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

Annotations already cover read-only/idempotent, but the description adds substantial context beyond them: cost transparency ('Free, no credits consumed'), auth-dependent output shape, and an error-recovery role for 401/403. That is exactly the behavioral information the agent cannot get from structured fields.

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?

Four short sentences, front-loaded with the core purpose, then the two auth branches, then the cost guarantee, then the highest-value routing tip. No filler and no restatement of the title.

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 not be explained. For a parameterless, read-only introspection tool, the description supplies purpose, both auth modes, cost, and an error-recovery trigger — nothing an agent needs is missing.

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?

There are zero parameters, so the baseline is 4. The description correctly doesn't waste space documenting nonexistent inputs, but it also doesn't clarify why auth state changes output (a behavioral note rather than a parameter note).

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?

Names a specific resource (plan/entitlements/pricing) and states the scope precisely: available model_versions and speaker ids. It also splits behavior by auth state ('With an API key' vs 'Without a key'), which no sibling tool covers, so an agent can distinguish it immediately.

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?

Explicitly prescribes the trigger condition: 'Call this FIRST when any tool returns a 401 or 403'. It also tells you when the no-key variant applies, giving both a routing rule and an alternative-path explanation.

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