Computer Use helper SIGTRAPs while traversing a SwiftUI accessibility tree; AppShot cannot attach

Additional A/B isolation: complex SwiftUI grouping around mixed AX subtrees

This appears to be the same failure class as #32293 and the topology-sensitive behavior reported in #28933. A new A/B investigation isolated a narrower application-side trigger while preserving the full accessibility tree.

Failure signature

A get_app_state observation failed with:

Sky Computer Use native pipe closed before response

Confirmed crash reports contained:

EXC_BREAKPOINT / SIGTRAP
Array.remove(at:)

The triggered stack repeatedly traversed the accessibility hierarchy through Sequence.compactMap, consistent with an out-of-bounds mutation while transforming or invalidating AX children. The inspected application remained alive.

Isolated structural trigger

The smallest confirmed trigger was a SwiftUI semantic grouping container around an AppKit-backed text subtree:

GroupBox {
    AppKitBackedTextView(...)
}

The represented view returns an NSScrollView containing an NSTextView.

The text view itself is not sufficient to trigger the failure:

• The same represented text view succeeds when mounted without the grouping wrapper.
• The isolated section fails when the wrapper is present.
• Replacing only the semantic grouping wrapper with a visually equivalent layout container makes the same controls return a complete tree.

Successful output after that single substitution included:

text Input
button Writing Tools
scroll area
    text entry area (settable)
    scroll bar

No AX children were hidden, combined, replaced with a proxy, or removed.

Related affected structures

The same substitution was needed for several complex dynamic sections containing combinations of:

• represented AppKit text/scroll views;
• nested scrollable content;
• dynamically generated pipeline controls;
• graph/canvas content;
• segmented diff/output content.

This does not mean every GroupBox is broken. Simple groups containing ordinary SwiftUI content continued to work. The narrower common pattern is semantic grouping around a mixed or deeply dynamic accessibility subtree.

Full-tree controls after the workaround

Two independently built variants were tested.

Variant A:

AX nodes:       165
AX text:        11,064 characters
History rows:   54
Screenshot:     returned
New helper crash: no

Variant B:

AX nodes:       209
AX text:        20,383 characters
History total:  250 records
Screenshot:     returned
New helper crash: no

The returned trees included the large history, AppKit-backed input editor, dynamic pipeline/graph controls, output/diff controls, inspector content, and their interactive descendants.

Interpretation

This is not simply a response-size failure: the failing isolated subtree was much smaller than the successful complete trees.

The result strengthens the conclusion that the trap occurs in the helper's accessibility-tree transformation or invalidation logic. It does not appear to require a screenshot failure, target-app crash, missing permission, duplicate installation, or exact-path routing.

The application-side grouping-to-layout substitution is only a workaround. Equivalent SwiftUI AX topologies are valid and may occur in unrelated applications. The helper should revalidate collection indices/tree revisions, rebuild from a fresh snapshot, or skip the unsupported subtree with a controlled warning instead of trapping in Array.remove(at:).

Cross-reference differential

I read the full issue bodies and available comments for the related open reports below. They share the broad AppShot/Computer Use surface, but not all share this crash mechanism:

• #32293 — same failure class. EXC_BREAKPOINT / SIGTRAP during SwiftUI AX transformation, native pipe closure, and later disassembly showing an attempted collection operation at index 5 with count 4. Its controlled routing and duplicate-installation checks also rule out those conditions as necessary causes.
• #28933 — same topology-sensitive class. A minimal SwiftUI Form / Section / Toggle / Stepper / LabeledContent hierarchy triggers a helper SIGTRAP, supporting the conclusion that valid but specific semantic container structures can expose the transformer defect.
• #38760 — different lifecycle defect. A desktop-managed helper spawn storm exhausts LaunchServices and can escalate to desktop OOM or a WindowServer watchdog panic. Its on-demand bundled path can remain stable; it does not report this isolated AX-child bounds trap.
• #38003 — different startup/packaging defect. Intel builds hit a 250 ms native-pipe handshake timeout and then lack a managed helper payload. The service fails to initialize before accessibility traversal.
• #35770 — different screenshot handoff defect. AX text and PNG generation succeed, but the desktop completes with completed_without_screenshot, apparently during temporary-file URL conversion or cleanup.
• #34788 — different mode/session-state defect. AppShots work in Work mode but become unavailable in Chat mode after focus/background transitions; no helper SIGTRAP or AX-transform crash is reported.
• #34296 — symptom-only report. The hotkey produces no result and provides no crash receipt or layer-specific evidence, so it cannot currently be classified as the same bug.
• #20683 — related traversal safety goal but different crash signature. It reports EXC_BAD_ACCESS / SIGBUS and explicit stack exhaustion from excessive recursion, rather than this deterministic EXC_BREAKPOINT / SIGTRAP collection-bounds failure.

The shared product requirement across these reports is graceful, diagnosable failure. For this issue specifically, the actionable fingerprint remains: valid SwiftUI AX topology → transformation/invalidation → out-of-bounds collection mutation → SIGTRAP → native pipe closure.

UI symptom and underlying service error were the same capture attempt

The failure was observed at two layers:

• In the desktop UI, the user saw the AppShot attachment error: Unable to attach appshot.
• Through the underlying Computer Use API for the same target state, get_app_state returned: Sky Computer Use native pipe closed before response.
• A fresh helper crash receipt then confirmed EXC_BREAKPOINT / SIGTRAP in the accessibility-transformation path.

Therefore the UI attachment error in this reproduction was the presentation-layer consequence of the helper crashing before it could return a complete AppShot payload. It was not merely a visual-capture failure.

This distinction matters for #35770: that report reaches screenshot and AX updates, creates valid PNG files, and only later settles as completed_without_screenshot. Here, the helper itself terminates during AX transformation, so the desktop has no completed payload to attach even though both cases display a similar user-facing attachment error.