React to a message

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

With no annotations supplied, the description carries the whole burden and does meaningful work: it discloses the acting identity, that sending is paced/throttled like a reply, that an empty emoji removes the reaction, and that a new emoji replaces the old. It stops short of covering failure modes (missing message, no permission) and whether repeated calls are idempotent.

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, front-loaded with the action, then behavior, then edge cases. Every clause carries information about how the tool behaves; nothing is redundant with the schema or title.

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 five-parameter mutation tool with no annotations and no output schema, the description covers identity, pacing, replacement and removal semantics adequately. It is slightly incomplete on the undocumented idempotency_key and on what the call returns or how it fails, which an agent must infer.

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 80%, so the baseline is 3, but the description adds real semantics beyond the schema: emoji acts as a replacement rather than an append, and empty removes the reaction. The idempotency_key parameter is left unexplained in both schema and description, which is the remaining gap.

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 ("React to a message with an emoji") and clarifies the acting identity ("as the account holder would from their phone"), which cleanly separates it from sibling tools like send_message or delete_message. An agent can identify the operation without opening the 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 clear operational context: the reaction is sent as the account holder and is "paced like a reply," and it explains the removal/replacement cases. It does not explicitly say when to prefer this over send_message or what to do if the message is gone, but the intended context is unambiguous.

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