any]? {
+    static func replaceSelection(_ element: AXUIElement, with text: String) throws -> [String: Any]? {
         guard isSettable(element, kAXValueAttribute as String),
               let current = rawStringAttribute(element, kAXValueAttribute as String)
         else {
@@ -2825,7 +2911,13 @@ private enum TextInput {
         let start = String.Index(utf16Offset: startOffset, in: current)
         let end = String.Index(utf16Offset: endOffset, in: current)
         let next = String(current[..<start]) + text + String(current[end...])
-        guard AXUIElementSetAttributeValue(element, kAXValueAttribute as CFString, next as CFString) == .success else {
+        let writeStart = CFAbsoluteTimeGetCurrent()
+        let result = AXUIElementSetAttributeValue(element, kAXValueAttribute as CFString, next as CFString)
+        // Why: typing the text as keys after an unanswered write could enter it twice.
+        if AccessibilityMessaging.timedOut(result, since: writeStart) {
+            throw AccessibilityMessaging.unanswered("the text write")
+        }
+        guard result == .success else {
             return nil
         }
         setSelectedTextRange(element, CFRange(location: startOffset + text.utf16.count, length: 0))
diff --git a/native/computer-use-macos/Sources/OrcaComputerUseMacOSCore/AccessibilityWindowIdentity.swift b/native/computer-use-macos/Sources/OrcaComputerUseMacOSCore/AccessibilityWindowIdentity.swift
new file mode 100644
index 00000000000..444bd527bcc
--- /dev/null
+++ b/native/computer-use-macos/Sources/OrcaComputerUseMacOSCore/AccessibilityWindowIdentity.swift
@@ -0,0 +1,45 @@
+import ApplicationServices
+import CoreGraphics
+import Darwin
+import Foundation
+
+public enum AccessibilityWindowIdentity {
+    private enum Failure: Error {
+        case unreadable
+    }
+
+    /// Nil permits geometry fallback; unreadable identity must refuse input.
+    public static func windowID(of element: AXUIElement) throws -> CGWindowID? {
+        var nativeAbsent = true
+        if let getWindow {
+            var windowID: CGWindowID = 0
+            let status = getWindow(element, &windowID)
+            if status == .success, windowID != 0 { return windowID }
+            nativeAbsent = status == .attributeUnsupported || status == .noValue
+        }
+
+        var value: CFTypeRef?
+        switch AXUIElementCopyAttributeValue(element, "AXWindowNumber" as CFString, &value) {
+        case .success:
+            guard let number = value as? NSNumber,
+                  CFGetTypeID(number) != CFBooleanGetTypeID(),
+                  let windowID = CGWindowID(exactly: number.doubleValue), windowID != 0
+            else { throw Failure.unreadable }
+            return windowID
+        case .attributeUnsupported, .noValue:
+            guard nativeAbsent else { throw Failure.unreadable }
+            return nil
+        default:
+            throw Failure.unreadable
+        }
+    }
+
+    private typealias GetWindow = @convention(c) (AXUIElement, UnsafeMutablePointer<CGWindowID>) -> AXError
+
+    // AppKit and WebKit windows often omit AXWindowNumber.
+    private static let getWindow: GetWindow? = {
+        guard let symbol = dlsym(UnsafeMutableRawPointer(bitPattern: -2), "_AXUIElementGetWindow") else { return nil }
+        // SAFETY: HIServices exports AXError _AXUIElementGetWindow(AXUIElementRef, CGWindowID *).
+        return unsafeBitCast(symbol, to: GetWindow.self)
+    }()
+}
diff --git a/native/computer-use-macos/Sources/OrcaComputerUseMacOSCore/BackgroundInputTransport.swift b/native/computer-use-macos/Sources/OrcaComputerUseMacOSCore/BackgroundInputTransport.swift
index 5c49f131599..c66fe0402ca 100644
--- a/native/computer-use-macos/Sources/OrcaComputerUseMacOSCore/BackgroundInputTransport.swift
+++ b/native/computer-use-macos/Sources/OrcaComputerUseMacOSCore/BackgroundInputTransport.swift
@@ -210,9 +210,8 @@ public enum BackgroundInputTransport {
             else { throw Failure.targetWindowNotKey }
             // SAFETY: the CFTypeID check above proves focused is an AXUIElement.
             let window = focused as! AXUIElement
-            var number: CFTypeRef?
