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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds genuinely non-obvious behavior: a per-call price ($0.005 USDC via x402) and a payment endpoint, which an agent must know before calling. It stops short of explaining failure modes when payment is missing or how granularity is auto-detected.

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?

Two tight sentences: capability and return shapes first, the zero-arg default second, with the cost/payment note clearly appended as a call-to-action. Nothing redundant and no filler.

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?

With no output schema, the description carries the burden by naming all four returned formats, and it covers payment prerequisites and the default-input case. Together with the annotations, an agent has everything needed to decide and call 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 baseline is 3, but the description adds the semantic that an absent 'value' means 'now' rather than an error, which the schema only calls 'Optional'. The accepted formats (seconds, ms, ISO) are repeated from the schema rather than extended.

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 ('Convert a unix timestamp ... or ISO date') and enumerates the exact output representations, which no sibling (e.g. units_convert) claims. An agent can tell instantly that this is the timestamp-normalization tool rather than a generic unit converter.

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?

'Omit input for the current time' gives a concrete invocation condition for the zero-argument case, and the schema note explains when to omit payment. It does not, however, contrast itself with the adjacent units_convert tool or state any exclusions, so it stops short of full routing guidance.

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