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

Annotations already cover readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is handled. The description adds that the tool returns fund instructions and specifies a prerequisite and price, which is useful context. However, it does not clarify whether any funds are deducted at this step or what the returned fund instructions contain, so behavioral transparency is only partially enriched.

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?

The description is two tight, front-loaded sentences: main action, free preview alternative, specific listing, price, and return behavior. It contains no filler and does not repeat what the schema or annotations already provide.

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, read-only, idempotent tool with rich annotations, the description covers purpose, prerequisite, price, and the fact that fund instructions are returned. The lack of an output schema is partially mitigated by that return statement, but the exact structure of the fund instructions and the next action to actually fund the session are left unspecified.

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 100% for the single required parameter, session_id, which is already described as the UUID returned from data_session_open. The description reinforces that prerequisite but adds no additional parameter-specific meaning beyond the schema, so the baseline of 3 applies.

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 a concrete verb+object ('Buy per-query access to live data listings'), names the specific listing and price, and even references the free preview path. It distinguishes itself from data_preview and data_session_open, but does not contrast itself with the closely named sibling data_session_fund, leaving some sibling differentiation unresolved.

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 clear sequence context: invoke after data_session_open, and use data_preview for a free first taste. This helps an agent decide between a paid purchase and a free preview, though it stops short of explicit when-not-to-use guidance or naming the actual funding step (e.g., data_session_fund).

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