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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds genuine non-schema behavior: that daylight saving transitions are accounted for so the real elapsed hour is counted, and that output granularity is days/hours/minutes rather than a raw timestamp. It does not mention limits on span size or handling of an end before start, which keeps it short of a 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?

Three sentences, each earning its place: purpose first, then use cases, then sibling routing. No filler and the core definition is front-loaded ahead of the disambiguation.

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 an output schema present, return-value detail is unnecessary, and annotations cover the safety profile while the schema fully documents all five parameters. The description supplies the remaining DST and granularity context an agent needs to call this 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 description coverage is 100%, including the format patterns and an explicit explanation of the time_zone enum and the 'none' option, so the schema already does the heavy lifting and the baseline is 3. The description adds only that the times are read in a US zone with DST adjustment, which largely restates the time_zone schema text; it says nothing extra about how start_date/start_time pair together.

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 opens with a specific verb-plus-resource statement: it computes elapsed time between a start and an end date-time, returned in days, hours and minutes, and it flags DST-awareness. An agent can distinguish it immediately from siblings like days_between_dates or add_time_to_datetime without opening any 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 gives concrete scenarios (shift lengths crossing midnight, ticket elapsed time, on-call windows, billing) and then explicitly routes three named alternatives with the condition that selects each. This is the when-to-use / when-not-to-use pattern in full.

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