[krsnak]

Summary

• Recover from macOS AXUIElementCopyAttributeValue(AXSubrole) returning AXError(-25200) without discarding the entire accessibility tree.
• Treat an unreadable or unexpectedly typed AX subrole as unverified and protected, suppress identifying and text metadata for that element, and propagate protection through its descendants.
• Preserve fail-closed behavior for other AX failures and existing permission-denied semantics. Include the fixed AX attribute name in diagnostic errors without exposing its value.
• Add focused redaction/identity unit tests and an opt-in live macOS Calculator regression.

Observed reproduction

Directly reproduced with WebCodex 0.5.0 on macOS (Apple M1). observe_computer(action=accessibility_tree) on a Calculator window failed with:

accessibility_failed: AXUIElementCopyAttributeValue failed with AXError(-25200)

On isolated current main (d8e437b4), diagnostic instrumentation identified attribute=AXSubrole. An opt-in Calculator test passed at tree depths 0 and 1, but failed at depth 2 before this fix. After this change it passes at depths 0–3.

The root cause inside Apple's AX provider is not established; the fix conservatively handles a bounded failure case rather than assuming the subrole is absent.

Safety

• Unverified-subrole elements are marked protected; their text/values are not read, they lack positive identifying metadata, and they cannot be used for native control or text entry. Descendant protection is inherited; verified siblings remain unaffected.
• A successful AX response carrying a non-CFString (tested using CFBoolean) is also protected, not treated as a benign missing subrole.
• AXError::APIDisabled remains permission_denied; unrelated failures remain failures. No changes to authorization, installation, Server, or Runner process lifecycle.

Validation

• cargo fmt --all -- --check — passed
• git diff --check — passed
• cargo test --locked --offline -p webcodex-computer --lib — 90 passed, 0 failed, 6 ignored
• cargo test --locked --offline -p webcodex-computer computer_macos_accessibility_calculator_live_regression -- --ignored --nocapture — 1 passed
• Read-only cross-application check on the same Mac: Chrome, Terminal and System Settings still pass; Calculator now passes. TextEdit exposes a separate AXDescription AXError(-25200), deliberately left outside this PR.
• Production WebCodex 0.5.0 was not replaced or restarted and its /healthz returned HTTP 200.

Related to #1161 (Computer Use ergonomics/reliability); this patch does not close the broader issue.

[swjnxyf]

Reviewed all three changed files and the macOS AX read / fingerprint / tree / scoped-correlation / native control paths at 87b8d18 on mini. AXSubrole provider Failure and unexpected CF value types conservatively mark the element protected; inherited protection suppresses title, description, placeholder, AXValue, native press/focus/scroll and text input. Unverified nodes lack positive identity evidence; ordinary NoValue/unsupported, API-disabled, and other errors retain their respective original fail-closed handling. Exact AX attribute names improve diagnostics without exposing values. On mini (macOS arm64), cargo test --locked --offline -p webcodex-computer --lib passed: 90 passed, 0 failed, 6 intentionally ignored; cargo fmt --all -- --check and git diff --check passed. The optional live Calculator test is intentionally not run here because no Calculator window is open; the PR documents successful author's opt-in live test. Hosted CI was still running at review time, with no failures observed. No blocking findings or follow-up commits required. Approve; do not merge.