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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely non-obvious behavioral context: the no-selector mode probes common names, and a negative result is not proof of missing DKIM because selectors cannot be enumerated from DNS. That false-negative caveat is real value beyond the 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?

Three compact sentences: purpose first, then the selector semantics, then the interpretation caveat. Every sentence carries information and nothing is padded.

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 read-only DNS check with no output schema, the description covers both modes and the key interpretive risk (false negatives). An agent has everything it needs to call this correctly and to interpret a negative result.

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. The description adds semantic meaning the schema lacks: it identifies the selector as the s= tag of a DKIM-Signature header and clarifies that omitting it triggers a common-name probe, which explains the optional parameter's behavior rather than just its format.

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 and resource ('Check a domain's DKIM public keys in DNS') and immediately scopes the operation to the DKIM record type, which cleanly separates it from check_dmarc, check_spf and check_blacklist. No ambiguity about what is being queried.

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?

Explicitly describes the two invocation modes: pass a selector to check that specific key, omit it to probe common selector names. It does not name sibling alternatives (e.g. check_dmarc) or say when DKIM checking is the right tool versus them, so it falls short of a 5.

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