-            guard AXUIElementCopyAttributeValue(window, "AXWindowNumber" as CFString, &number) == .success,
-                  let number = number as? NSNumber, number.uint32Value == target.windowID
+            guard let windowID = try? AccessibilityWindowIdentity.windowID(of: window),
+                  windowID == target.windowID
             else { throw Failure.targetWindowNotKey }
             try body()
             return
diff --git a/skill-guides/computer-use.md b/skill-guides/computer-use.md
index af8c7bead13..7ec6d2d92b0 100644
--- a/skill-guides/computer-use.md
+++ b/skill-guides/computer-use.md
@@ -87,6 +87,7 @@ printf '%s' "$TEXT" | ORCA computer set-value --app <app> --element-index <index
 - An action's verification is separate from whether its provider call succeeded:
   - `verified` means the changed value was read back.
   - `unverified (accessibility action unasserted)` means the accessibility call succeeded but no post-state assertion was made.
+  - `unverified (suspected noop)` means the accessibility call succeeded but the window's tree and the app's windows were unchanged about a second later. Web and Electron controls often ignore it. Do not repeat the same element click; if the effect you expect would show in the window, retry once with `click --x --y` at the control from a fresh screenshot. If the effect lives elsewhere (clipboard, another app), check there first.
   - `unverified (synthetic input)` means input was fired into the void and is unverifiable.
   - Missing verification metadata is unverified, including responses from older runtimes.
   - Never report an unverified action as success. If it could have sent, submitted, bought, or deleted something, say the effect is unproven.
@@ -164,6 +165,7 @@ Slack: the accessibility tree may be shallow while the screenshot contains usefu
 - `element_not_clickable`: the element has no actionable frame; use a parent/child element with a frame or choose window-local coordinates from the latest screenshot.
 - `invalid_argument`: fix the command flags; do not retry the same command unchanged.
 - `action_timeout`: inspect current state before retrying, then use a simpler semantic action or `--no-screenshot` if observation is slow.
+- `verification_failed`: the write was read back and the element holds a different value, so the action failed. Run `get-app-state` to see which field changed. For editable text, click the intended field and confirm focus, select all (`cmd+a` on macOS, `ctrl+a` on Windows/Linux), then use `type-text` and inspect the resulting value instead of repeating `set-value`.
 - `screenshot_failed`: use `--no-screenshot` if tree state is enough; if the message names Screen Recording or screenshots permission, run `ORCA computer permissions --id screenshots --json`.
 - `accessibility_error`: run `ORCA computer capabilities --json`; if the message names Accessibility permission, run `ORCA computer permissions --id accessibility --json`.
 - Empty tree or no screenshot: app may have no visible window, be minimized, or need permissions.
