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

Despite annotations already marking readOnly, openWorld, and idempotent, the description adds substantial behavioral context: checks hostname and parent domains against specific public databases, uses hostname shape signals, returns clear | caution | risky | unknown with language-independent signal_keys, and explicitly warns that a clear verdict does not mean safe. 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.

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

The description is longer than a one-liner but every sentence earns its place: purpose, workflow context, checks performed, verdict output, important caveat, companion tool, and cost/auth note. The main purpose and differentiation are front-loaded.

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 explains the output vocabulary (clear | caution | risky | unknown) and signal_keys. It also covers the key caveat ('NOT that the endpoint is safe'), free usage, no API key requirement, and input format via the schema. An agent has enough to call it correctly and interpret results.

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 description coverage is 100%, so the schema already documents both url and lang. The description's contextual framing of the endpoint as a payment endpoint is helpful, but it does not add parameter-level meaning beyond what the schema provides, which matches the baseline.

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?

The description opens with a specific verb and resource: 'Screen a payment ENDPOINT (URL or domain) before calling it.' It immediately distinguishes itself from address screening and names the complementary sibling screen_address, so an agent can tell exactly what this tool covers.

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?

It gives explicit situational context: in x402 the agent discovers a URL, receives a 402, and then must screen the endpoint before paying. It also states 'The half that address screening cannot answer' and 'Use together with screen_address,' providing clear when-to-use and companion-tool guidance.

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