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

Beyond the annotations, it discloses the two behaviors that actually change how an agent interprets results: reactions are replaced per person rather than accumulated, so the list is a current snapshot and not a history, and an empty list is a normal answer rather than an error. Those are exactly the non-obvious traits annotations cannot express.

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 paragraphs, each earning its place: purpose first, then the replacement-versus-history semantic, then the empty-result case. No redundancy with the title or schema, and the most decision-relevant fact is front-loaded.

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?

The tool is simple (one required param, output schema present), and the description covers the semantics an output schema cannot convey, such as snapshot behavior and empty-list handling. The only omission is routing guidance relative to react_to_message and get_message, which keeps it short of a 5.

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% — message_id and number_alias are both fully documented in the schema, including the msg_/num_ formats and the list_numbers pointer. The description adds no additional meaning about either parameter, so the baseline 3 applies.

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 opens with a specific verb and resource: 'List the emoji reactions on a message, and who left each one.' That verb contrast (list) against the write-side sibling react_to_message makes the tool's role unambiguous without needing to open either 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?

Usage is implied by the purpose statement rather than stated: an agent can infer this is the read path for reactions, but the description never names read/write alternatives such as react_to_message (to add or remove a reaction) or get_message (for the message body itself). No explicit when-to-use or when-not-to-use guidance is given.

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