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

Annotations already declare readOnlyHint, idempotentHint and a closed-world scope, so the safety profile is covered. The description adds real behavioral substance by disclosing the exact accounting identity being applied and the derived metrics (quick ratio, NRR/GRR) rather than leaving the computation opaque.

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 sentences, front-loaded with the purpose and equation, then the outputs. The trailing calculator URL is marginal but plausible as a reference; otherwise there is no 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 an output schema present, return values need not be detailed, and the description still enumerates them briefly. Five parameters at full schema coverage plus read-only annotations leave little an agent needs to know that is missing; handling of omitted optional terms is the only unstated detail.

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?

Schema coverage is 100%, so a baseline of 3 applies, but the description goes further by placing each term in the equation with its sign — clarifying that contraction and churned ARR are subtracted and that expansion only counts existing customers. This removes the main ambiguity between contraction and churn beyond what the schema alone conveys.

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 states a specific verb and resource ('Explain how ARR moved over a period') and gives the exact bridge identity, so the tool's job is unmistakable. It does not explicitly distinguish itself from the sibling calculate_arr or calculate_churn, which is the only thing keeping it off a 5.

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 period-comparison framing implies when to reach for this over a point-in-time ARR calculation, but the description never names the alternatives or states exclusions. An agent must infer the choice from the formula rather than being routed explicitly.

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