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

Annotations already declare idempotentHint=true and destructiveHint=false, and the description explains the replace-on-second-react semantics that back the idempotency claim, plus platform availability. It doesn't address auth/permission requirements beyond 'your business account', but adds genuine context over the structured fields.

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 tight sentences, front-loaded with the action, then platform scope, then per-platform parameter rule, then the replace behavior. No filler; every sentence carries information.

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 mutation tool with full annotations and no output schema, the description covers platform scope, parameter rules, and the idempotent replace behavior. Remaining gaps (success/return shape, error cases) are minor against the annotation coverage.

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 baseline is 3, but the description adds cross-parameter behavior the schema doesn't: WhatsApp ignores 'reaction' and requires 'emoji', and a second react replaces the prior one. That is meaningful semantic value beyond the per-field descriptions.

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+resource ('Add a reaction from your business account to a message') and scopes it to three named platforms with explicit exclusions (not Telegram or Bluesky). Together with the sibling dm_message_unreact, the write direction is unambiguous.

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 platform-conditional guidance: on WhatsApp pass emoji and ignore reaction names, while Instagram/Messenger use named reactions. It does not explicitly route the agent to dm_message_unreact or state prerequisites, but the platform context is clear enough to invoke correctly.

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