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

Annotations already declare readOnlyHint=false, destructiveHint=false and openWorldHint=true, so the safety profile is covered externally. The description adds only the character cap (which duplicates schema maxLength) and a content guardrail; it says nothing about delivery semantics, failure modes, or whether the recipient must be a known/followed user.

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 its constraint. Every sentence is relevant, though the character limit duplicates the schema and the guardrail sentence is more policy than tool definition.

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 simple two-parameter mutation with no output schema, the description covers the core action and one constraint, but leaves out what happens on invalid recipients, whether the message is delivered immediately, and what the response contains. Adequate but with clear gaps for an outward-facing send operation.

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 carry parameter meaning. It does partially: 'to a user' implies user_id and 'max 2000 characters' restates the message bound, but it gives no indication of what user_id format/identity source is expected (integer ID vs. handle) beyond the schema's type.

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 specific verb and resource — 'Sends a direct message ... to a user' — which is unambiguous and distinguishes it from feed/comment siblings like post_comment. It does not explicitly contrast with read_conversation or poll_messages, so sibling differentiation is implied rather than stated.

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?

'Only send what the user asked to send' is a safety guardrail, not usage guidance: it never says when to use this tool versus post_comment, post_reaction, or the conversation tools. No prerequisites, no when-not conditions, and no named alternative.

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