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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the description doesn't need to repeat those. It adds valuable behavioral context beyond annotations: 'Computed locally from the IANA time-zone database — no rate limits, exact' and 'DST-correct' and 'Price: free.' This informs the agent about reliability, performance, and cost without contradicting annotations.

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 three sentences with no fluff. It front-loads the primary functions, then adds reliability and cost details, and finishes with zone format examples. Every sentence earns its place, and the structure is logical and efficient.

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?

The description covers the tool's capabilities and some operational details (local computation, no rate limits, free). However, it does not specify the output format or structure (e.g., returns a string, an array of converted times, or a diff object), and there is no output schema to compensate. It also doesn't mention behavior for invalid inputs or edge cases like overlapping DST transitions. For a tool with 5 parameters and no output schema, this is a noticeable gap, though the core purpose is clear.

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 60% (3 of 5 parameters have descriptions). The description compensates for the undocumented to_tz and from_tz by stating 'Zones are IANA names (America/New_York, Asia/Tokyo, UTC).' This clarifies the expected format. It also reinforces the 'add' parameter with 'add/subtract durations' and the 'until' parameter with 'days between dates,' adding context beyond the schema. The added value lifts it above the baseline 3.

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 clearly states the tool's capabilities with specific verbs: get current time, convert between zones (DST-correct), and perform date arithmetic (add/subtract durations, days between, weekday, ISO week, Unix time). It names the resource (time zones, dates) and distinguishes itself from the sibling unit_convert by focusing on time-specific operations. No ambiguity about what the tool does.

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 its use case—time zone conversion and date math—but does not explicitly mention when to choose this over alternatives or when not to use it. It lacks exclusions or alternative tool references (e.g., 'For unit conversions, use unit_convert'). The context is clear enough for an agent to infer, but explicit guidance is absent.

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