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

Without relying on the annotations, it discloses the key non-obvious behavior: the call BILLS THE ACCOUNT per hour unless the plan covers it, plus the 402 (insufficient balance) and 409 (inactive entity, with retry parameter) failure modes. It does not say how the subscription is terminated or that cancellation is a separate tool, and it does not explicitly frame the operation as destructive. These gaps keep it short of 5, but it goes well beyond what the annotations convey.

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 dense clauses, front-loaded with the core action and effect, then cost, then failure modes. No filler; every clause carries operational information.

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 no output schema and only two parameters, the description covers effect (fills from now on), cost, and error handling. It omits how to stop the subscription and what exactly a successful response looks like, both of which matter for a billed, non-idempotent write.

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 only 50% — acceptInactive has no schema description — and the description supplies its semantics ('retry with acceptInactive' after a 409). It also reinforces entityId's accepted forms via the schema. It stops short of explaining what accepting an inactive entity implies.

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?

States a concrete verb+resource ('Follow a trader: their fills start arriving from now on') and immediately scopes it in time, which separates it from the historical export tools named in the same sentence. An agent can distinguish this from 'subscriptions', 'fills', or 'export_files' 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?

Gives the when ('their fills start arriving from now on'), the when-not/alternative ('earlier fills: the export tools'), and low-level recovery guidance ('409 = no fill for 7 days (retry with acceptInactive)'). This is explicit routing rather than implied usage.

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