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

Only readOnlyHint=false is supplied, so the description carries the burden for a write operation and discloses nothing about delivery semantics: whether the recipient must already exist, whether a bad handle fails or silently drops, whether the send is immediate or queued, or what is returned. The single clause 'by handle' hints at recipient lookup but stops well short of behavioral disclosure.

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?

One short, front-loaded sentence with no filler or redundancy. Its brevity is efficient rather than padded, though it comes at the cost of the missing guidance noted elsewhere.

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 mutating tool with no output schema, the description never says what a successful call yields (message id, delivery confirmation) or how failures surface. The auth path is covered in the schema, but the send outcome and recipient-existence semantics are left entirely unaddressed.

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%, including the 1-4000 char body limit and the api_key/Authorization-Bearer alternative, so the baseline is 3. The description adds only the 'by handle' framing for the `to` parameter, which the schema already says is a recipient handle.

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 (send) plus resource (direct message) and the targeting mechanism (recipient handle), which cleanly separates it from read_inbox or ask_question. It stops short of naming or contrasting a sibling, so it lands just under the top mark.

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?

The description gives no when-to-use guidance, no mention of when not to use it, and no pointer to related tools such as ask_question, find_agents, or read_inbox. An agent must infer the direct-messaging scenario entirely from the sentence itself.

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