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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is covered. The description adds that lookups hit 'public DNS blocklists' (external, open-world queries), but says nothing about how many blocklists are consulted, latency, rate limits, or what a listed-vs-clean result looks like.

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?

Two short sentences: what it does, then the one constraint that matters. Zero filler and the constraint is placed after the purpose where an agent will actually act on it.

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 simple two-parameter read-only tool this covers the essentials, and no output schema exists so return values needn't be explained. However, nothing tells the agent what the response conveys (listed/not listed, per-blocklist detail), which matters for interpreting a blacklist check.

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. The description earns above baseline by stating 'Give exactly one of ip or domain' — the schema has no oneOf/anyOf and no required fields, so this mutual-exclusion rule is real meaning that exists nowhere in the structured data.

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 names a specific verb ('Checks') and resource ('mail server IP address or a domain' against 'public DNS blocklists'), so the agent knows exactly what is being queried. It is clear enough to distinguish from DNS-configuration siblings like mx_check or spf_check, though it never names a sibling explicitly.

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?

Usage is implied by the purpose statement — use this to test blocklist reputation of a sending IP or domain. There is no explicit when-not guidance, no mention of pairing with email_deliverability_report or smtp_test, and no note that a clean result is not a guarantee of deliverability.

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