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

Annotations already declare readOnlyHint=false, destructiveHint=false, openWorldHint=false and idempotentHint=true. The description usefully adds that correspondence is bound to a subject so the next agent discovers it, but it never addresses the non-obvious idempotency claim (can the same post be safely retried?) nor any auth/visibility constraints.

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?

Front-loaded with the core action, then rationale, then kind guidance, then follow-up – a logical progression with little waste. The middle sentence about discoverability is slightly padded but earns its place by explaining why subjects are used.

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 6-parameter write tool with no output schema, it covers the action, the subject binding, the kind vocabulary, and the follow-up read path, so an agent has enough to call it correctly. It could add what happens on response or how retries interact with the idempotent annotation.

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% so the baseline is 3, but the description adds semantic meaning to the kind values beyond the schema's bare enumeration ('question to ask, answer to reply, handoff to pass work on, blocker to say what stopped you') and frames the purpose of subjectId as binding correspondence to an existing entity.

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?

Specific verb+resource ('POST A MESSAGE onto the thread of one specific subject') with an explicit scope and the subject types enumerated. It also names its sibling follow-up (read_thread), letting an agent distinguish this write tool from read_thread without opening either 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?

Gives clear guidance on choosing a kind (question to ask, answer to reply, handoff to pass work, blocker to state what stopped you) and a next step (read_thread to see the reply). It stops short of stating when NOT to use post_message versus sibling posting tools like submit_finding or post_reality_request.

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