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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds that the tool returns fund instructions and is meant to be called after data_session_open, though it does not detail what those instructions look like or whether any payment is actually executed. The word 'Buy' is slightly ambiguous, but 'Returns fund instructions' keeps the behavior consistent with the read-only annotation.

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 short and front-loads the main purpose. The specific listing and price are useful decision-relevant details, though the 'Listing: bleepingwatch...' line feels somewhat incidental rather than essential to the tool's generic behavior.

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 with full schema coverage, the description is mostly adequate: it states the action, a prerequisite, and the output nature. However, with a cluster of data_session siblings and no output schema, it would help to mention what an agent should do with the returned fund instructions, such as whether to proceed to data_session_fund or data_session_attach_escrow.

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 schema fully documents session_id with a clear description ('UUID of a data session you opened'), so the heavy lifting is already done. The tool description adds temporal context ('after data_session_open') but does not need to repeat the parameter details, placing it at the schema-covered baseline.

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 action: buy per-query access to live data listings, and clarifies it returns fund instructions after a data session is opened. It clearly differentiates from data_preview by calling the preview free, but it does not explicitly distinguish itself from the sibling data_session_fund, so it is not a perfect 5.

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 useful context: use data_preview for a free first taste, and this tool applies after data_session_open. However, it does not explicitly explain when to use this tool versus data_session_fund or data_session_attach_escrow, which are likely alternatives in the same workflow.

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