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

Annotations already declare readOnlyHint, idempotentHint, and non-destructive behavior, so the description only needs to add non-safety context. It adds that the tool returns fund instructions after data_session_open, which is useful, but the word 'Buy' creates mild tension with readOnlyHint and the description does not explain what the instructions contain.

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 purpose is front-loaded in the first sentence and the structure is compact. However, the second sentence includes listing-specific advertising detail (awswhatsnew, 0.01 USDC/query) that is helpful for discovery but not necessary for correct tool invocation, so some content is not maximally earned.

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 1-parameter tool with rich annotations and no output schema, the description gives a minimum viable picture: what it does, when it is used relative to opening a session, and that it returns fund instructions. It still omits what the fund instructions look like or what the next step is, which would be valuable without an output schema.

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 description coverage is 100%, so the schema already fully documents the single session_id parameter, including its source from data_session_open. The description adds no further semantic detail about the parameter beyond what the schema provides, making the baseline 3 appropriate.

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 states a specific verb ('Buy') and resource ('per-query access to live data listings'), names a concrete listing, and tags the free preview path via data_preview. It distinguishes itself from data_preview and data_session_open, though it does not clearly differentiate from the actual funding sibling data_session_fund.

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 implies the tool is used after data_session_open and suggests data_preview for a free trial, giving some context. However, it never explicitly states when to choose this over data_session_fund or data_session_attach_escrow, leaving usage boundaries to inference.

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