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

Beyond the readOnlyHint annotation, the description adds significant behavioral detail: deduplication to one row per fund rather than per venue listing, the ordering by fee, the default UCITS domicile restriction, the fact that fees are percentage points, and the explicit statement that the response reports how many funds publish no fee so a 'cheapest' claim is never made over silently omitted funds. This is excellent transparency about the tool's logic and limitations.

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 concise yet information-dense. Every sentence adds value: the core purpose, the dedup rule, the domicile default, the optional indexKey behavior, and the fee reporting nuance. It is front-loaded with the main question and efficiently covers the key behaviors without redundancy.

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, the description needs not explain return structure, but it covers all essential behavioral context: what is returned, how results are ordered and deduplicated, the default scope, the optional indexKey behavior, and the handling of missing fee data. This is a complete and self-sufficient description for an agent to select and invoke the tool correctly.

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 each parameter is already documented. The description reinforces the indexKey behavior ('Omit indexKey to list the available index families') and mentions the UCITS default that maps to the scope parameter, but it does not add substantial new parameter-level meaning beyond what the schema already states. This meets the baseline for full schema coverage.

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 opens with a concrete question it answers ('what is the cheapest way to track <index>?') and then states exactly what it returns: every fund tracking one index, ordered by fee, deduplicated by fund rather than venue listing. This clearly distinguishes it from siblings like compare_etfs or search_etfs by specifying fund-level deduplication and fee ordering.

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 intended use case is immediately clear ('Answer...'), and it provides helpful context such as the UCITS-buyable domicile default and the optional indexKey omission for listing index families. However, it does not explicitly name alternative tools or state when not to use this tool, leaving that to the agent to infer from sibling names.

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