fix(ios): preserve session on AX snapshot failure

Author: support

docs/adr/0003-ios-ax-snapshot-failure.md

@@ -0,0 +1,70 @@
+# ADR 0003: iOS AX Snapshot Failure Handling
+
+## Status
+
+Accepted
+
+## Context
+
+iOS XCTest can fail hierarchy capture with `kAXErrorIllegalArgument` when an accessibility tree is
+too deep to serialize. Appium's XCUITest guidance documents the practical depth limit: callers may
+raise `snapshotMaxDepth` only up to `62`, and elements at depth `63` or greater cannot be returned by
+XCTest. React Native screens are a common source of this shape.
+
+Before this ADR, Agent Device let that XCTest failure escape as a slow runner command. The daemon
+could wait for the command deadline, invalidate or kill the runner session as a transport failure,
+and then later commands reported `SESSION_NOT_FOUND`. The app tree may still need flattening, but a
+snapshot limitation should not break screenshot, logs, app lifecycle, or direct selector commands in
+the same runner session.
+
+Maestro handles this class of failure in its iOS view hierarchy route by using a depth cap of `60`,
+detecting `kAXErrorIllegalArgument`, and retrying from a child/window subtree when the app root
+cannot be serialized.
+
+## Decision
+
+Agent Device treats iOS AX snapshot serialization failure as a typed snapshot failure, not as a
+runner transport failure.
+
+The runner snapshot path now:
+
+- caps traversal depth at `60`, with lower user-provided `--depth` values still honored
+- catches Swift errors and Objective-C exceptions from `XCUIElement.snapshot()`
+- classifies `kAXErrorIllegalArgument` as `IOS_AX_SNAPSHOT_FAILED`
+- retries app-root failures from `windows.firstMatch`, first child, and first `.other` subtree
+- returns a partial snapshot with a warning when fallback succeeds
+- returns `IOS_AX_SNAPSHOT_FAILED` with an app-side flattening hint when fallback fails
+
+Daemon and CLI output preserve runner warnings and runner error hints. Because the error code is not
+`COMMAND_FAILED`, runner-session retry and invalidation policy does not treat this typed failure as a
+dead transport.
+
+Direct iOS selector interaction remains the first path for simple selector clicks, and `find id
+<value> click` now probes the runner `querySelector` path before taking a full snapshot. If the
+direct probe misses or has a transport fallback condition, the normal snapshot-based find path still
+executes.
+
+## Alternatives Considered
+
+- Flatten every problematic app screen: still useful when the screen must be fully inspectable, but
+  it moves a tooling failure mode into each app codebase and does not protect other sessions.
+- Copy WebDriverAgent/Appium source generation: too broad for Agent Device. The immediate need is
+  typed fast failure, partial recovery, and session preservation.
+- Copy Maestro's hierarchy implementation wholesale: Maestro builds a different AX model and has
+  its own swizzled max-depth path. Agent Device keeps its existing snapshot model and adopts only the
+  small recovery behavior that fits the runner protocol.
+- Always return an empty snapshot on AX failure: simple, but ambiguous. Users need to know this is an
+  iOS AX serialization limit and that app-side flattening may be required.
+
+## Consequences
+
+Partial fallback snapshots are explicitly marked `truncated` and include a warning. Selectors may be
+less accurate against partial trees, so callers should treat screenshot as visual truth and flatten
+the app-side accessibility tree when full inspectability is required.
+
+`IOS_AX_SNAPSHOT_FAILED` should remain a snapshot-domain error. Do not add it to generic retryable
+runner transport errors, and do not invalidate the runner session for it.
+
+Future improvements can add a dedicated regression fixture for a minimal React Native tree that
+reproduces the XCTest depth failure. Until then, TypeScript tests guard warning propagation, typed
+error preservation, and direct `find id ... click` routing.

ios-runner/AgentDeviceRunner/AgentDeviceRunnerUITests/RunnerTests+CommandExecution.swift

@@ -646,12 +646,25 @@ extension RunnerTests {
         scope: command.scope,
         raw: command.raw ?? false
       )
-      if options.raw {
+      do {
+        let payload: DataPayload
+        if options.raw {
+          payload = try snapshotRaw(app: activeApp, options: options)
+        } else {
+          payload = try snapshotFast(app: activeApp, options: options)
+        }
         needsPostSnapshotInteractionDelay = true
-        return Response(ok: true, data: snapshotRaw(app: activeApp, options: options))
+        return Response(ok: true, data: payload)
+      } catch let failure as SnapshotCaptureFailure {
+        return Response(
+          ok: false,
+          error: ErrorPayload(
+            code: failure.code,
+            message: failure.message,
+            hint: failure.hint
+          )
+        )
       }
-      needsPostSnapshotInteractionDelay = true
-      return Response(ok: true, data: snapshotFast(app: activeApp, options: options))
     case .screenshot:
       let screenshot: XCUIScreenshot
 #if os(macOS)

ios-runner/AgentDeviceRunner/AgentDeviceRunnerUITests/RunnerTests+Models.swift

@@ -94,6 +94,7 @@ struct DataPayload: Codable {
   let items: [String]?
   let nodes: [SnapshotNode]?
   let truncated: Bool?
+  let warnings: [String]?
   let gestureStartUptimeMs: Double?
   let gestureEndUptimeMs: Double?
   let x: Double?
@@ -115,6 +116,7 @@ struct DataPayload: Codable {
     items: [String]? = nil,
     nodes: [SnapshotNode]? = nil,
     truncated: Bool? = nil,
+    warnings: [String]? = nil,
     gestureStartUptimeMs: Double? = nil,
     gestureEndUptimeMs: Double? = nil,
     x: Double? = nil,
@@ -135,6 +137,7 @@ struct DataPayload: Codable {
     self.items = items
     self.nodes = nodes
     self.truncated = truncated
+    self.warnings = warnings
     self.gestureStartUptimeMs = gestureStartUptimeMs
     self.gestureEndUptimeMs = gestureEndUptimeMs
     self.x = x
@@ -154,10 +157,12 @@ struct DataPayload: Codable {
 struct ErrorPayload: Codable {
   let code: String?
   let message: String
+  let hint: String?
 
-  init(code: String? = nil, message: String) {
+  init(code: String? = nil, message: String, hint: String? = nil) {
     self.code = code
     self.message = message
+    self.hint = hint
   }
 }