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

With only a title and destructiveHint=false in annotations, the description carries the behavioral burden and does so richly: it explains the posting identity, why a bot-added emoji differs from a person's acknowledgement, how duplicate reactions are treated as success, the required scope, and the reconnect behavior for older workspaces. It also discloses that the result's reacted_as field reveals what actually happened.

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?

The description is dense but front-loaded with the core action and every subsequent sentence earns its place by covering identity, duplicate handling, auth, and compatibility. There is no filler or redundancy.

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?

Even without an output schema, the description explains the key result field reacted_as and covers the important behavioral and auth contexts an agent needs. For a 4-parameter, 3-required mutation with no output schema, this is complete enough to call correctly.

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 four parameters in detail. The description restates the channel composite external_id behavior but adds no syntax or format details beyond what the schema provides. A baseline of 3 is appropriate when the schema does the heavy lifting.

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 — 'React to a Slack message with an emoji' — which an agent can immediately distinguish from sibling tools like remove_reaction, get_reactions, and list_my_reactions. The scope is precise and there is no ambiguity about what the tool does.

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?

The description clearly states the auth condition that changes behavior: it posts as the connecting person when the workspace has user-token reactions:write, otherwise as the app. It also notes that needing reactions:write and older workspaces return a reconnect hint. However, it does not explicitly name alternatives such as remove_reaction for undoing a reaction or get_reactions for viewing them, so it falls short of a full when/when-not/alternatives guide.

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