Accessibility testing

Goal

Expose an MCP tool that runs fast a11y checks using the existing AccessibilityService, with optional deeper checks via ATF.

Proposed MCP tool

runA11yChecks({
  scope: "visible" | "screen",
  includeContrast: boolean,
  includeFocusOrder: boolean,
  minTapTargetDp: 48
})

Return structure should include:

• violations: array of rule IDs, node references, and summaries
• supported: per-rule capability flags

Android implementation

Baseline checks (fast, AccessibilityService):

• Content description on image-only controls
• Minimum tap target size (dp -> px from density)
• Clickable without label
• Duplicate or ambiguous labels
• Focusability issues for editable text

Contrast checks:

• Use a recent screenshot and view bounds from AccessibilityNodeInfo.
• Sample foreground and background colors and compute WCAG contrast ratio.
• Flag nodes under a threshold (e.g., 4.5:1).

Optional ATF integration:

• Dependency: com.google.android.apps.common.testing.accessibility:accessibility-test-framework
• Use AccessibilityCheckRunner.runChecks() on the current node tree.
• Map ATF results into the MCP violation schema.

Plan

1. Implement baseline heuristic rules using accessibility nodes.
2. Add contrast checks with screenshot sampling.
3. Add optional ATF integration behind a feature flag.

Risks

• Contrast sampling can be noisy on transparent backgrounds.
• ATF may increase build size and dependency surface.