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

Annotations already declare the read-only, idempotent, non-destructive nature of the tool. The description adds the return-value scope but does not disclose additional behavioral details such as whether pricing data is cached, whether authentication is required, or whether plan availability changes.

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 compact sentences: the first states what the tool returns and front-loads the key output fields; the second adds practical next-step guidance. Every sentence contributes value with no redundancy or filler.

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 present, the description carries the burden of explaining return values, and it does so by listing the fields returned per plan. For a zero-parameter read-only tool, this is complete enough for an agent to call it successfully and understand the result.

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 coverage is 100%, so there is nothing for the description to explain about parameters. The baseline of 4 applies because no parameter semantics are needed.

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?

Description states a specific action ('returns') and resource ('per-plan') and enumerates the exact output fields: monthly fee, included connection minutes, concurrent sessions, max session length, and avatar count. This clearly distinguishes it from sibling list_* tools like list_sessions and list_avatars.

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 description gives workflow context: try the Free signup plan first, then use create_checkout if the included limits are insufficient. This implies list_plans is the tool to consult before deciding on a plan, though it does not explicitly state exclusions or when not to use it.

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