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 modest behavioral context beyond that: it is a free public API (implies no auth) and it parses the public key and flags issues, hinting at the kind of analysis returned. It stops short of richer detail like rate limits or what categories of issues are flagged.

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 terse clauses with zero padding, and the core verb is front-loaded. 'Free public API' is a brief preamble that still earns its place by signaling no authentication is needed. Nothing is wasted, but there is no structural ordering beyond a single run-on sentence.

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 lookup tool with an output schema present and annotations covering the safety profile, the description supplies everything needed to invoke it correctly: the resource, the DNS-name construction, and the fact that it returns issue flags. The only gaps — precise issue categories and error behavior — are plausibly covered by the 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?

With 100% schema description coverage, the baseline is 3, but the description genuinely adds meaning by showing how the two parameters compose into the DNS lookup name ('<selector>._domainkey.<domain>'), clarifying the selector/domain relationship the schema states only separately. It does not add format or syntax rules beyond that construction.

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 and resource ('look up the DKIM record') and even gives the exact DNS name it queries ('<selector>._domainkey.<domain>'), plus the follow-on actions ('parse the public key, flag issues'). It does not explicitly differentiate itself from the DKIM-adjacent siblings like tools_dmarc_record_checker or tools_spf_record_checker, which keeps it just short of a 5.

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 — an agent can infer this is the tool for checking a specific domain's DKIM record — but there is no explicit when-to-use, when-not-to-use, or routing to alternatives such as the DMARC/SPF checkers or the generic tools_txt_record_lookup. No prerequisites or exclusion conditions are stated.

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