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

Annotations already cover read-only, idempotent, non-destructive, closed-world behavior, so the bar is lower. The description adds real context beyond them: 'No auth' and the gating rules ('Plans stay date-gated until Nov 16; the T-Rex is a separate sale'), which materially affect how the agent interprets the returned state.

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?

Front-loading 'Plans: list' followed by 'WHEN:' and 'Next:' is good structure. However, the parenthetical enumerating exact prices ($4.99, $14.99, $1.99, 'edition of 500, on sale now') reads as promotional copy rather than decision-relevant guidance, since the tool returns that data anyway.

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 must convey what comes back, and it does: plan tiers, add-ons, top-ups, the limited pet drop, the free tier, and a checkout-open flag. The date-gating and separate-sale caveats pre-empt the most likely misinterpretation of the response.

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 the baseline is 4. The 'Example: plan_list {}' line confirms the empty-argument call shape, which is the only parameter-level information an agent needs here.

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 verb and resource ('Plans: list') and then enumerates exactly what is returned: plans and add-ons with prices, the T-Rex pet drop, the free tier, and checkout-open status. It is clearly distinguishable from plan_checkout and plan_weekly_drop, which are named as the follow-up tools.

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?

The 'WHEN:' clause gives a concrete trigger — the agent wants to see plans, add-ons, the T-Rex drop, the free tier, or whether checkout is open. It also routes onward to plan_checkout and plan_weekly_drop. It stops short of explicit when-not guidance, but the scope is narrow enough that little is left to inference.

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