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

Annotations only say this is not read-only, but the description adds critical behavioral context: the operation costs 0.01 USDC per query, is prepaid, and is capped at 20 queries per session. This is exactly the kind of real-world consequence an agent must know before calling. It does not mention any refund or failure behavior, so it is not a perfect 5.

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?

Two dense sentences with no filler. The primary action and free alternative are front-loaded, followed by the specific listing, price, cap, and session sequence. Every clause earns its place.

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?

Despite a strong top-level pitch, the definition is incomplete for a complex financial operation with 6 parameters and no output schema. It does not mention what the tool returns (e.g., a session identifier) or explain open_tx_hash and proof_escrow_id, which are likely essential to opening the session. The input schema adds some detail, but the overall definition still leaves important call-time information to inference.

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 provides descriptions for listing_id, listing_slug, and buyer_address, and the tool description adds meaning for max_queries by stating the 20-query session cap. However, open_tx_hash and proof_escrow_id remain unexplained in both the schema and the description. With 50% schema coverage, the description only partially compensates.

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 and resource: "Buy per-query access to live data listings" and "Open a prepaid session." It also names the exact listing and price, and distinguishes itself from data_preview by framing it as the free first taste. This is immediately clear to an agent selecting among sibling 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?

The description explicitly routes free trials to data_preview with "first taste free via data_preview," which is a clear when-not-to-use signal. It also orders the lifecycle with "then fund and query," telling the agent that this tool precedes data_session_fund and data_session_query. This is explicit usage guidance.

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