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

Despite only a readOnlyHint annotation, the description discloses substantial behavior beyond structured data: opaque=true with a `reason` for ETFs with no filed holdings, the rule that a store outage returns DATA_UNAVAILABLE and is never opaque, the ~60-day SEC EDGAR lag, and that caveats ride in tool_notes. This is exactly the behavioral context an agent needs.

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?

Dense but front-loaded with the core purpose before the return-shape detail; nearly every clause earns its place (return keys, opaque handling, source, cost). It is on the long side but not padded or repetitive.

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 carries the full burden of describing returns and does so thoroughly: it names the top-level keys ({summary, count, etfs, units}), the per-ETF fields, and the per-holding fields including the derived-vs-filed distinction for weight_pct/pct_of_nav. Nothing material is missing.

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 coverage is 100%, so the schema already documents both parameters (baseline 3). The description adds meaning beyond the schema by stating that top_holdings is 'value-ranked' and confirming the default/hard-cap and per-ETF ticker cap, which usefully characterizes the selection order of the returned rows.

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?

Opens with a specific verb+resource ('ETF look-through: expand ... ETF tickers into the underlying holdings') and immediately scopes the source (fund's own Form N-PORT filings), which distinguishes it from siblings like get_13f_holdings and get_institutional_holders. An agent can tell what this does without opening the schema.

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 scope note 'HOLDINGS-ONLY: no yield, NAV or premium' usefully tells the agent what this tool is NOT for, and the ~60-day lag + FREE notes help set expectations. However, no sibling alternative is named and there is no explicit 'use X when Y' routing, so usage guidance is only implied.

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