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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), but the description adds real behavioral context: results are nearest-first with exact angular separation in degrees, and it correctly handles the spatial query on an endpoint lacking ADQL geometry and Q3C. The performance caveat is left to the schema rather than repeated here.

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?

Front-loaded with the core action, then routing guidance, then the endpoint caveat. Slightly heavy on emphatic framing ('AUTHORITATIVE', 'PREFER OVER WEB SEARCH') but every sentence carries usable information.

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 usefully explains the return shape (nearest-first with angular separation) and the backend constraint. For a 6-parameter spatial tool it is nearly complete, though it does not restate pagination/limit interaction with the billion-row warning.

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 ra, dec, radius, table, limit and columns are all fully documented in the schema, including units and the small-radius performance warning. The description adds nothing parameter-specific beyond that, 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 specific verb+resource ('Find catalogue sources within a radius of a sky position') and names the concrete table families (Gaia DR3, Legacy Surveys, DES, AllWISE). It also distinguishes itself from siblings like datalab_query/resolve_entity by claiming to read the survey catalogue itself rather than a resolver.

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?

Explicit when-to-use guidance: 'what is at these coordinates', cross-identification, and counting sources around a target, with a direct directive to prefer it over web search and over finder charts/name resolvers. The agent knows which path to take without inference.

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