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

Annotations already mark it read-only and idempotent; the description adds that it returns fund instructions, depends on an open session, and names the specific paid listing. The word 'Buy' is outcome-oriented and could be slightly misleading, but the stated return behavior keeps it from contradicting the readOnlyHint.

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 short sentences carry the purpose, free alternative, exact listing/price, prerequisite, and return behavior without filler. The key decision information is front-loaded.

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 single-parameter tool with strong annotations and no output schema, the description is complete: it tells the agent what it does, when to use it, what the input must come from, and what it returns. No critical invocation detail is missing.

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 already fully documents session_id as the UUID from data_session_open, and the description only echoes that prerequisite. No additional parameter-level meaning is added, so the high-coverage 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 states a clear action and object ('Buy per-query access to live data listings') and identifies the exact listing (cvewatch) with price. It does not explicitly distinguish itself from the sibling data_session_fund, so it falls just short of a 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?

It gives an explicit use case for paid access and points to data_preview as the free alternative, plus the prerequisite data_session_open. It does not explicitly say when to prefer this over data_session_fund, but the context is clear enough for the common path.

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