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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the description's burden is lighter. It adds useful scope transparency by naming the returned billing-date fields and clarifying that this tool does not cover quotas and usage. It does not discuss auth expectations or edge cases like an absent plan, but that is minor for a zero-parameter read-only tool.

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 purposeful sentences with no wasted words: output definition, example triggers, and alternative-tool routing. The most important scope information is front-loaded in the first sentence.

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 zero-parameter, read-only, idempotent tool with an output schema and clear sibling differentiation, the description covers everything an agent needs to invoke it correctly. It states what it returns, when to use it, and when to use get_plan_limits instead.

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 has zero parameters and the schema is empty, so there are no parameter semantics to explain. The description still adds value by specifying what the returned data includes, which is sufficient given the absence of parameters.

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 opens with a specific verb and resource: 'Return the user's plan + billing-date info', then enumerates the exact fields (tier, renewal date, trial end date, cancel-at-period-end state). It clearly distinguishes itself from get_plan_limits by stating this tool is 'purely about plan identity and billing dates'.

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 provides concrete triggering user queries ('when does my plan renew?', 'how many days left on my trial?', 'is my subscription cancelling?') and explicitly excludes quotas/usage requests, routing them to get_plan_limits. This leaves no ambiguity about when to select this tool over siblings.

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