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

Annotations already flag non-read-only, non-idempotent behavior; the description adds economic and workflow context beyond that: 0.01 USDC/query, max 20 queries/session, and the need to fund before querying. No contradiction with annotations.

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 sentences, front-loaded with the purpose, and every clause earns its place: free alternative, exact listing, price, per-session cap, and the fund/query sequence. No fluff or repetition of schema constraints.

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 financial session-opening tool with no output schema, the description conveys the workflow and cost but omits what the call returns (e.g., how to reference the new session in subsequent fund/query calls) and leaves open_tx_hash/proof_escrow_id unexplained. The rich input-schema note and annotations cover part of the gap but not the return contract.

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 descriptions cover 3 of 6 parameters; the tool description adds meaning for max_queries via 'max 20 queries/session' and ties the listing to a concrete slug/name. However, open_tx_hash and proof_escrow_id remain unexplained by both schema and description, so the description only partially compensates for the 50% schema coverage gap.

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 specific action and resource: 'Buy per-query access' and 'Open a prepaid session', pointing at a concrete listing with price and session cap. It also differentiates from the data_preview sibling ('first taste free via data_preview') and positions itself as the first step before funding and querying, so an agent can pick it out of the sibling list.

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 a clear workflow: try data_preview first, then open a prepaid session, then fund and query. It explicitly names the free-preview alternative but does not spell out when to prefer data_session_attach_escrow or data_session_funding_package, so it stops short of full exclusion guidance.

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