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

Annotations already declare readOnlyHint=true and that the tool performs no external writes, but the description adds important behavioral boundaries: it parses text without external DNS or HTTP lookups, does not independently verify DKIM signatures, and cannot establish the receiving provider's exact spam-filter decision. These caveats go well beyond the annotation and prevent an agent from over-trusting the results.

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 front-load the core purpose, then add output details and critical caveats without any wasted words. Every sentence contributes either function, output scope, or behavioral limits, and the exclusion of body text is placed last as a final input constraint.

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 required string parameter with no output schema, the description is complete: it explains what the tool does, what it returns, what it does not do (no external lookups, no DKIM verification), and what not to supply. No output schema exists, so the enumerated return fields fully cover the result scope.

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 parameter's type and constraints are fully documented in the schema. The description reinforces the parameter's meaning by clarifying that headers must be provided as raw text and not the body, adding modest value. Baseline 3 is appropriate when the schema already handles the parameter definition.

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 (analyze) and resource (raw email headers) with a clear goal: investigate authentication or delivery issues. It also enumerates the concrete outputs (SPF, DKIM, DMARC results, alignment, sending IP, detected issues), making its function unmistakable and distinct from siblings like diagnose_domain.

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 description specifies the input context (raw headers copied from 'Show original') and explicitly excludes situation-dependent alternatives ('Provide headers only, not the message body'). It gives clear when-to-use context (auth/delivery issues) but does not name a specific sibling to use instead, so it falls just short of a 5.

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