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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the description need not repeat those. It adds value by specifying what the summary includes (volume, pass rate, top sources, rua inbox) and the prerequisite of an existing domain and DMARC ingest plan, which are not captured in structured fields.

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 main purpose and followed by key details and prerequisites. Every phrase earns its place; no redundant or filler 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?

The tool has no output schema, so the description compensates by enumerating the summary components (report volume, pass rate, top sending sources, rua inbox). It also states prerequisites. Missing are error/edge-case behaviors (e.g., no reports available) and any response format details, but for a read-only summary with strong annotations, this is nearly complete.

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 description coverage is 100% — both 'domain' and 'days' have descriptive text. The description adds no additional parameter-level detail beyond the schema. It mentions account/plan prerequisites but these are contextual, not parameter semantics. Baseline 3 is appropriate.

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 uses a specific verb ('Summarize') and clearly identifies the resource ('ingested DMARC aggregate (RUA) reports for a domain'). It lists concrete output elements (report volume, pass rate, top sending sources, rua inbox) and distinguishes this DMARC-specific tool from sibling tools like get_deliverability_report or get_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 provides clear prerequisites: the domain must already be added to the account and the plan must include DMARC ingest. It implies the tool is for DMARC summary data specifically, but it does not explicitly name alternatives or state when not to use it. Still, the context is clear enough to guide selection.

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