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

Annotations already provide readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false. The description adds valuable behavioral context: SPF's ten-term limit, direct vs nested lookup counting, the 255-character limit, and that it is 'computation only' with nothing looked up or stored. No contradiction.

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?

The description is information-dense but every sentence serves a purpose: purpose, parameter explanation, constraints, return values, and sibling routing. It front-loads the core concept and packs nuance efficiently without redundancy.

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?

Covers all necessary aspects: what it does, how to use each parameter, limitations (10-term, 255-char), what it returns, and when to use alternatives. Since it has no output schema, it also enumerates the return fields. Nothing an agent needs to call it correctly is missing.

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%, but the description adds substantial meaning to each parameter: concrete examples for include providers, explanations of ip4/ip6, useMx/useA, and the policy enum with real-world guidance (e.g., 'pass...should never be published'). This goes far beyond the 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 states a specific verb ('Build') and a specific resource ('SPF record') and clarifies it is a DNS TXT record. It distinguishes itself from siblings by naming check_spf and flatten_spf as alternatives for different tasks. Purpose is unmistakable.

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 states when to use this tool (building/validating a record) and when not to ('Use check_spf to resolve and validate a live record, and flatten_spf only when an existing record is over the limit'). It also warns to validate with check_spf before relying on the count, and clarifies this is computation only. Clear when/where to use.

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