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

The annotation destructiveHint=true already carries the safety profile, so the bar is lower. The description usefully adds a precondition (the sender must be an existing user) and the underlying endpoint, but says nothing about reversibility, rate limits, notification fan-out, or what happens to the message after posting.

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?

A single front-loaded sentence led by a WRITE warning, then the action, the actor constraint, and the endpoint — no filler and nothing buried.

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 5-parameter write tool with full schema coverage and no output schema, the description covers the required inputs and the key precondition; the optional mentioned_users parameter and any return detail are left to the schema, which is acceptable given 100% coverage.

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 100%, so all five parameters are already documented in the schema, establishing a baseline of 3. The description only restates the user_id precondition (already in the schema as 'Existing user id to send the message as') and mirrors type/id through the endpoint path, adding no new meaning.

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 gives a specific verb and resource — 'post a new message to a channel' — which clearly separates it from the read-oriented get_*/query_* siblings. It does not, however, name the nearest alternative (stream_send_reaction) or otherwise explicitly differentiate from siblings, so it stops short of a 5.

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?

Usage is only implied by the WRITE marker and the verb 'post'; there is no explicit when-to-use/when-not guidance and no named alternative. The one precondition given ('user_id must be an existing user in the app') is a prerequisite rather than routing guidance.

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