diff --git a/src/cli/bundled-skill-guides.ts b/src/cli/bundled-skill-guides.ts
index 5f83dffa691..3c38e17802b 100644
--- a/src/cli/bundled-skill-guides.ts
+++ b/src/cli/bundled-skill-guides.ts
@@ -104,6 +104,7 @@ const COMPUTER_USE_MARKDOWN = [
   "- An action's verification is separate from whether its provider call succeeded:",
   "  - `verified` means the changed value was read back.",
   "  - `unverified (accessibility action unasserted)` means the accessibility call succeeded but no post-state assertion was made.",
+  "  - `unverified (suspected noop)` means the accessibility call succeeded but the window's tree and the app's windows were unchanged about a second later. Web and Electron controls often ignore it. Do not repeat the same element click; if the effect you expect would show in the window, retry once with `click --x --y` at the control from a fresh screenshot. If the effect lives elsewhere (clipboard, another app), check there first.",
   "  - `unverified (synthetic input)` means input was fired into the void and is unverifiable.",
   "  - Missing verification metadata is unverified, including responses from older runtimes.",
   "  - Never report an unverified action as success. If it could have sent, submitted, bought, or deleted something, say the effect is unproven.",
@@ -181,6 +182,7 @@ const COMPUTER_USE_MARKDOWN = [
   "- `element_not_clickable`: the element has no actionable frame; use a parent/child element with a frame or choose window-local coordinates from the latest screenshot.",
   "- `invalid_argument`: fix the command flags; do not retry the same command unchanged.",
   "- `action_timeout`: inspect current state before retrying, then use a simpler semantic action or `--no-screenshot` if observation is slow.",
+  "- `verification_failed`: the write was read back and the element holds a different value, so the action failed. Run `get-app-state` to see which field changed. For editable text, click the intended field and confirm focus, select all (`cmd+a` on macOS, `ctrl+a` on Windows/Linux), then use `type-text` and inspect the resulting value instead of repeating `set-value`.",
   "- `screenshot_failed`: use `--no-screenshot` if tree state is enough; if the message names Screen Recording or screenshots permission, run `ORCA computer permissions --id screenshots --json`.",
   "- `accessibility_error`: run `ORCA computer capabilities --json`; if the message names Accessibility permission, run `ORCA computer permissions --id accessibility --json`.",
   "- Empty tree or no screenshot: app may have no visible window, be minimized, or need permissions.",
diff --git a/src/main/computer/computer-action-verification-normalization.ts b/src/main/computer/computer-action-verification-normalization.ts
index a1a510a5282..f3b896ec95a 100644
--- a/src/main/computer/computer-action-verification-normalization.ts
+++ b/src/main/computer/computer-action-verification-normalization.ts
@@ -1,4 +1,5 @@
 import type { ComputerActionResult } from '../../shared/runtime-types'
+import { RuntimeClientError } from './runtime-client-error'
 
 export function normalizeComputerActionResult(result: ComputerActionResult): ComputerActionResult {
   const action = result.action
@@ -24,3 +25,21 @@ export function normalizeComputerActionResult(result: ComputerActionResult): Com
     }
   }
 }
+
+// Why: a read-back showing another value proves the write failed; returned as ok, callers that check only the exit status took it as success (#33511).
+export function rejectContradictedComputerAction(
+  result: ComputerActionResult
+): ComputerActionResult {
+  const verification = result.action?.verification
+  if (verification?.state !== 'unverified' || verification.reason !== 'value_mismatch') {
+    return result
+  }
+  const actual =
+    typeof verification.actualPreview === 'string'
+      ? JSON.stringify(verification.actualPreview)
+      : 'a different value'
+  throw new RuntimeClientError(
+    'verification_failed',
+    `set-value did not take effect: after the write the element reads ${actual}, not the requested value`
+  )
+}
diff --git a/src/main/computer/sidecar-client.test.ts b/src/main/computer/sidecar-client.test.ts
index f6b01e82f21..9bb2d57a101 100644
--- a/src/main/computer/sidecar-client.test.ts
+++ b/src/main/computer/sidecar-client.test.ts
@@ -285,6 +285,51 @@ describe('computer sidecar client', () => {
     })
   })
 
+  it('fails set-value when the provider read the write back as a different value', async () => {
+    const call = callComputerSidecarAction('setValue', {
+      app: 'Safari',
+      elementIndex: 15,
+      value: 'pf1300-awllch'
+    })
+    const child = children[0]!
+    const request = child.sent[0]!
