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

Annotations already declare readOnly, openWorld, and non-destructive behavior. The description adds endpoint paths and the full status_id ladder plus aggregate buckets (passed/failed/alive), which is real domain behavior beyond annotations. It does not discuss auth, rate limits, or response shape, but with annotations present this is solid.

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?

A single dense paragraph front-loads purpose, then endpoint, prerequisite, and domain mappings; every clause is useful for interpreting counts. There is no filler.

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 two-parameter count tool with no output schema, it explains grouping modes, status codes, and bucket semantics, which is enough to call and interpret. It stops short of describing exact response shape or subject identifiers, but those are minor gaps.

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 baseline is 3; the description adds meaning by explaining by='status' vs by='subject' and giving the full status_id mapping and aggregate buckets. It also tells where session_id comes from, exceeding the schema's terse descriptions.

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 states a specific verb ('Bill count') and resource ('per status ... or per subject') scoped to one session, and the title reinforces it. It does not differentiate from siblings such as get_session_dashboard or get_weekly_stage_breakdown, so 4 rather than 5.

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 the prerequisite 'Get session_id from list_sessions' and implies use for session-level bill counts, but never says when to choose this over get_session_dashboard, get_weekly_stage_breakdown, or list_bills_by_stage. No explicit exclusions or alternatives are provided.

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