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

The description says 'Returns fund instructions' which correctly indicates a non-mutating read-only behavior, aligning with the readOnlyHint and destructiveHint annotations. The word 'Buy' could imply a purchase action, but the elaboration that it returns instructions avoids a direct contradiction.

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 with a dash for a side note, efficiently conveying the core purpose, the listing, and the prerequisite. No unnecessary fluff.

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 simple one-parameter tool, it adequately mentions the prerequisite session and the free preview alternative, but it does not explicitly reference related funding actions (e.g., data_session_fund) or explain how the returned instructions are to be used, which would make the context more complete.

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 only parameter, session_id, is fully documented in the schema: it is a UUID of a data session opened via data_session_open. The description covers 100% of the schema and provides clear context on how to obtain the value.

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 clearly states the tool returns fund instructions for per-query access to a specific live data listing (NASA Image of the Day). The phrase 'first taste free via data_preview' provides a contrast that helps anchor its purpose, though 'Buy' is slightly ambiguous as it actually provides instructions rather than executing a purchase.

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 mentions the prerequisite data_session_open and the alternative data_preview for a free taste, but it does not explicitly distinguish itself from sibling tools like data_session_fund or data_session_attach_escrow, leaving some ambiguity about when to use this vs. those tools.

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