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by DevZonayed
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JavaScript
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browser_assert

Assert conditions on a page: check URL, title, element presence, or text content with CSS selectors.

Instructions

Assert a condition is true on the page. Returns {ok, got}. Kinds: url-contains, url-equals, title-contains, element-exists, element-missing, text-contains, text-equals (text kinds need a target selector; element kinds too).

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "kind": "<kind>"
    },
    "name": "browser_assert"
  }
}

Input Schema

NameRequiredDescriptionDefault
kindYes
targetNoCSS selector for element/text kinds
valueNoExpected value (URL fragment, text, etc.)
intentNoOptional human description for the trace/workflow.
tabIdNo

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A4.2/5.0
Behavior4/5

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

The description discloses return format {ok, got} and lists all assertion kinds with their parameter dependencies. However, it does not mention timeouts, error handling, or whether assertions are soft or hard. Given no annotations, this is adequate but not exhaustive.

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

Conciseness5/5

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

Two sentences efficiently convey purpose and key details without redundancy. Every sentence adds value.

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

Completeness4/5

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

The description covers all assertion kinds and return format, which is sufficient for a read-only tool. No output schema exists, so return format disclosure is helpful. Could mention retry or polling behavior, but not essential.

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?

The schema has 60% description coverage; the description adds meaning by explaining which kinds require a target selector and how the 'value' parameter is used (URL fragment, text). This compensates for schema gaps.

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 clearly states the tool asserts a condition on the page, listing specific kinds (url-contains, url-equals, etc.) and noting requirements for some kinds. This distinguishes it from sibling tools like browser_navigate or browser_click.

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

Usage Guidelines3/5

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

The description implies use for checking page state but does not provide explicit when-to-use or when-not-to-use guidance relative to alternatives. No exclusions or comparisons with sibling tools are given.

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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