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

With no annotations, the description carries the full burden and does well: it discloses ordering, returned fields, timezone handling, and that failed sends stay visible with state 'failed' and the reason. It does not discuss pagination or permissions, but for a read-only listing tool the behavioral picture is largely complete.

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?

The definition is front-loaded with the core behavior and then adds only necessary lifecycle context: what has left, what failed, and when it disappears. Every sentence contributes useful information without repetition.

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 no-required-parameter list tool with no output schema and no annotations, the description is quite complete: it explains return fields, ordering, timezone, and failed-send behavior. It could additionally mention pagination or result limits, but those are minor gaps for this tool.

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?

There is only one optional parameter and the schema description coverage is 100%, so the schema already explains the address filter. The description does not add syntax or selection guidance beyond the schema, making the baseline 3 appropriate.

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: it lists messages with a 'send_at' that have not left yet, ordered soonest first. It also distinguishes this from messages already sent, which appear in their thread rather than here, so an agent can identify the tool's scope without opening the schema.

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?

It gives clear context for when this tool applies: pending scheduled messages, including failed ones that remain until cancelled. It does not explicitly name alternatives such as list_drafts or cancel_scheduled_send, but the lifecycle explanation implies when this list is the right source.

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