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 structurally. The description still adds useful domain context: result ordering, the fields present on each item, and an explicit guarantee that nothing is mailed — a meaningful assurance given siblings like send_letter exist. It does not describe pagination behavior beyond what the cursor parameter implies.

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 short sentences, front-loaded with what is returned before the usage hint and the safety disclaimer. No filler and nothing repeated.

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 return structure need not be spelled out, and the annotations fully cover the safety profile. Purpose, usage context, ordering and the no-side-effect guarantee are all present for a simple two-parameter list tool.

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%: 'limit' documents its default of 20 and 'starting_after' explains it takes the next_cursor from the previous call. The description adds no further parameter meaning, which is the expected baseline 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?

States a specific verb and resource ('Return this account's letters') plus the ordering ('newest first') and the per-item fields returned ('status, mail date and total'). The line 'Use this to answer questions about what has already been sent' and the negative framing 'creates nothing, changes nothing and mails nothing' separate it from the mutating siblings like create_draft and send_letter.

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?

'Use this to answer questions about what has already been sent' gives a clear use context, and the closing sentence implicitly excludes the create/send paths. It stops short of naming the closest alternative (get_letter_status for a single letter) or stating 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.