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

Beyond the readOnlyHint annotation, the description discloses non-obvious calculation behavior: spend is valued at a fixed list price, subscription fees are excluded, and campaignId narrows only the outcome side while spend remains workspace-level. This is exactly the kind of behavioral context an agent needs and could not infer from the schema alone.

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 dense sentences carry all the essential information with no filler. Metric groups, exclusions, the campaignId caveat, and the intended use case are each given their due, and the most important scoping context is front-loaded.

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 read-only reporting tool without an output schema, this description covers the metric families, pricing basis, exclusions, and attribution semantics. It could go slightly further by stating default time-window behavior and return shape, but the provided information is sufficient for an agent to select and invoke the tool correctly.

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 the schema already defines all three parameters. The description adds useful context about campaignId's partial-attribution caveat, but it mostly reinforces what the schema already states about spend remaining workspace-level rather than introducing substantial new parameter meaning.

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 opens with a specific operation—mapping money spent to results for a time window—and enumerates the exact metric groups included. The closing 'Use it to answer what did we spend and what did we get' makes the tool's purpose unmistakable and clearly separates it from campaign-stat or credit-balance tools.

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?

It provides a clear when-to-use signal and includes important exclusions: subscription fees are not included, and campaignId produces partial attribution rather than a true campaign cost. However, it does not explicitly name sibling alternatives or say when NOT to use this tool, so it stops just short of a 5.

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