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

Annotations already convey readOnly=false, idempotent=false, and destructive=false. The description adds valuable behavior beyond that: the operation is a paid/prepaid purchase, requires API key auth, cannot be called by guests, and hits a specific REST endpoint. It does not describe potential charging/refund side effects, but it adds meaningful context.

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 compact sentences, front-loaded with the core purpose and value proposition. Every sentence adds useful information: what the tool does, how to access it, and where it fits relative to data_preview and the REST API.

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?

The description covers auth, guest restrictions, and the endpoint, which is helpful. However, there is no output schema and the description does not mention what the response contains (e.g., a session ID), nor does it explain the optional parameters. Given five parameters and zero schema description coverage, more completeness would be expected.

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 0%, so the description carries the burden of explaining parameters, but it does not address listing_id, buyer_address, max_queries, open_tx_hash, or proof_escrow_id. Phrases like 'public data listing' and 'buyer session' vaguely map to listing_id and buyer_address, but optional parameters remain fully unexplained.

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 action: 'Buy per-query access to live data listings' and 'Open a prepaid buyer session against a public data listing.' It names the resource (data session) and the verb (open/buy), and it differentiates itself from data_preview by noting the 'first taste free via data_preview.'

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 provides explicit context: requires an agent API key, is not guest-callable, and points to data_preview as the free first taste. It does not explicitly say when to prefer data_session_open over other data-session siblings like data_session_fund or data_session_query, but the overall flow is reasonably inferable.

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