what_changed

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

With no annotations, the description carries the full burden for behavioral disclosure, and it does provide a key behavioral constraint: only complete billing periods are compared, and it explains the rationale (avoiding calendar artifacts). It does not cover every trait (e.g., permissions, rate limits), but the given nuance is valuable and non-obvious.

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 two sentences: the first front-loads the purpose, the second adds a necessary caveat about complete periods. Every sentence earns its place, with no filler or repetition.

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?

The output schema likely covers return values, but the description lacks parameter semantics and any guidance on when to use this tool versus siblings like what_is_wasting_money. It provides the core purpose and a key limitation, but the operational context is incomplete.

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?

The input schema has two parameters ('cur', 'profile') with no descriptions (0% coverage), and the description does not mention them at all. The general purpose hints at what they might mean (current period, billing profile), but the agent is left guessing, and the description does not compensate for the missing schema descriptions.

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 uses a specific verb ('Explain') and resource ('a bill that moved'), and explicitly lists the key outputs (growth, services driving it, spikes). This clearly distinguishes it from sibling tools like what_is_wasting_money, which focus on waste rather than change analysis.

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 a clear usage context (explaining bill changes) and provides an explicit when-not-to-use rule ('Compares complete billing periods only... a month still running is not comparable'). It does not explicitly name alternative tools, so it stops 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.