React to fmsg message

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

Goes beyond the annotations (idempotentHint, destructiveHint=false) by disclosing the replace-on-new-emoji semantics, the one-reaction-per-person limit, the side effect of notifying other participants, and the error cases. That is meaningful behavioral context a write operation needs.

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, fully front-loaded: the primary action, its semantics, the side effect, and the failure modes. No filler.

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?

With an output schema present, return values need not be explained, and the description covers the remaining gaps: replacement semantics, notification side effect, and error conditions. Nothing needed to 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 coverage is 100%, so the baseline is 3; the description adds the effect of the emoji parameter (a new emoji replaces the previous, so setting is effectively an upsert) that the schema does not convey. It does not restate the null-clears-your-reaction convention, which the schema already covers.

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 ('Set or clear your emoji reaction on a message') with scope that clearly separates it from siblings like mark_read, send_message, or reply. An agent can select it 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 explicit failure conditions (drafts and terminal messages) and the one-reaction-per-person rule, which tells the agent when the call will not succeed. It does not name sibling alternatives, but the usage context is otherwise clear.

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