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

Annotations already declare readOnly, idempotent, and non-destructive behavior. The description adds useful context beyond annotations: it explains that the tool names the top-moving models/features/customers and that the baseline defaults to the same number of days just before. It doesn't cover rate limits or auth needs, but those are lower priority for a read-only analytical tool.

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 sentences with no wasted words: the first describes the core comparison and output, the second clarifies default baseline behavior. The purpose is front-loaded and easy to parse.

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?

Given the output schema and annotations, the description covers the essential non-schema behavior (default baseline, output content). It lacks explicit sibling differentiation and edge-case constraints (e.g., date inclusivity, maximum range), but it is sufficient for a simple read-only comparison tool.

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 coverage is 100%, so the baseline is 3. The description adds meaning beyond the schema by explaining the default behavior when baseline_start and baseline_end are omitted—they default to the same number of days just before the main period. This is valuable for correct invocation.

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 ('compares') and resource ('two periods') and specifies the output ('names the models, features and customers that moved most'). This clearly distinguishes it from simple spend retrieval tools, though it doesn't explicitly contrast with the sibling compare_models.

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 by stating it compares periods and identifies top movers, but it doesn't explicitly say when to use this tool versus alternatives like compare_models or get_spend. No exclusions or routing guidance are provided.

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