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

Annotations already cover the safety profile (read-only, idempotent, non-destructive), so the bar is lower. The description adds genuine domain context beyond them: the RUWE flag's meaning and threshold (>~1.4 means a poor single-star fit, often a binary) is decision-relevant behavior an agent could not infer from the schema.

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?

One front-loaded sentence that leads with the verb and resource before the field enumeration. The long return-value list is dense but each item earns its place given there is no output schema.

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 return-value burden and does so thoroughly, itemizing the fields and interpreting RUWE for the agent. Nothing needed to call it correctly 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 description coverage is 100% and the single parameter is fully documented in the schema (including the string-precision caveat). The description adds no syntax or format detail beyond the schema, so the baseline 3 is correct.

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 and resource — look up one Gaia DR3 star by source_id — and enumerates exactly what comes back (astrometry, parallax, distance, magnitudes, RUWE). This cleanly distinguishes it from the sibling spatial lookup gaia_cone_search and the query-building gaia_adql_query.

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 'one star ... by its source_id' scoping signals when this single-object lookup is appropriate versus the cone/ADQL siblings, but it never names those alternatives or states exclusions explicitly. Clear context, no routing rules.

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