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webability

by io.github.snayyar00
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Visual accessibility audit

visual_audit

Pixel-level accessibility audit using Claude vision. Catches issues that DOM scanners miss: icon contrast (1.4.11), focus visibility (2.4.7), "looks like a button but isn't" (4.1.2), text rendered as images (1.4.5), visual hierarchy mismatches. Takes a URL, opens it in a headless browser, screenshots, and runs vision-based detection. Complements scan_page — run both for full coverage. Sign in with a free WebAbility account for vision and full audits (sign in by connecting https://mcp.webability.io/mcp/auth, or run webability login). Fair-use rate limits apply. NOTE: on this HOSTED server, localhost and private addresses are refused — it runs in our cloud and cannot reach your machine. Two ways to scan a local dev server: run the MCP locally (npx -y @webability/mcp, simplest — nothing leaves the machine), or open a tunnel (webability-tunnel --port 3000) and pass its URL as url together with the printed secret as tunnel_secret.

Input Schema

NameRequiredDescriptionDefault
urlYesURL to audit visually
contextNoExplain in 15-25 words, in third person, why this tool is called and how it supports the user's goal. For analytics only. You MUST describe only the abstract purpose of the tool call. NEVER include, repeat, paraphrase, or infer personal, sensitive, or identifying information from the user request or tool results, including names, emails, phone numbers, IPs, IDs, or credentials. You MUST generalize specific entities into roles such as "a user", "the customer", or "an account". Example: "Retrieving a customer's recent orders to investigate a billing issue and help support determine the appropriate resolution."
fullPageNoCapture full scrolled page instead of just viewport (default: false)
viewportNoViewport size (default: desktop)
llm_modelNoThe exact model identifier you (the assistant) are running as, taken from your system prompt or environment (e.g. "claude-opus-4-8", "gpt-5.2"). Used for analytics only. If you do not know your model identifier with certainty, pass "unknown" — never guess.
brandColorsNoBrand hex colors for context-aware filtering
tunnel_secretNoSecret printed by `webability-tunnel`. Required when `url` is a tunnel URL; the URL alone will be refused by the relay. Ignored otherwise.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changed
    • removedInput schema / properties / conversation_id
      Removed value: -{
-  "description": "Echo the conversation_id from the server's previous response. The server provides it on the first call — never invent one, and do not issue parallel tool calls until you have it.",
-  "type": "string"
-}
  2. Changed1 schema field changed
    • changedInput schema / required
      Previous value: -[
-  "url",
-  "context",
-  "llm_model"
-]New value: +[
+  "url"
+]
  3. First observed

TDQS

A4.8/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations only declare readOnly/openWorld/idempotent/destructive flags; the description goes far beyond by disclosing the auth requirement, fair-use rate limits, the cloud-vs-local reachability limitation, and the mechanism (headless browser screenshot then vision detection). No contradiction with annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loads what the tool does and its differentiator before moving to prerequisites and the localhost/tunnel caveat. Dense but each sentence carries operational information; the numbered issue list is arguably slightly long but defensible.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema and a 7-parameter openWorld tool, the description supplies the missing context: auth, rate limits, reachability, and local-dev workarounds. An agent has everything needed to decide and call correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3; the description adds cross-parameter semantics by explaining that a tunnel URL must be paired with the printed secret as `tunnel_secret` and that the URL alone will be refused. It does not add much for viewport/fullPage/brandColors, hence not a 5.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb+resource (pixel-level accessibility audit via vision) and enumerates the exact WCAG issues it catches, which distinguishes it from generic DOM scanners. It also explicitly positions itself against the sibling scan_page ('Complements scan_page — run both for full coverage').

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives explicit when-to-use (issues DOM scanners miss), when-to-run-both (with scan_page), prerequisites (sign-in to WebAbility), and hard constraints (localhost/private addresses refused on hosted server). It even routes the agent to two concrete alternatives for local dev servers.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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