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

Annotations already declare readOnlyHint and openWorldHint, and the description adds real value on top: deterministic/no-AI behavior, RFC limits applied, presence of permerror causes and warnings, and the x402 USDC payment model with a 5-calls-per-day free tier. Only latency/timeout or failure-mode caveats are absent.

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?

Front-loaded with the core behavior, followed by return contents and cost. Dense but every sentence carries information an agent needs; the pricing sentence is slightly tangential but genuinely decision-relevant.

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 does summarize what comes back (per-term lookup costs, problems, warnings) and explains the counting rules. For a one-parameter, read-only tool this is essentially complete, though it omits any statement about error responses when a domain has no SPF record.

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?

There is a single required 'domain' parameter with 100% schema description coverage and an example, so the schema already carries the semantics. The description adds nothing about accepted domain formats (bare domain vs URL vs trailing dot), so the baseline 3 is appropriate.

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 (expands recursively) and resource (SPF record + RFC 7208 lookup/void-lookup counts), including include/redirect traversal. This clearly separates it from the siblings dmarc_check, domain_expiry and email_auth_report, which cover different resources.

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 rather than stated: an agent can infer this is the tool for auditing SPF lookup limits, but the description never says when to prefer it over email_auth_report or how it relates to dmarc_check. No explicit when-not or prerequisite guidance is given.

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