Check DKIM keys

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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and non-destructive. The description adds valuable behavioral context by noting the selector sourcing limitation and the fallback to common selectors. This goes beyond the annotations, though it doesn't describe error handling or response shape, which is acceptable given the output schema exists.

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 sentences, no fluff. The first sentence front-loads the core function and output details; the second provides a crucial usage hint. Every clause earns its place.

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 read-only lookup with an output schema, the description covers purpose, selector sourcing, and parameter behavior. It omits explicit alternative routing, but the domain-specific nature and sibling names make it sufficiently complete for an agent to invoke correctly.

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 50%: the domain parameter is described in the schema, but the selectors array items are not. The description compensates by explaining that selectors are optional and default to 30 common provider selectors, clarifying the semantic role of the selectors parameter beyond the raw schema.

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 clearly states the tool's function: it looks up DKIM keys at specified selectors, detailing the key attributes it returns (type, size, flags, problems). It distinguishes itself from siblings like check_spf (SPF-specific) and dns_lookup (generic DNS) by focusing on DKIM and its selector-based lookup.

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 description provides important usage context: it explains that selectors cannot be discovered via DNS and must come from a sent message's DKIM-Signature header. It also mentions the fallback to 30 common provider selectors when none are given. However, it doesn't explicitly compare to alternatives like check_spf or dns_lookup, though the domain-specific nature makes it clear enough.

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