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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so safety is covered. The description adds genuinely new context beyond them: the call is free and does not consume the assistant allowance, which directly affects an agent's decision to call it speculatively, plus the fact that the quota is scoped to 'today'.

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, with the return content front-loaded before the cost caveat. Every clause carries information an agent would otherwise have to guess at.

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-argument read tool with no output schema, the description gives enough of the return shape (plan tier plus remaining assistant actions) and the cost profile to call it correctly. It stops short of naming the concrete fields or what a quota-exhausted state looks like, which would have made it fully self-sufficient.

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?

The tool takes zero parameters, so there is no parameter meaning to convey and the baseline is 4. The description correctly signals this by describing a self-scoped lookup of 'the reader's' account with no arguments required.

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 names the specific resource (the reader's Readlink plan) and the specific measurement (assistant actions remaining today), so an agent knows exactly what comes back. It is clearly distinct from every sibling, which deal with books, shelves, notes and reading progress, though it never says so explicitly.

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 only implied: an agent can infer this is the call to make when it needs to know the plan tier or remaining assistant allowance. There is no explicit when-to-use statement, no prerequisites, and no named alternative, but the zero-parameter shape makes the invocation unambiguous once the intent exists.

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