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

Annotations already declare this a safe read (readOnlyHint/destructiveHint=false), yet the description adds material context beyond them: required scopes (domains:read, reports:read), empty-result behavior (zero totals when no reports), and the error case (domain not in account). It also clarifies it mutates nothing, closing any ambiguity about report generation.

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 dense sentences, front-loaded with the purpose, then window/return shape, then permissions and edge cases. No filler clauses; each sentence carries distinct information an agent needs.

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?

With no output schema, the description still enumerates what is returned (report and message totals, DMARC pass/fail counts, per-source SPF/DKIM alignment and receiver actions) plus empty and error behavior. Nothing needed to invoke or interpret the call 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 description coverage is 100%, so the schema already documents both parameters, including the 'one of the user's domains' constraint and the day bounds. The description largely restates the 1-to-90-day/default-7 window and repeats the account-scoped domain constraint without adding new format or syntax guidance, 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 specific verb (Summarize) and resource (received DMARC aggregate reports) with scope qualifier (for a domain already in the user's account). This clearly separates it from siblings like add_domain, generate_dns_records, or analyze_email_headers, which operate on different objects.

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 the precondition (domain must already exist in the account) and a when-not clause ('Does not add domains or generate report traffic'), which implicitly routes users to add_domain/generate_dns_records for those tasks. It stops short of naming an explicit alternative for domain-level diagnostics such as diagnose_domain.

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