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

Annotations already declare readOnly and closed-world, so the safety profile is covered; the description adds the meaningful behavioral fact that no DNS lookup occurs, which is exactly the distinction an agent needs. It also previews the response shape (valid/invalid, score, grade, parsed breakdown), though that partly duplicates the existing output schema.

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, front-loaded with the scope constraint and followed by the trigger condition. No filler or repetition.

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-param read-only validator with an output schema, the description covers what it does, what it doesn't do (DNS), and when to reach for it. The only gap is not explicitly contrasting with DNS-based sibling tools.

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%, so both the record string and the spf/dmarc/auto enum are fully documented in the schema. The description adds nothing about the type parameter or auto-detection behavior, so the baseline 3 applies.

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 precise verb (Validate) and resource (SPF/DMARC record string) and immediately bounds scope to SYNTAX with 'no DNS lookup'. This distinguishes it from DNS-performing siblings like spf_analyze and dmarc_check without needing the schema.

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?

Gives a concrete trigger: 'Use when the user pastes an SPF/DMARC record and asks "is this correct?"'. That is clear usage context, but it never names the alternative DNS-based tools (spf_analyze, dmarc_check) that an agent might otherwise pick, so routing is implied rather than explicit.

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