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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=true, and destructiveHint=false, so the safety profile is covered. The description adds modest context with 'bounded page' and the metric-focused answer framing, but does not disclose rate limits, authorization specifics, or pagination mechanics beyond what the schema already states.

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?

Two sentences, both informative, with the core verb-resource first and no filler. It stays within a reasonable length for a 9-parameter tool.

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 read-only list endpoint without an output schema, the description covers the key return categories (recipients, channel, parent, timestamps) and the metric-filtering intent. It omits details like default limit/start cursor behavior, but those are present in the schema, so the description is adequate for its complexity.

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 description coverage is 67%, and the description does not materially explain any parameter. Terms like 'channel' and 'campaign or newsletter parent' loosely map to message_type and campaign_id but never instruct the agent how to set them, leaving the least-documented parameters (campaign_id, message_type, metric) without added semantic value.

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 opens with a concrete verb-resource pair ('Return a bounded page of Customer.io message deliveries') and enumerates the returned fields (recipients, channel, parent, engagement timestamps). It clearly implies a distinct resource from sibling tools like cio_list_campaigns and cio_campaign_metrics, though it never names them explicitly, so it falls just short of full sibling differentiation.

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?

No guidance is given on when to choose this tool over cio_campaign_metrics, cio_list_campaigns, or cio_list_segments. The phrase 'Answers which messages reached...' implies a delivery-outcome use case, but there are no explicit when-to-use conditions, exclusions, or alternative routing.

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