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

The annotation destructiveHint=true already signals side effects, but the description goes further with the explicit warning '⚠️ SENDS A REAL EMAIL', immediate delivery, and the underlying App API endpoint. It adds meaningful behavioral context beyond the annotation, though it omits details like retention or consent implications.

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, purposeful sentences with the critical side-effect warning front-loaded. There is no filler, and the endpoint/delivery details earn their place.

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 side-effecting tool with 9 parameters, nested objects, no output schema, and a sibling broadcast tool, this description is thin. It warns about real delivery but does not describe the return/ack behavior, error expectations, or how to choose between a message id and trigger name. Schema descriptions compensate for parameters, but the overall operational context remains incomplete.

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?

Input schema description coverage is 100%, so the baseline is 3. The description only echoes the `to` recipient and template concept without adding new meaning beyond the schema's per-parameter documentation.

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 uses specific language: 'Send a transactional email using a transactional message template' and names the concrete recipient behavior ('Delivers immediately to the `to` recipient'). The 'transactional' qualifier differentiates it from the sibling broadcast tool even without naming it explicitly.

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?

There is no explicit guidance on when to prefer this tool over alternatives such as customerio_trigger_broadcast. The description implies transactional use but never states exclusions or routing criteria, leaving the agent to infer the right choice from the tool name and sibling list.

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