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

With no annotations, the description carries the full burden, and it does disclose two important traits: the output shape ('plain findings and an A-F grade') and the payment requirement ('Costs $0.002 USDC on Base via x402'), which is critical pre-invocation context. It says nothing about behavior for domains with missing/partial records, timeouts, or idempotency, and never explicitly states it is a read-only operation — meaningful gaps for a paid check with zero annotation coverage.

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?

Three tight sentences: capability enumeration first, then the decision question it answers, then cost last. Every clause names a distinct checkable record type; nothing is repetitive or filler.

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?

For a single-parameter, no-output-schema tool, the description covers what is inspected, what is returned (findings plus letter grade), and the cost. The only real omission is edge-case behavior when a domain lacks some or all records, which matters because findings are per-record and partial results are likely.

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?

Only one parameter with 100% schema description coverage, so the schema already defines accepted forms (domain, email address, or URL, with an example). The description's phrasing 'Check a domain's email authentication' reinforces domain-first input but adds no parsing or normalization detail beyond the schema, which is fine at this 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?

States a specific verb ('Check') and resource ('a domain's email authentication') and then enumerates exactly what is examined: SPF all-mechanism and lookup count, DMARC policy, DKIM selectors, MX/mail provider, and BIMI, plus the A-F grade output. An agent can distinguish this from address-level siblings like email_validate without opening the schema.

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?

Gives the deciding question the tool answers: 'whether mail from the domain can be spoofed and whether it is set up to reach inboxes,' which tells an agent when this is the right call. It offers no explicit exclusions or named alternative for domain-level checks, so it stops short of full when/when-not routing.

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