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

Annotations already declare non-read-only, idempotent, non-destructive, open-world. The description adds genuinely non-structured context: a plan entitlement requirement (Plus/Student), the user-wide blast radius, and that existing schedules support patching rather than replace. It does not state what happens on entitlement failure or how conflicts between frequency and weekday are resolved.

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 tight sentences: purpose first, then eligibility, then parameter requirements. No filler, and the constraints are front-loaded before the per-field guidance.

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?

An output schema exists, so return values need no explanation, and the mutation profile is covered by annotations. The remaining gap is edge-case semantics for a fully-optional 4-param mutation (all defaults, partial patch interactions), which matters slightly given required=0.

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 0% and the schema carries only bare types/min-max, so the description has to carry the load — and it largely does: hour 0–23, weekday with Sunday=0, timezone as IANA, frequency of daily/weekly, plus the conditional rule that weekday applies 'when weekly'. It omits defaults and what happens to weekday when frequency is daily.

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?

Specific verb + resource: 'Set the user-wide Following email digest schedule'. The 'user-wide' scope qualifier and the schedule object distinguish it cleanly from sibling reads like following.digest.get and from following.pause/resume, which act on delivery rather than schedule.

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?

'after the user requests it' and 'Requires Plus or Student' give a clear consent gate plus an eligibility prerequisite, and the description tells the agent which fields a new schedule needs versus patching an existing one. It stops short of naming an explicit alternative (e.g. following.digest.get for reading the current schedule), so it is a strong 4 rather than a 5.

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