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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds meaningful behavioral context beyond annotations: the exclusive counting convention (start date is day 0), the 'today' resolution in tz, and the negative-days behavior implied by the schema. It doesn't describe return format, but the output schema exists and the operation is a pure calculation, so the added context is sufficient.

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 sentences, each earning its place: the core operation and counting rule, the 'today' behavior, and sibling routing. The most important semantic detail (exclusive counting) is front-loaded. No wasted words.

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 pure read-only date calculation with full schema coverage, an output schema, and annotations covering safety, the description is complete. It explains the counting convention, the 'today' resolution, and routes to alternatives. An agent has everything needed to select and invoke this 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 documents all three parameters (tz, date, days) with types, defaults, and constraints. The description adds the key semantic detail that counting is exclusive (start date is day 0), which clarifies the 'days' parameter's meaning. However, it doesn't add much beyond that because the schema already carries the parameter documentation. Baseline 3 is appropriate.

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 states a specific verb ('add') and resource ('calendar days to a date'), and immediately clarifies the counting convention ('exclusive counting: the start date is day 0'). It also distinguishes itself from siblings by naming business_days and days_between as alternatives. This is a clear, specific purpose statement.

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 explicitly says when to use this tool vs alternatives: 'For working days use business_days; for the gap between two dates use days_between.' It also explains the 'today' behavior in tz. This is explicit routing guidance, leaving nothing to inference.

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