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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so the safety bar is low. The description goes further by disclosing the shape of the response and, importantly, the conditional behaviour for multi-dimensional zarr variables (axes/labels, prescribed slice, xarray expression for other slices) – useful context beyond the 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?

It is compact and front-loads the returned-fields summary, but the second sentence is telegraphic to the point of ambiguity – 'the slice /query serves (prescribed)' and 'an xarray line for any other slice' require guesswork about what 'access zarr' refers to.

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 re-explained, and annotations cover the safety profile. The description adds the conditional zarr branch, leaving only minor gaps such as behaviour on an unknown id and explicit routing from search_variables.

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 is a single required parameter and the schema already documents it at 100% coverage ('a variable id from search_variables'). The description adds no further constraint, format or example for the id, 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?

The description states a concrete verb+resource: it returns the full description of one variable, and enumerates what that description contains (names, unit, dataset, coverage, quality flag, licence, DOI). It is clearly the detail-fetch counterpart to search_variables, though it never names that sibling explicitly.

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: the id parameter (documented in the schema, not the description) says 'a variable id from search_variables', which suggests the search-then-describe flow. The description itself gives no when-to-use guidance and does not distinguish this from get_data, list_datasets or aggregate.

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