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

Annotations already declare readOnlyHint=true, openWorldHint=false, and destructiveHint=false, so safety is covered. The description confirms a pure listing/audit operation but adds nothing on pagination behavior, result ceilings beyond the schema's limit, or latency, which is the bar for going above baseline here.

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 tight sentences: the first front-loads what is listed and its channel scope, the second covers filtering and intended use cases. No filler, no repetition of the title.

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 schema fully documents all 8 parameters including the required context field. What remains thin is disambiguation from the inbound-listing and single-get siblings, which the agent would ideally have to call this tool confidently.

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 100%, so every parameter is already documented in the input schema, including search semantics, status enum meaning, and the metadata filter. The description's mention of 'channel, template, delivery status, or metadata' only restates filter dimensions already in the schema, so baseline 3 applies.

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 (List) and resource (sent and scheduled messages) plus the channel scope (email, SMS, push, web-push). The 'sent and scheduled' framing implicitly separates it from the inbound siblings, but it never names senderkit_messages_get or senderkit_inbound_messages_list, so the differentiation is inferred rather than stated.

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 concrete reasons to call it — monitor deliverability, debug failed sends, audit transactional history — and enumerates the filter dimensions. It stops short of saying when NOT to use it or which sibling handles a single-message lookup, so the routing guidance is one-sided.

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