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

Beyond the readOnlyHint and openWorldHint annotations, the description discloses the exact records checked, the policy fields inspected, what is reported (presence and version validity), and the pricing model ($0.002 per successful call via x402, failed calls free). This adds substantial behavioral context that annotations alone do not provide.

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?

Three sentences, each earning its place: first the core check, then the exclusion, then the pricing. Front-loaded with the main purpose and no redundant text.

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 single-parameter read-only validator with an output schema, the description covers what is checked, what is reported, the scope limitation, and payment terms. Nothing an agent needs in order 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%, so the single 'domain' parameter is fully documented in the schema. The description adds only the example 'gmail.com', which is already present in the schema, and does not contribute additional syntax or meaning.

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 ('Checks MTA-STS for a mail domain') and enumerates exactly what is examined: the _mta-sts TXT record and the HTTPS policy file. The exclusion 'Does not test the mail servers themselves' further distinguishes it from siblings like security_mx_server_test.

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?

Provides clear context for when this tool applies (MTA-STS validation) and explicitly notes what it does not cover ('Does not test the mail servers themselves'). However, it does not name a specific alternative sibling to use for mail-server testing, so the guidance stops short of explicit routing.

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