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

Annotations already declare readOnlyHint=true, so safety is covered. The description adds a meaningful scope constraint beyond that: results are limited to reactions made by the authenticated user ('YOU'), not all reactions in the workspace, and it discloses the composition of each returned item. It stops short of noting pagination size behavior, which the schema covers.

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?

One tight sentence for the core behavior plus one short contrast sentence. The scope (what, who, and returned fields) is front-loaded and nothing is redundant.

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 read-only list tool with full schema coverage and no output schema, the description supplies what structure does not: user-scoped semantics, item composition, and sibling differentiation. Nothing an agent needs to select or invoke it 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 description coverage is 100%, so limit, cursor, and account are already fully documented in the schema. The description adds no parameter-level detail, which is the expected baseline 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?

States a specific verb and resource ('List the Slack messages/files YOU have reacted to') and names the underlying API call (reactions.list). It also previews the returned item shape (type, location, reaction set), so an agent knows exactly what comes back without opening a 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?

Explicitly routes the agent away from the nearest sibling: 'Different from get_reactions, which looks up one specific message's reactions.' That is a clear usage condition, but it gives no guidance on when this tool is the wrong choice for other related siblings (add_reaction, remove_reaction) or edge cases like empty reaction sets.

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