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

The annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description goes further than the structured fields by disclosing that results carry 'delta-coded EA item identifiers with sparse price and token metadata' and that 'zero/absent values' are preserved with source wire fields intact — real behavioral context an agent needs to interpret results correctly. It says nothing about data volume or freshness, which keeps it from a 5.

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 compact sentences, front-loaded with the resource and purpose before the disambiguation clause. The jargon ('delta-coded EA item identifiers') is dense but each clause carries information; 'Takes no input' mildly duplicates the empty schema but is a cheap and useful confirmation.

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 not be fully spelled out, yet the description usefully flags the wire-format quirks (delta-coded IDs, sparse metadata, preserved zeros) that a schema alone may not convey. The notable residual gap is the absence of any pointer to the related prices or historical endpoints, which would complete the routing picture.

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?

There are zero parameters, so the baseline is 4 per the rubric. The description reinforces this with 'Takes no input', leaving no ambiguity about whether arguments are required, but adds no further parameter meaning because none exists.

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 names a specific verb and resource ('Get the published FC27 card-price index') and explicitly scopes it as 'a current-price index, not a historical price series', which lets an agent separate it from history-style siblings. It does not, however, distinguish it from the closely related futgg_fc27_prices endpoint, so sibling differentiation is only partial.

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 line 'This is a current-price index, not a historical price series' functions as a when-not condition, and 'Takes no input' sets a clear precondition. Beyond that there is no explicit guidance naming an alternative tool (e.g. a price-history or full-prices endpoint) or stating when this index is preferable, so usage is implied rather than prescribed.

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