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

Annotations only declare the safety profile (not read-only, open-world, non-destructive). The description adds real behavioral context the annotations cannot: the two timestamps are coupled, local resolution precedence, derivation of gmtTimestamp from the resolved instant, and that supplied strings are re-formatted rather than forwarded verbatim with naive strings interpreted as LOCAL vs UTC.

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 core differentiator is front-loaded in the first clause, and every subsequent sentence addresses a distinct timing edge case rather than restating the schema. It is dense with jargon and one long paragraph, but no sentence is 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?

For a mutation tool with no output schema and 5 params, the description covers the genuinely hard part (timestamp coupling and interpretation) thoroughly. It omits what the call returns and any unit-conversion note, which are minor gaps given the raw-value rule is stated.

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 only 20% (only 'when' is documented), so the description must compensate, and it does: it explains the precedence of dateTimestamp over when, the new Date() default, and the coupling to gmtTimestamp. It leaves weightValue and unitKey (the required value and its enum unit) unexplained, so it is not fully compensating.

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 names the specific operation ('addWeighIn with explicit local and GMT timestamps'), identifying both verb and resource, and implicitly contrasts it with the sibling add_weigh_in by noting the extra timestamp handling. An agent can tell which of the two weigh-in adders to pick without opening either 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?

It states the distinguishing condition (you supply explicit local and GMT timestamps) and ties behavior back to the base addWeighIn ('same raw-value, no-conversion rule'), which routes the agent correctly. It never spells out an explicit when-not or a named alternative to avoid, so it stops short of a 5.

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