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

With no annotations, the description carries the behavioral burden. It adds valuable detail about the status_id ladder and aggregate buckets (passed, failed, alive), which goes beyond the schema. However, it does not disclose the response shape, pagination, or how subject values are sourced, so transparency is incomplete.

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 description is dense but well-structured: main behavior first, then endpoint, prerequisite, status mapping, and aggregate buckets. Every sentence adds useful information with no filler or redundancy.

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 simple count tool with two required parameters, the description covers the main behavior, parameter semantics, and a prerequisite. It lacks an explicit output shape and subject-id source, but the tool's purpose is simple enough that an agent can call it correctly with the provided information.

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 100%, so the baseline is 3. The description adds meaning beyond the schema by mapping status_id values to labels and defining aggregate buckets, which helps an agent construct correct queries. It does not elaborate on subject parameter values, but the enum and endpoint hint are sufficient.

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 clearly states the tool returns bill counts grouped by status or subject for a single session, with explicit endpoint paths. It is specific enough to distinguish from sibling tools like get_session_dashboard or get_weekly_stage_breakdown, though it does not name any alternative directly.

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 provides a useful prerequisite ('Get session_id from list_sessions') and explains the two grouping modes, so an agent knows when this tool is relevant. However, it does not explicitly state when not to use it or which sibling tool might be preferred for other breakdowns, leaving some inference to the agent.

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