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

With no annotations, the description carries the full burden and does well: it discloses that DKIM verification only works when the body is included, that SPF/DMARC policy and receiving-server records are consulted, and that the tool is free. It omits rate limits, whether input is retained, and any failure modes.

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 cost and purpose, then the checks, then the return shape. Every clause carries information an agent needs; no filler or restatement of the tool name.

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 tool with no output schema, the description covers input forms, the checks performed, the headers-versus-body caveat, and the shape of the result ('reasons in plain words'). Nothing essential for correct invocation 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 already 100%, so the baseline is 3, but the description adds real semantic value: it explains the two acceptable input forms and the consequence of each (headers-only degrades DKIM verification). That is meaning the schema description does not convey.

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: explain why a message passed or failed authentication or landed in spam. It enumerates exactly what is examined (DKIM, SPF authorization, DMARC alignment/policy, receiving-server records, per-hop delays), which clearly separates it from siblings like verify_email or check_blocklists.

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?

The diagnostic trigger is clear: use it when you need to know why a message was rejected or spam-filtered, and it tells you the input can be the raw message or headers only. However, it never names an alternative tool (e.g. verify_email) or states when not to use it.

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