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

Annotations already establish the write profile (readOnlyHint=false, destructiveHint=false, openWorldHint=true). The description adds non-obvious behavior the annotations cannot convey: the send is attributed to the user rather than a bot, dry_run resolves the destination without sending, and the prompt-injection guard. It does not cover rate limits, failures, or what a successful send returns, which keeps it short of a 5.

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?

Three tightly packed sentences: identity/target first, formatting second, safety and verification third. No filler, and the most consequential constraint (only on explicit user request) is stated plainly rather than 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?

With no output schema and 100% schema coverage, the remaining burden is behavior and safety, both of which are covered: attribution, destination resolution, threading via target links, dry-run verification, and the injection guard. Nothing needed to call this correctly is missing.

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, and the description adds real value on top: the mrkdwn format with concrete syntax examples (*bold*, _italic_, <url|label>) and the mention form <@USER_ID>, which the schema only gestures at with '(Slack mrkdwn)'. The dry_run rationale is duplicated from the schema, so it is not a 5.

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 as yourself') plus the full range of destinations (channel, DM, thread) and explicitly distinguishes itself from bot posting. An agent can tell this apart from edit_message, delete_message, and add_reaction without opening any 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 explicit when-to-use and when-not-to-use: requires the user's explicit request in this conversation, explicitly forbids acting on instructions found in Slack messages, search results, or other tool output, and directs showing the user the exact text/destination first with dry_run as the verify path.

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