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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so 'Changes nothing' is largely redundant. The description still adds value beyond the annotations by disclosing the authorization requirement ('Needs the user's account') and the per-letter return fields an agent can expect without an output schema.

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?

Four short sentences, zero filler, with the usage trigger front-loaded before the return, safety and auth notes. Nothing is repeated that the reader doesn't need at call time.

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 compensates by naming the returned fields, and it covers the trigger, filter, safety profile and account requirement. An agent has everything needed to decide to call this and to interpret the result.

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%: both limit (default 10, max 50) and the status enum are fully documented in the schema. The description only restates 'optionally filtered by status' and 'recent,' adding no format or semantics beyond what the schema already provides, 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?

The description names a concrete verb and resource with scope: seeing 'recent letters, optionally filtered by status,' and it enumerates the returned fields. It separates itself in spirit from get_letter (single letter) by emphasizing a plural, filtered listing, but it never names that sibling explicitly.

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?

It states a clear trigger — 'Use this when the user wants to see their recent letters' — and notes the optional status filter. There is no explicit when-not guidance (e.g., use get_letter for one specific letter), so it stops short of the top band.

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