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

Annotations already declare the write and non-destructive profile; the description adds real value by disclosing that delivery is immediate and irreversible once sent, hence the mandatory approval step. It does not mention the 4000-character cap consequence, rate limits, or what happens on failure or duplicate sends (idempotentHint=false goes unremarked).

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: purpose first, then the safety-critical constraint in caps and front-loaded before the call. No filler or repetition.

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?

Covers the riskiest aspect (human approval, immediacy) but leaves three required params undocumented in both schema and description, and says nothing about the result of a successful send. Adequate for safety, incomplete for correct invocation.

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 carries full burden, and it only glosses "message" as "the exact text." It never explains what conversationId or accountId are, where to obtain them (e.g., list_conversations or get_conversation), or how the length limit applies.

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 message") plus the scoping constraint that it must be an existing conversation, which distinguishes it from siblings like send_broadcast, send_business_email, and reply_to_comment. It stops short of naming 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?

Gives an explicit precondition: always show the user the exact text and obtain approval before calling, because it reaches a real person immediately. That is strong when-to-use guidance for a send path, but there is no guidance on alternatives for non-conversation delivery (broadcast, email) or on picking the conversation.

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