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

Beyond the annotations (readOnlyHint, idempotentHint, destructiveHint), the description discloses crucial behavioral details: 'Pure UTC date-only math; no time-of-day/DST/timezone' clarifies timezone handling, 'inclusive of both endpoints' specifies diff semantics, 'negative days = before' explains subtraction behavior, and 'No signup, no API key' sets expectations for auth. These are significant behavioral traits not covered by annotations. No contradiction with annotations exists.

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 a compact three-sentence structure that is front-loaded with the main purpose ('Business-day date math'), then expands with mode details and context. Every sentence contributes essential information—mode logic, optional parameters, and operational constraints—with no filler or repetition. It is appropriately sized for the tool's complexity.

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 6 parameters, no output schema, and no siblings, the description is remarkably complete. It explains what each mode returns (date, count, boolean), how to handle optional parameters, and edge cases like negative days and inclusive endpoints. It also addresses operational context (no auth, UTC-only) and parameter combinations. No significant gaps remain.

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?

Although the input schema already describes each parameter at 100% coverage, the description adds substantial meaning by explaining how parameters combine into modes, the default weekend ('default sat,sun'), the holiday format ('comma list of YYYY-MM-DD'), and the inclusive diff behavior. This goes far beyond the schema's individual field descriptions and clarifies the inter-parameter logic.

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 the tool performs 'Business-day date math' and explicitly enumerates three distinct modes (add, diff, is) with input-output mappings. This is a specific verb+resource definition that fully clarifies what the tool does. Although there are no sibling tools to differentiate from, the description is unambiguous and comprehensive.

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 provides explicit when-to-use guidance for each mode: 'add (start+days -> the date N business days away, negative days = before)' describes when to use add; 'diff (start+end -> count of business days between...)' describes diff; 'is (date -> is it a business day)' describes is. It also explains optional parameters and defaults ('weekend (default sat,sun)'), giving clear context for invocation. No alternatives are needed since there are no siblings, but the mode descriptions function as clear usage rules.

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