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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is established. The description adds valuable behavioral context beyond annotations: it clarifies the 'covered' universe limitation and specifies exactly what is measured (e.g., weekly returns, three overlap measures).

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?

A single, dense sentence front-loads the scope ('Any two covered US ETFs') and then enumerates all outputs in a clear list. Every clause adds information and there is no filler or repetition.

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, the description takes on the job of explaining what the tool returns, and it does so thoroughly by itemizing overlap metrics, shared holdings, correlations, and fund characteristics. It could further clarify response formatting or exact behavior for uncovered/unrecognized tickers, but for a read-only comparison tool the coverage is strong.

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 parameters are already fully documented in the schema. The description confirms both are US ETF tickers but does not add meaningful semantic detail beyond that, matching the baseline expectation.

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 clearly identifies the tool's action (compare), the resource (US ETFs), and the specific outputs (overlap, shared holdings, correlation/covariance, cost/return/risk). The focus on 'US ETFs' and overlap/risk metrics distinguishes it from siblings like compare_stocks and overlap_matrix.

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 phrase 'Any two covered US ETFs' gives clear context for when to use the tool and implicitly scopes it to the available ETF universe. It stops short of explicitly naming alternatives or saying when not to use it, so it misses the top score.

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