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ViewportWitness by Apex Labs

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verify_page

Destructive

Choose this only when the caller supplies explicit website assertions that need pass/fail results across three viewports. For broad exploratory browser QA use check_page instead. Assertion types: noHorizontalOverflow, noConsoleErrors, textVisible, titleIncludes, selectorExists, selectorVisible. Costs $0.10 USDC via x402. Successful paid calls settle the authorized USDC payment and create a stored report; settlement reduces the payer's balance and cannot be undone by this tool. The target website is not modified. Requires an x402-aware client; standard AI assistants cannot automatically sign x402.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
urlYesPublic HTTPS page URL. Private, loopback, credentialed, and non-HTTPS URLs are rejected.
assertionsYesOne to twenty explicit pass/fail assertions to evaluate across all three viewports.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changed
    • addedInput schema / properties / assertions / description
      Added value: +"One to twenty explicit pass/fail assertions to evaluate across all three viewports."
    • changedInput schema / properties / assertions / items / anyOf
      Previous value: -[
-  {
-    "additionalProperties": false,
-    "properties": {
-      "type": {
-        "const": "noHorizontalOverflow",
-        "type": "string"
-      }
-    },
-    "required": [
-      "type"
-    ],
-    "type": "object"
-  },
-  {
-    "additionalProperties": false,
-    "properties": {
-      "type": {
-        "const": "noConsoleErrors",
-        "type": "string"
-      }
-    },
-    "required": [
-      "type"
-    ],
-    "type": "object"
-  },
-  {
-    "additionalProperties": false,
-    "properties": {
-      "type": {
-        "const": "textVisible",
-        "type": "string"
-      },
-      "value": {
-        "maxLength": 200,
-        "minLength": 1,
-        "type": "string"
-      }
-    },
-    "required": [
-      "type",
-      "value"
-    ],
-    "type": "object"
-  },
-  {
-    "additionalProperties": false,
-    "properties": {
-      "type": {
-        "const": "titleIncludes",
-        "type": "string"
-      },
-      "value": {
-        "maxLength": 200,
-        "minLength": 1,
-        "type": "string"
-      }
-    },
-    "required": [
-      "type",
-      "value"
-    ],
-    "type": "object"
-  },
-  {
-    "additionalProperties": false,
-    "properties": {
-      "selector": {
-        "maxLength": 300,
-        "minLength": 1,
-        "type": "string"
-      },
-      "type": {
-        "const": "selectorExists",
-        "type": "string"
-      }
-    },
-    "required": [
-      "type",
-      "selector"
-    ],
-    "type": "object"
-  },
-  {
-    "additionalProperties": false,
-    "properties": {
-      "selector": {
-        "maxLength": 300,
-        "minLength": 1,
-        "type": "string"
-      },
-      "type": {
-        "const": "selectorVisible",
-        "type": "string"
-      }
-    },
-    "required": [
-      "type",
-      "selector"
-    ],
-    "type": "object"
-  }
-]New value: +[
+  {
+    "additionalProperties": false,
+    "properties": {
+      "type": {
+        "const": "noHorizontalOverflow",
+        "type": "string"
+      }
+    },
+    "required": [
+      "type"
+    ],
+    "type": "object"
+  },
+  {
+    "additionalProperties": false,
+    "properties": {
+      "type": {
+        "const": "noConsoleErrors",
+        "type": "string"
+      }
+    },
+    "required": [
+      "type"
+    ],
+    "type": "object"
+  },
+  {
+    "additionalProperties": false,
+    "properties": {
+      "type": {
+        "const": "textVisible",
+        "type": "string"
+      },
+      "value": {
+        "description": "Visible text to find on the rendered page.",
+        "maxLength": 200,
+        "minLength": 1,
+        "type": "string"
+      }
+    },
+    "required": [
+      "type",
+      "value"
+    ],
+    "type": "object"
+  },
+  {
+    "additionalProperties": false,
+    "properties": {
+      "type": {
+        "const": "titleIncludes",
+        "type": "string"
+      },
+      "value": {
+        "description": "Text that the document title must include.",
+        "maxLength": 200,
+        "minLength": 1,
+        "type": "string"
+      }
+    },
+    "required": [
+      "type",
+      "value"
+    ],
+    "type": "object"
+  },
+  {
+    "additionalProperties": false,
+    "properties": {
+      "selector": {
+        "description": "CSS selector that must exist in the DOM.",
+        "maxLength": 300,
+        "minLength": 1,
+        "type": "string"
+      },
+      "type": {
+        "const": "selectorExists",
+        "type": "string"
+      }
+    },
+    "required": [
+      "type",
+      "selector"
+    ],
+    "type": "object"
+  },
+  {
+    "additionalProperties": false,
+    "properties": {
+      "selector": {
+        "description": "CSS selector that must be visibly rendered.",
+        "maxLength": 300,
+        "minLength": 1,
+        "type": "string"
+      },
+      "type": {
+        "const": "selectorVisible",
+        "type": "string"
+      }
+    },
+    "required": [
+      "type",
+      "selector"
+    ],
+    "type": "object"
+  }
+]
    • addedInput schema / properties / url / description
      Added value: +"Public HTTPS page URL. Private, loopback, credentialed, and non-HTTPS URLs are rejected."
  2. First observed

TDQS

A4.8/5.0
Behavior5/5

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

Annotations declare readOnlyHint=false and destructiveHint=true, and the description explains exactly why: a $0.10 USDC x402 charge settles irrevocably against the payer's balance, while the target website itself is not modified. It also discloses the auth prerequisite (x402-aware client) and the side effect of creating a stored report, all beyond what annotations carry.

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?

The routing decision is front-loaded in the first sentence, and the assertion list, cost, settlement, and prerequisite follow in a logical order. Payment is covered in two separate sentences ('Costs $0.10 USDC via x402' and 'Successful paid calls settle...') that could be merged without loss.

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?

For a paid, mutating-side-effect tool with no output schema, the description covers cost, irreversibility, auth requirements, scope (three viewports), and alternatives. An agent has everything needed to decide whether it can and should call it.

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, but the description adds one genuinely new semantic: assertions are evaluated across all three viewports, a behavior absent from the schema. It also enumerates the six assertion types, though those are already expressed via const in the schema.

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?

The description states a specific verb and resource: evaluating explicit website assertions for pass/fail across three viewports. It immediately distinguishes itself from the sibling check_page, so an agent can route without opening either schema.

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?

It gives an explicit selection condition ('only when the caller supplies explicit website assertions') and names the alternative ('For broad exploratory browser QA use check_page instead'), plus a hard prerequisite (x402-aware client, standard assistants cannot sign). Nothing is left to inference.

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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