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

With no annotations, the description carries the full behavioral burden, and it only partially delivers: 'using the mobile user session' is a genuinely useful disclosure of execution context (implying a session must be established, cf. session_status). It says nothing about permissions, rate limits, whether the message notifies recipients, or whether failures are recoverable.

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 with no wasted words. Brevity is appropriate, though it comes at the cost of the missing behavioral and parameter detail noted above.

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 mutation tool with no annotations, no output schema, and two undocumented parameters, the description is too thin. An agent cannot tell what a successful call returns, what happens on failure, or how the required parameters should be formatted.

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 only 33% — channel_id and message_text are both undocumented in schema and description. The description adds no guidance on channel_id format (ID vs. name) or message_text content rules, which is a meaningful gap for a posting tool.

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 ('Post a message to a Slack channel or DM'), so an agent knows the core action immediately. However, it does not distinguish itself from the closely related siblings post_message_rich and reply_thread, which is exactly where ambiguity would arise.

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?

No indication of when to use this versus post_message_rich (rich formatting) or reply_thread (threaded replies), nor any prerequisite such as requiring an active session. The agent must infer selection from the sibling names alone.

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