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

Adds real behavioral context beyond the annotations: the message is visible to only one user, disappears on navigation, and 'DELIVERS immediately' (i.e., it is not a draft or scheduled send). The destructiveHint=true annotation is left unexplained, but nothing contradicts it.

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 compact sentences, with the core behavior (visibility, ephemerality) front-loaded and the operational warning placed last. No filler.

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 three-required-param messaging tool with a fully documented schema and no output schema, the description covers the essential traits an agent needs: scope, immediacy, and the confirm-before-sending caution. Minor gap is that it doesn't address the destructiveHint flag or failure modes.

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 the schema already documents all five parameters, including the 'user' being the only viewer and 'thread_ts'. The description reinforces the required triple (channel, user, text) but adds no syntax or format detail beyond the schema, making the baseline 3 correct.

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 precise verb+resource ('Post a Slack message') with the defining qualifier 'visible ONLY to one specific user in a channel' and even names the underlying API (chat.postEphemeral). This distinguishes it cleanly from send_message and schedule_message among the siblings.

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 usage context: it delivers immediately, so the agent should confirm channel, target user, and text first. It does not explicitly name the alternative (send_message) or state when NOT to use this tool, so it falls short of a 5.

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