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

The readOnlyHint annotation is reinforced by the 'Read-only' statement. The description adds valuable context beyond the annotation by explaining the comparative methodology ('Compares the same gross-return path with and without costs') and describing outputs ('reports direct charges plus ending-value drag'). It also discloses limitations (exclusions). This exceeds the baseline transparency expected from a read-only annotation.

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?

The description is three sentences long, front-loaded with the core purpose, and every sentence adds value (purpose, methodology, exclusions, read-only note). No fluff or repetition of schema 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?

Given the tool's complexity (9 parameters, output schema, read-only annotation), the description adequately covers the purpose, methodology, and scope. It does not need to explain return values due to the output schema. The main gap is lack of usage alternatives or clarification of edge cases, but the description is largely complete for a simulation tool.

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 schema description coverage is only 44%, and the description does not compensate sufficiently. It lists broad input categories ('expense-ratio, assumed bid/ask spread, commissions, and recurring contributions') but does not add meaningful detail about individual parameters, their interactions, or the meaning of 'ending-value drag'. The schema already describes a few parameters (e.g., spreadPct, expenseRatioPct), so the description adds little beyond a high-level summary.

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 uses a specific verb ('Simulate') and clearly identifies the resource (ETF cost) and scope ('expense-ratio, assumed bid/ask spread, commissions, and recurring contributions over a holding period'). It also differentiates from sibling tools by emphasizing the comparison of gross-return paths with and without costs, which is unique among the listed getters and portfolio tools.

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 provides clear context for when to use the tool (to simulate ETF costs over a holding period) and explicitly states exclusions ('Taxes, FX, market impact and brokerage-specific fees are excluded unless represented by the inputs'). However, it does not name alternative tools or explicitly state when NOT to use it, so it falls short of a 5.

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