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

Annotations already establish readOnly, idempotent, and non-destructive behavior; the description adds that this tool returns fund instructions rather than executing payment, and it discloses pricing and the prior-session dependency. The word 'Buy' slightly overstates mutability, but 'Returns fund instructions' prevents a real contradiction.

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 tight sentences front-load the core purpose and then add the free-preview pointer, listing name, price, and precondition. Every clause carries useful routing information with no filler or repetition.

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 one-parameter tool with no output schema, the description supplies the precondition, the result (fund instructions), and pricing. It does not describe the contents of the fund instructions or the next step after receiving them, but the schema and sibling tools are sufficient to infer the workflow.

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?

The input schema fully documents session_id as the UUID from data_session_open, so the description does not need to repeat parameter details. The phrase 'after data_session_open' reinforces the parameter's source but adds no new semantic information beyond the schema.

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 names a concrete function: return a funding package/instructions for per-query access to a specific live data listing (vergewatch), with price and a pointer to the free preview alternative. It clearly separates this from data_preview and from data_session_open, which is a precondition rather than the same action.

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 encodes the workflow: open a data session first via data_session_open, try the free taste via data_preview, then use this tool to get paid access. It does not explicitly enumerate when to use data_session_fund or data_session_query instead, so it lacks full when-not-to-use guidance.

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