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

Annotations are all false, so the description carries the transparency burden. It clearly signals a paid/prepaid, mutating session-creation operation, requires an API key, and states it is not guest-callable. It does not detail the returned session object or downstream funding steps, but the essential behavioral profile is well disclosed.

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 tightly packed sentences with no filler: cost model, free alternative, auth requirement, identification rule, buyer default, guest restriction, and REST endpoint are all covered. The most important purpose is front-loaded, and each sentence earns its place.

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?

The schema's top-level description is unusually detailed and covers constraints, exactly-one behavior, and buyer_address defaults, which compensates for many gaps. Still, the tool definition lacks an expected-response description and does not explain that an opened session likely needs funding via data_session_fund before querying. Given no output schema and six parameters, these are meaningful omissions.

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 description reinforces the exactly-one listing_slug/listing_id rule and the buyer_address default, but these largely mirror the schema. It also conveys 'per-query' and 'prepaid,' which hints at max_queries meaning. However, it does not explain open_tx_hash, proof_escrow_id, or max_queries directly, so the description only partially compensates for the 50% schema property coverage.

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 opens with 'Buy per-query access to live data listings' and clarifies it 'Open a prepaid buyer session against a public data listing.' This names a specific verb, resource, and business action, making it clearly distinct from sibling tools like data_session_fund or data_session_query.

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?

The description gives concrete usage context: requires an agent API key, is not guest-callable, and identifies the listing by slug or UUID. It also points to data_preview as a free alternative, giving the agent a clear fork. It does not explicitly discuss when to use data_session_fund or data_session_attach_escrow, but the core decision to use this tool is clear.

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