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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is covered. The description adds meaningful context by saying the tool 'returns fund instructions' rather than executing a payment, clarifying that 'buy' is not an immediate transfer, and it identifies the specific listing. No contradiction with annotations is present.

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 a single compact sentence that front-loads the action, then efficiently covers the free alternative, listing identity, price, and expected output. Every clause earns its place and there is no filler or redundant restatement of the tool name.

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, the description is mostly self-contained: it names the required session concept, the listing, and the output type ('fund instructions'). The main gap is that the contents or format of those fund instructions are not described, and with no output schema, an agent has limited guidance on what to do with the result.

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 the single session_id parameter, including its UUID format and origin from data_session_open, giving 100% schema coverage. The description reinforces the sequencing but adds no additional parameter-level meaning, 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 specific action verb ('Buy per-query access') and a concrete resource ('live data listings'), names a specific listing (pubmedwatch), and differentiates itself from data_preview by calling out the free first taste. This clearly distinguishes the tool from sibling data_session_open/fund/query tools.

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 explicitly points the agent to data_preview for a free trial and states that fund instructions come 'after data_session_open', giving clear sequencing context. However, it does not explicitly state when to use this tool over data_session_fund or data_session_attach_escrow, leaving exclusions implicit rather than explicit.

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