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

Annotations already declare readOnly, idempotent, non-destructive, non-open-world, so the safety profile is covered. The description adds genuinely useful behavior: billed hours are excluded and rate lines are never averaged. It stops short of noting that exclusions depend on entry_mark_billed state or what the response shape is, so it clears the annotated baseline without being rich.

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?

One dense sentence front-loads the core action and output, with the exclusion rule and plan limit following. No filler, though the rate-averaging rationale is worded slightly awkwardly and the plan note sits flat at the end.

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 read-only, four-parameter tool with no output schema, the description conveys both the computation and the shape of each returned line (hours, rate, amount, total). The main remaining gap is that nothing says how to obtain project identifiers or what happens when no unbilled time exists.

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 all four parameters are already documented, including the unbilled_only default and the from/to free-vs-Pro bounds. The description restates the billed-hours exclusion, which maps to unbilled_only but adds no format or syntax detail beyond the schema. Baseline 3 is appropriate.

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?

Specific verb and resource: turns tracked billable time into invoice lines for one project, and enumerates exactly what each line contains (hours, hourly rate, amount per task, total). The 'one line per rate so two rates never average' clause differentiates it from summary/report siblings that aggregate differently.

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 invoicing framing implies when to reach for it, and the 'Free: last 7 days' note sets a scope boundary, but it never names when to prefer report, export_csv, or summary_client instead, nor states prerequisites such as needing a rate set via project_set_rate.

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