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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=true and destructiveHint=false, so the safety profile is covered. The description adds real behavioral content by disclosing that l2 performs live DNS/SPF/DKIM/DMARC/DNSSEC/FCrDNS lookups and l3 performs an SMTP RCPT handshake — network-touching steps that can be slow or blocked. However, it says nothing about latency, rate limits, or what happens when a deeper level fails, which matters for a tool that reaches external mail servers.

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?

It is a single dense sentence with zero filler, and the level list is ordered from cheapest to most expensive, which front-loads the useful structure. The run-on packaging and heavy acronym load (FCrDNS, DKIM, DMARC) make it slightly harder to scan than a short bulleted list would be.

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 two-parameter validation tool with full schema coverage and no output schema, the description covers purpose, depth semantics, and network-touching behavior well. The remaining gap is that it never hints at the result shape (boolean verdict vs. per-level report), which an agent cannot infer since there is no output schema.

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 would be 3, but the description goes further by expanding each enum value into the actual checks it performs, which the bare enum names (l1_syntax … l5_smell) do not convey. The 'up to 5 progressive levels' phrasing also clarifies that levels nest rather than being mutually exclusive modes.

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 gives a specific verb ('Validate') and resource ('an email address') plus the exact mechanism: five named progressive levels with their technical content (syntax, DNS/MX/SPF/DKIM/DMARC, SMTP RCPT handshake, provider databases, AI credibility). None of the siblings (get_balance, lookup_ip, search_nearby_poi) overlap, so an agent can select this tool unambiguously.

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?

The word 'progressive' plus the level names tells the agent that depth is cumulative and cheap-to-expensive, and the schema default (l4_dbs) and the l5 opt-in for AI scoring are reinforced in the text. What is missing is explicit guidance on when to pick a shallower level — e.g. cost, latency, or failure tradeoffs — and no exclusions or alternatives are named.

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