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

Annotations already declare read-only, idempotent, non-destructive and open-world. The description adds meaningful behavior beyond them: the ratio provenance (Yahoo Finance's listing of the latest annual report), that no recommendation is made, and that non-fund or unpublished-ratio lines are explicitly named rather than silently dropped.

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?

Single paragraph with the computation and formula front-loaded, followed by caveats. Several parentheticals (the provenance clause especially) make it denser than strictly necessary, but no sentence is wasted.

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?

An output schema exists so return values need no explanation; the description still covers the calculation, provenance and degradation behavior for unpublished ratios. What is missing is only minor: how the weighted ratio is computed and what happens with mixed or non-USD currencies.

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 ticker, amount, currency and the 50-item cap are all documented in the schema. The description only restates 'funds and amounts the user gives' and adds no format, currency or edge-case detail beyond the schema, so the baseline 3 applies.

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 precise verb and resource ('Computes ... what each fund's expense ratio takes a year'), gives the formula (amount × expense ratio), the outputs (per-fund cost, total, weighted ratio) and the data source. This is clearly distinguishable from siblings like etf_holdings or analyze_ticker.

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?

Ends with an explicit trigger phrase ('Use it for "how much do my ETFs cost me a year"') and frames itself as a fact-finder, not advice. It does not name an alternative tool or state when not to use it, so it falls short of the top score.

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