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

Annotations already declare readOnly, idempotent and non-destructive, so restating those adds little; however the description contributes a genuinely new behavior contract, namely that malformed input never raises a protocol error but returns an {"error": ...} payload, which is not visible in the annotations or schema. The concrete error example makes recovery behavior predictable.

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?

Front-loaded with purpose, then mode selection, then a compact example, then routing and error contract — every block is useful. Minor waste from the 'FREE' tag and from restating read-only/idempotent already covered by annotations.

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 4-parameter, output-schema-backed tool this covers all an agent needs: when to use it, which mutually exclusive mode to pick, a concrete example of the call and result, sibling exclusions, and error/retry behavior. Nothing material is missing.

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% and already documents every_days vs weekly_on exclusivity and the count clamp, so the schema carries the baseline; the description adds a worked input/output example showing how start_date, every_days and count combine, which clarifies semantics beyond the field 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?

States a specific verb and resource ('Expand a recurrence rule into a concrete list of ISO dates') and explicitly names what it is not for, distinguishing it from business_days and meeting_overlap. An agent can route to it without opening the 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?

Gives both positive conditions ('use when a recurrence rule has to become concrete dates') and explicit exclusions with the correct alternatives named ('not for counting working days in a span (business_days)', 'not for finding a slot across zones (meeting_overlap)').

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