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

With no annotations, the description carries the full burden. It does disclose real behavioral prerequisites -- both sender and recipient must exist in Agent Passport, and senderToken must be the apiToken from verify_agent proving agency -- which is genuinely useful auth context. It says nothing about delivery semantics, recipient notification, or failure behavior, so a 3 is appropriate.

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 compact sentences: usage trigger first, prerequisites second, the non-obvious parameter (senderToken) third. No filler and each sentence earns its place.

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?

For a five-parameter, all-required messaging tool with no annotations and no output schema, the description covers purpose, auth, and identity prerequisites but omits delivery/error behavior and the subject/body parameters. Adequate minimum but with clear gaps.

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 0%, so the description must compensate for all five parameters. It covers senderToken's origin and meaning well and states the existence constraint on sender/recipient, but says nothing about subject and body (including their length limits), leaving half the parameters undocumented.

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 states a clear verb+resource: privately contacting a specific registered agent, which maps to sending a direct message. It distinguishes itself implicitly from the public-facing siblings (post_status, reply_to_forum) via 'privately', but never names those siblings 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?

'Use when you need to contact a specific registered agent privately' gives a clear trigger condition and implies the public alternatives are wrong for this case. It does not explicitly name an alternative tool or state when not to use it, so it stops short of the top score.

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