+
+    child.emit('message', {
+      id: request.id,
+      ok: true,
+      result: {
+        snapshot: {
+          id: 'snap-1',
+          app: { name: 'Safari', bundleId: 'com.apple.Safari', pid: 100 },
+          window: { title: 'Form', id: 63, width: 800, height: 600 },
+          coordinateSpace: 'window',
+          treeText: 'tree',
+          elementCount: 16,
+          focusedElementId: null
+        },
+        screenshot: null,
+        screenshotStatus: { state: 'skipped', reason: 'no_screenshot_flag' },
+        action: {
+          path: 'accessibility',
+          actionName: 'AXSetValue',
+          fallbackReason: null,
+          targetWindowId: 63,
+          verification: {
+            state: 'unverified',
+            reason: 'value_mismatch',
+            expected: 'pf1300-awllch',
+            actualPreview: 'orig-objective-1300'
+          }
+        }
+      }
+    })
+
+    await expect(call).rejects.toMatchObject({
+      code: 'verification_failed',
+      message: expect.stringContaining('"orig-objective-1300"')
+    })
+  })
+
   it('rejects oversized pasteText payloads before forking the sidecar', async () => {
     const secret = 'sidecar-secret-token'
 
diff --git a/src/main/computer/sidecar-client.ts b/src/main/computer/sidecar-client.ts
index 57fd6689857..4a86a108632 100644
--- a/src/main/computer/sidecar-client.ts
+++ b/src/main/computer/sidecar-client.ts
@@ -7,7 +7,10 @@ import type {
   ComputerProviderCapabilities,
   ComputerSnapshotResult
 } from '../../shared/runtime-types'
-import { normalizeComputerActionResult } from './computer-action-verification-normalization'
+import {
+  normalizeComputerActionResult,
+  rejectContradictedComputerAction
+} from './computer-action-verification-normalization'
 import { isComputerSidecarDiagnostic, logComputerDiagnostic } from './computer-sidecar-diagnostics'
 import { validateComputerSidecarPasteText } from './computer-sidecar-paste-validation'
 import { RuntimeClientError } from './runtime-client-error'
@@ -85,8 +88,10 @@ export async function callComputerSidecarAction(
   if (validation) {
     await validation
   }
-  return normalizeComputerActionResult(
-    (await getComputerSidecar().call(method, params)) as ComputerActionResult
+  return rejectContradictedComputerAction(
+    normalizeComputerActionResult(
+      (await getComputerSidecar().call(method, params)) as ComputerActionResult
+    )
   )
 }
 
diff --git a/src/shared/computer-use-error-recovery.ts b/src/shared/computer-use-error-recovery.ts
index 86b7c18d262..43721c5d28b 100644
--- a/src/shared/computer-use-error-recovery.ts
+++ b/src/shared/computer-use-error-recovery.ts
@@ -90,6 +90,11 @@ export function computerUseErrorRecoveryData(
         'Run `orca computer get-app-state --app <app> --json` before retrying so you know whether the UI changed.',
         'Retry with a simpler semantic action or `--no-screenshot` if observation is slow; do not repeat the same timed-out action blindly.'
       )
+    case 'verification_failed':
+      return recoverWith(
+        'Run `orca computer get-app-state --app <app> --json` and check which field holds the value; another field may have changed instead.',
+        'Do not retry the same `set-value` unchanged. For editable text, click the field and confirm focus, select all (`cmd+a` on macOS, `ctrl+a` on Windows/Linux), then use `type-text` and inspect the resulting value.'
+      )
     case 'screenshot_failed':
       return recoverWith(
         'If the accessibility tree is sufficient, rerun the command with `--no-screenshot` instead of retrying the same screenshot capture.',
diff --git a/src/shared/computer-use-runtime-types.ts b/src/shared/computer-use-runtime-types.ts
index e2e7cbebc4a..ff077347ae4 100644
--- a/src/shared/computer-use-runtime-types.ts
+++ b/src/shared/computer-use-runtime-types.ts
@@ -14,6 +14,7 @@ export const COMPUTER_ERROR_CODES = {
   value_not_settable: 'value_not_settable',
   invalid_argument: 'invalid_argument',
   action_timeout: 'action_timeout',
+  verification_failed: 'verification_failed',
   screenshot_failed: 'screenshot_failed',
   accessibility_error: 'accessibility_error'
 } as const
@@ -112,6 +113,8 @@ export type ComputerActionVerification =
         | 'readback_unsupported'
         | 'window_changed'
         | 'value_mismatch'
+        // The accessibility call succeeded but the window and the app's windows looked the same about a second later.
+        | 'suspected_noop'
       expected?: string | null
       actualPreview?: string | null
     }