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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description usefully adds the modeling assumptions (three-year horizon, compounding on existing base, zero new customers), but says nothing about the returned projection shape or numeric conventions.

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 core behavior and the key modeling assumption. The trailing promotional link is short but is the least load-bearing element and could be dropped without loss.

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 three-parameter, read-only calculator with a fully documented schema and safe annotations, the description supplies the essentials plus the non-obvious assumption of no new customers. The only gap is that the return value is not described, which is minor given there is no output schema.

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 description coverage is 100%, so the three parameters are already documented at the schema level, establishing a baseline of 3. The description's mention of 'gross churn and expansion' and 'existing base' loosely maps to the inputs but adds no units, ranges, or handling of implausible values.

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 ('Project ARR'), a horizon ('over three years'), and the mechanism ('net revenue retention (gross churn and expansion)'). The constraint 'compounded on the existing base with no new customers' meaningfully separates it from expansion-oriented siblings like expansion_revenue_impact_simulator, though no sibling is named directly.

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?

There is no guidance on when to pick this tool over the many adjacent ARR/retention siblings (mrr_bridge_planner, customer_expansion_revenue_matrix, arr_multiple_calculator). The only routing pointer is an outbound URL, not a when-to-use condition or exclusion.

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