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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so safety is covered by structured data. The description adds genuine behavioral context beyond that: the result includes a prior-period comparison, and the output is intended for reasoning rather than rendering. It stops short of return-format or pagination detail, but with output schema present that gap is minor.

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?

Two tight sentences with zero filler; the scope statement comes first and the routing hint second. Every clause earns its place.

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 an output schema carrying return values and full schema coverage on parameters, the description only needs to establish purpose and routing, which it does. It is nearly complete, though it could have noted the default window behavior.

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 both parameters (period, summary) are already documented with defaults and the enum list. The description adds nothing about either parameter, 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 resource (categorized spending) plus a distinguishing capability (prior-period comparison), and explicitly distinguishes itself from the sibling spending_breakdown. An agent can tell the two apart without opening either schema.

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?

'Numbers to reason with — to display, use spending_breakdown' names the alternative and the exact condition that routes to it. This mirrors the ideal pattern of stating when this tool is correct and when the sibling is.

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