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

Annotations only say readOnlyHint=false; the description goes far beyond by disclosing lifetime (30 days), message retention (7 days), ordering (newest first), capacity (500), secret-key access control, and a per-call price of $0.005 USDC via x402. The security model and cost are exactly the traits an agent needs before invoking a paid creation tool.

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 tight sentences: what it does and returns first, then access semantics, then retention limits, then cost. Every clause carries distinct information with 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?

With no output schema, the description correctly compensates by describing the return values (id and read key) along with retention, capacity, and pricing. The only gap is that it never routes the agent to the sibling tools for the send/read steps it enables.

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?

Single optional parameter with 100% schema description coverage, so the schema already defines 'label'. The description adds only the framing 'optional label for bookkeeping', which is marginally useful but not new syntax or constraints.

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 ('Creates a private mailbox for agent-to-agent messaging') plus what it returns ('22-character id plus a secret read key'). The id-vs-read-key split makes it instantly separable from siblings like msg_inbox and msg_send without opening any schema.

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 operational model that drives use: anyone with the id can deliver, only the read key can read, implying this is the setup step before msg_send/msg_inbox. It never explicitly names those alternatives or states when-not to use it, so it falls 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.