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

Annotations already declare readOnlyHint=true, destructiveHint=false, and openWorldHint=false, so safety is covered. The description adds real value beyond that: it discloses the two result buckets, the outcome vocabulary (sent/cancelled/failed with reasons), and the presence of an id and draft uid per entry.

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, zero filler, front-loaded with the primary resource (messages waiting to be sent). The supplementary detail about ids and draft uids follows naturally without padding.

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 no output schema, the description takes responsibility for describing return contents, and it does so by naming the outcome states and the id/uid fields. Combined with annotations covering the safety profile, an agent has everything needed to call it correctly.

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% with a single documented 'mailbox' parameter, so the schema already fully carries parameter meaning. The description adds nothing about the mailbox argument, 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?

States a specific resource and its two-tier scope: pending messages plus the last week's finished ones. The verb is implied by the name but the description pins down exactly what is returned, and it identifies a sibling (cancel_scheduled) to distinguish its role in the workflow.

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?

Usage is implied: use this to inspect scheduled sends before cancelling them, signaled by the note that each id is 'for cancel_scheduled'. However, there is no explicit when-to-use statement or named alternative for listing, so routing remains inferential.

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