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

Annotations already declare this a safe, idempotent, closed-world read, so the safety profile is covered. The description adds a genuinely important behavioral fact beyond the annotations: the call is a stub that returns zero records in this build, which prevents the agent from mistaking an empty result for "no billed time exists."

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 short sentences, zero filler. The purpose comes first and the availability caveat follows immediately, so an agent reading only the opening words still gets the right signal.

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 parameterless stub with no output schema, the description covers everything an agent needs: what it nominally does and the fact that it yields nothing today. The only gap is lack of a pointer to whichever tool actually supplies billed time.

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?

With zero parameters and full schema coverage there is nothing to disambiguate, so the baseline of 4 applies. The description correctly implies no filtering is possible by advertising no scoping inputs.

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?

"Lists billed time" gives a specific verb plus resource, and no sibling tool covers billed time, so the agent can place it in the catalog without ambiguity. It stops short of distinguishing itself from nearby list_* tools (list_invoices, list_subscriptions) by scope, but the resource is distinct enough to be actionable.

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 explicitly tells the agent when NOT to use it: "Not available until phase 2, and this build returns no records." That is a clear, decisive usage constraint rather than implied guidance. It does not point at an alternative tool for getting billed time today, which is the only missing piece.

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