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

Annotations already declare readOnly, idempotent, non-destructive, closed-world. The description goes beyond that by disclosing the return payload — boolean fires, the local wall time, and DST gap/fold notes — which is meaningful since no output schema exists. It says nothing about invalid-expression error behavior, so it stops short of 5.

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 'Use when...', then a concrete example, then the return shape — a tight three-move structure with no filler. The example datetime repeats the schema's own example string, a minor redundancy, but the full call with all three parameters still earns its place.

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 three-parameter, read-only predicate tool with no output schema, the description covers purpose, triggering scenarios, a full example call, and the return values including DST edge-case output. An agent has everything needed to select and invoke it 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 coverage is 100%, so the schema already documents expression, at, and timezone including formats and defaults. The worked example adds a composite call pattern but no syntax or constraint detail beyond what the schema already states. Baseline 3 applies.

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?

Names a specific verb and resource: a yes/no check of whether a cron expression fires at a specific instant in a timezone. It is clearly distinguishable in intent from a next-runs lister, though it never names that sibling explicitly.

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 concrete when-to-use scenarios: validating a schedule that crosses a DST transition before shipping it, or confirming a single expected fire. It implies the single-instant use case vs listing upcoming runs, but does not state an explicit exclusion or name the sibling tool.

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