aggregate_across_instances

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

The description says 'Read-only', which is consistent with the annotations (readOnlyHint=true, destructiveHint=false, idempotentHint=true). It adds behavioral context by stating the operation 'fans out across instances' and produces 'combined grand totals', which is beyond what annotations provide. However, it does not disclose other behavioral aspects like potential latency, failure semantics, or how 'instances' is interpreted by default.

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?

The description is a single concise sentence with no filler words. It front-loads the key qualifier 'Read-only' and immediately conveys the core behavior. It is appropriately sized for the information it delivers, though it sacrifices detail for brevity.

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?

For a tool with cross-instance fan-out, five parameters, and no parameter descriptions, the one-line description is insufficient. It does not mention how instances are specified, what measures or group_by mean, or how domains are applied. An output schema exists, but the description alone leaves too many operational gaps for an agent to use the tool confidently.

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 0% and the description does not explain any of the five parameters (model, domain, group_by, measures, instances). The only hint is the phrase 'across instances', but it does not clarify parameter formats, defaults, or relationships. With no parameter documentation in either schema or description, the agent is left to guess semantics.

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 clearly indicates the tool performs an aggregation operation ('aggregate') that is 'fanned out across instances' with 'combined grand totals', distinguishing it from single-instance aggregation tools like aggregate_records and from cross-instance search tools like search_across_instances. However, it is phrased as a noun phrase rather than an explicit verb+resource construction, which slightly reduces clarity.

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 description implies usage is for cross-instance aggregation scenarios ('across instances', 'combined grand totals'), but it does not explicitly state when to use this tool instead of alternatives or when not to use it. Sibling tool names provide context, but the description itself offers no direct comparison or exclusion.

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