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

Annotations only cover readOnly/openWorld; the description adds real behavioral context: free, no login, no model call, no redirects tolerated, Cache-Control max-age>=60, and the important caveat that validation is not certification. It also discloses the response shape (errors, warnings, info each with a path and spec section), which no output schema provides.

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

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

Front-loads purpose, then inputs, then returns, then cost/trust caveats, then the raw endpoint. Dense and information-rich with no filler, though the long parenthetical check list and trailing 'JSON: GET /api/ucp/validate?url=' make it slightly cramped rather than crisp.

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

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

With no output schema, the description still enumerates return contents and severities, covers auth/cost constraints, and states the external-side-effect-free nature of a read-only validation. An agent has everything needed to call it correctly.

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

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 meaning beyond it: the /.well-known/ucp path append for a homepage URL and that the profile parameter validates 'without fetching'. Both parameters are covered with no ambiguity.

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

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

States a specific verb (validate) plus a specific resource (UCP business profile) and scopes it to the published UCP JSON Schema and spec rules. Names concrete checks (authority binding, https endpoints, orphaned extensions, hosting headers), so it is clearly distinguishable from siblings like precheck or find_agent_ready_stores.

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

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

Explains the two invocation modes and their selection condition ('Pass url ... or profile ...'), including the /.well-known/ucp auto-append behavior for homepages. It gives clear context but names no explicit alternative tool or when-not-to-use case.

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