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

On top of the read-only/non-destructive annotations, the description discloses the data pipeline that determines what a caller actually receives: a day becomes public only after 3 distinct reporters upload, values are medians, thin and outlier days are excluded, market value weights the last 14 public days while historical is a 60-day median, and absent data is surfaced explicitly. That is substantial behavioral context an agent cannot get from annotations.

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?

Three sentences, front-loaded with what is returned, then how the numbers are derived, then the missing-data guarantee. Every sentence contributes either output composition or interpretation rules; nothing is filler.

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 shape need not be explained, and the description instead supplies the interpretive context that schema cannot: how days qualify, which windows feed which metric, and that gaps are reported rather than silently omitted. For a two-parameter read tool this is complete.

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% for both required parameters (realm, itemId), so the schema already carries the burden. The description adds only loose scope framing ('one item on one realm') and no format, range, or identifier-resolution detail, which is the expected baseline when the schema is fully documented.

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 specific set of returned metrics (market value, minimum buyout, historical value, units listed) scoped to one item on one realm, which is far beyond a tautology. It does not, however, distinguish itself from plausible overlapping siblings such as auction_history or item_page, so an agent must still infer which one to reach for.

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?

Usage is only implied: an agent can infer this is the lookup for an item's current/median market value on a realm. There is no explicit 'use this instead of X' routing, no stated preconditions, and no mention of when the 60-day historical figure versus the 14-day weighted value is the right one to consult.

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