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

Annotations already cover the safety profile (readOnlyHint=true, idempotentHint=true, destructiveHint=false), and the description adds meaningful behavioral context beyond them: the specific event types returned and the semantic distinction between delivery state and recipient behaviour. It does not cover pagination or volume limits, but the output schema exists and the added semantics are genuinely useful.

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 sentences, each earning its place: the first states the core purpose, the second gives the required prerequisite, the third explains the sibling distinction. The most important scoping information is front-loaded and there is zero 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?

For a single-parameter, read-only tool with an output schema and full schema coverage, nothing an agent needs to call it correctly is missing. The description covers purpose, the dependency on the search tool, and the alternative to avoid; the annotations and output schema cover safety and return shape.

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, but the description adds value above that by telling the agent exactly where to source the value for the `id` parameter ('Get the message id from get_apollo_emailer_messages_search'). This cross-tool provenance is more helpful than typical parameter 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?

The description states a specific verb and resource ('Engagement events for one sent email: opens, clicks, replies and bounces, with timestamps') and immediately differentiates it from the message record tool, which 'carries delivery state but not recipient behaviour.' An agent can tell this apart from get_apollo_emailer_messages_search and the campaign tools without opening schemas.

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?

The description explicitly routes the workflow: 'Get the message id from get_apollo_emailer_messages_search.' It also names the alternative and explains why not to use it ('rather than inferring engagement from the message record itself'), giving a clear when-to-use and when-not-to-use signal with a concrete prerequisite.

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