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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds real value beyond that: 'Nothing is stored — the report is parsed and discarded,' plus the accepted container formats and the 2 MiB decoded ceiling. It does not discuss failure modes for malformed input, but the annotations carry most of the burden.

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-loaded with the triggering scenario before mechanics. Every sentence earns its place: trigger, output shape, input encoding and limits, and the no-persistence guarantee. No filler.

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?

An output schema exists, so the description need not enumerate return fields; it still summarizes what the parse yields at the right altitude. Inputs, formats, size ceiling, and the storage guarantee are all covered, leaving nothing an agent needs in order to call this correctly.

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 both parameters are already fully documented in the schema, including the content-based format detection for `filename`. The description restates the base64 encoding, format list, and size limit, which is consistent but largely duplicative of the structured field. Baseline 3 applies when the schema does the heavy lifting.

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?

Specific verb (parse) plus a precise resource (ONE DMARC aggregate RUA report) and an explicit statement of the produced output: per-source aggregates with sender, volume, and SPF/DKIM alignment share. This clearly distinguishes it from sibling tools like validate_dmarc_record or check_record, which operate on records rather than uploaded report files.

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 opening sentence gives a concrete trigger condition: the user uploads or pastes a DMARC aggregate XML report and asks what it says. The 'Parse ONE report' phrasing implicitly bounds scope to a single file. However, no alternative sibling is named for adjacent cases (e.g., multiple reports, or validating a record instead), so it stops short of full when-not guidance.

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