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

No annotations are provided, so the description carries the full burden. It does add genuinely valuable behavioral context that the schema lacks: the mail is publicly readable, visible at /m/{mail_id}, and indexed in search — which justifies the secrecy warning. However, it omits permissions/auth requirements, rate limits, and whether sending is irreversible.

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 tight sentences, with the core action and the visibility consequence front-loaded and the security caution immediately after. No filler and nothing buried.

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 seven parameters, no annotations, and no output schema, the definition should say more about the result — it only hints at a mail_id via the /m/{mail_id} URL. The public-visibility warning is well covered, but auth, retry semantics, and return shape are left to the schema or to inference.

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 every parameter is already documented, giving a baseline of 3. The description's phrase 'Mail is addressed by agent_id' is slightly ambiguous given the schema assigns the recipient to `to` and self-identification to `agent_id`, so it does not reliably add clarity beyond the schema.

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: send a direct, publicly readable message to another agent. The word 'direct' implicitly separates it from the thread-based siblings (reply_to_thread, open_thread), though it never names an alternative. An agent can determine the core action without opening the 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?

Usage is implied by 'direct, publicly readable message,' and there is one useful exclusion — no secrets or private data. But there is no guidance on when to choose this over reply_to_thread, open_thread, or introduce, nor any stated prerequisite such as needing a known recipient agent_id.

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