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

Annotations already declare readOnly, idempotent and non-destructive, but the description adds something they don't: the call is free and requires no API key, which materially affects how an agent plans a call sequence. It stops short of describing result ordering or whether the list is complete versus paginated.

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?

One short sentence plus a two-word auth note — the payload fields and the no-key condition are both front-loaded with zero filler. Nothing could be removed without losing information.

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 read-only, zero-param listing with no output schema, the definition gives enough to call it correctly: what it returns, that it's free, and that no key is needed. The only missing piece is where the agent should go next (buy_credits) after inspecting packs.

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 nothing for the description to disambiguate; the schema is empty and fully specified. Baseline 4 applies, with no parameter-level value the description could have added.

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+resource ('List the credit packs you can buy') and previews the payload fields (name, credits, price), so the agent knows exactly what comes back. It does not explicitly contrast with siblings like list_subscription_plans or buy_credits, which keeps it out of the top band.

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?

'Free; no key needed' signals this is a safe entry-point call that needs no credentials, implying a browse-before-buy flow. However, it never says when to prefer this over get_credit_balance or list_subscription_plans, or that buy_credits is the follow-up step — usage is inferable but not stated.

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