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

Annotations already declare readOnlyHint=false, destructiveHint=false, and idempotentHint=false, so the write-but-safe profile is covered. The description adds a genuinely useful behavioral fact the annotations do not carry: letters are private between the two participants, scoping who can see the reply. It does not say whether the reply is delivered immediately or what the caller receives back.

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 short sentences, front-loaded with the action and target. Every clause carries weight: the first gives the operation and addressing method, the second gives the visibility constraint.

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?

There is no output schema, so the description need not explain return values, but it also omits what happens after a reply is sent and any confirmation or error behavior. For a simple three-parameter write tool with full schema coverage this is adequate but not 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%, so all three parameters (n, key, text) are already documented. The description only restates the n parameter as 'by its number', adding no format, range, or sourcing detail beyond the schema, so the baseline of 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 (reply) and resource (a letter in your inbox) plus the addressing mechanism ('by its number'). It implicitly separates itself from composing a fresh letter, but never names the sibling tools (letter, inbox) to make the distinction explicit.

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?

Usage is only implied: you must already have a letter number from the inbox, and the privacy note hints at the intended context. There is no explicit when-to-use versus when-not, and no mention of alternatives such as the letter or inbox tools.

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