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

Annotations already declare the mutation/safety profile (readOnly=false, idempotent=false, destructive=false). The description adds useful behavior: the prepaid costing model (0.01 USDC/query, 20-query cap) and the multi-step open-then-fund lifecycle. It does not disclose what opening itself costs, what it returns, or how it relates to escrow/tx-hash flows visible in the schema.

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?

Purpose is front-loaded, which is good, but the nested parenthetical pricing and the hardcoded listing example ('awswhatsnew: ...') read like injected data and add parse friction rather than general guidance.

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 payment-enabled session-opening tool with no output schema, the description covers the purchase flow but omits what open returns (session id? tx hash?) and how the escrow/tx-hash params fit the lifecycle. Adequate as a minimum-viable overview but not 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?

Schema coverage is 50%, with listing_id/listing_slug/buyer_address documented in-schema and max_queries, open_tx_hash, proof_escrow_id undocumented. The description adds a query cap ('max 20 queries/session') loosely tied to max_queries but does not explain the escrow or tx-hash parameters, so it 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 verb+resource ('Open a prepaid session', 'Buy per-query access to live data listings') and names the sibling alternative data_preview for a free taste. It clearly positions the tool before data_session_fund and data_session_query. However, the embedded hardcoded listing sample ('awswhatsnew') muddies the general purpose.

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 clear context: use data_preview for a free first look, then open a prepaid session and fund/query. The workflow with sibling tools is implied. No explicit when-not or exclusion statements, so it falls short of a full 5.

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