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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive, so safety is covered. The description adds genuine value beyond that by noting the recursive lookup count and the 10-lookup failure threshold — the specific insight that makes SPF checks non-trivial. It stops short of describing the pass/fail output shape.

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?

One sentence, front-loaded with the primary action, followed by the supplementary lookup-count detail. No redundancy or filler.

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?

With no output schema, the description must hint at return value, and it does so by noting validation plus lookup-count reporting. An agent knows what it will get. Minor gap: it doesn't state whether results include raw record text or per-mechanism breakdown.

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?

Single parameter with 100% schema coverage, and the schema already supplies the 'acme.com' example. The description adds no syntax guidance beyond what the schema provides, which is the correct baseline-3 outcome when the schema does the heavy lifting.

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: 'Look up a domain's SPF record and validate it.' It adds a concrete behavioral detail (recursive DNS lookup count) that distinguishes it from generic DNS siblings like check_dns, lookup_txt, and check_dkim.

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 context is implied by 'domain's SPF record' — an agent can infer this is the SPF-specific checker among the check_dmarc/check_dkim/check_bimi family. However, it never explicitly says when to use this versus lookup_txt or check_dns, nor lists prerequisites such as the domain needing to be resolvable.

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