Send a Slack message

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

With no annotations, the description carries the full burden. It usefully discloses mrkdwn formatting support and the default-channel fallback behavior, but says nothing about auth requirements, rate limits, whether messages can be edited/deleted after posting, or what happens on delivery failure.

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 short sentences, each earning its place: purpose and targeting first, formatting second, default behavior last. No filler or restatement of the tool name.

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 two-parameter write tool with no annotations and no output schema, the description covers purpose, formatting, and targeting adequately, but omits the return shape (e.g., whether a message timestamp/id comes back) and permission expectations, which are the remaining gaps an agent would want closed.

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 already 100%, so the baseline is 3. The description goes slightly beyond the schema by disclosing the omitted-channel fallback to SLACK_DEFAULT_CHANNEL, a behavior the schema does not state, and restates the mrkdwn formatting allowance that also appears in the text parameter description.

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') and extends it to the DM case via user id, which the name alone doesn't convey. It is trivially distinguishable from the sibling set, which is entirely Gmail-based, so an agent can route between them immediately.

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 implied by the name and the description confirms the DM-vs-channel targeting, plus the default-channel fallback. However, there is no explicit when-to-use/when-not guidance, no mention of alternatives for richer Slack actions, and no prerequisites such as required scopes.

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