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

With no annotations, the description carries the full burden, and it does disclose the meaningful behavioral facts: the result includes a signed direction and a full unit breakdown, plus an explicit per-call cost of $0.003. It says nothing about timezone/offset handling or malformed-input behavior, which is the main remaining gap for a time-difference tool.

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 sentences, front-loaded with what is returned before the use cases, plus a short price note. Nothing is redundant and every clause adds information.

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 and no annotations, the description compensates well by describing the return content (direction, totals, breakdown) and the cost. For a two-parameter, read-only computation, the only meaningful omission is how timezones or offsets in the ISO-8601 inputs are handled.

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 100%, so both 'from' and 'to' are already documented as ISO-8601 instants, and the description adds no format or timezone detail beyond that. Baseline 3 applies; the description's mention of 'two instants' only echoes the schema.

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 computation (elapsed time between two instants) and enumerates the shape of the result: direction, per-unit totals, and a d/h/m/s breakdown. This clearly separates it from siblings like add_duration and convert_duration, though the closest sibling, business_days_between, is never named for contrast.

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 three concrete use cases (retry delay, rate-limit reset, deadline check) that tell the agent when this tool fits. It stops short of exclusions or pointing at alternatives, so it is clear context without routing guidance.

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