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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds useful context: it returns fund instructions (not actual execution), and it references a specific listing and price. However, it doesn't disclose what happens after fund instructions are returned, whether the purchase is reversible, or any rate limits. With annotations covering the core safety traits, a 3 is appropriate.

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 sentences with no filler. The core action ('Buy per-query access') is front-loaded, the free-trial alternative is mentioned, and the return behavior is stated. Every sentence 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?

For a single-parameter tool with rich annotations (readOnly, idempotent, non-destructive) and a clear prerequisite (data_session_open), the description covers the essential flow: what you buy, the price, and what you get back. It doesn't explain the full funding-package lifecycle or what to do with the fund instructions, but given the simplicity and the sibling set, this is nearly 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 description coverage is 100%, so the schema already documents the single parameter (session_id) as a UUID from data_session_open. The description adds the context that the session must be an existing buyer data session, but doesn't add new meaning beyond the schema. Baseline 3 is correct.

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 ('Buy per-query access') and resource ('live data listings'), and names the specific listing (devtowatch) with pricing. It distinguishes itself from siblings like data_session_fund and data_session_attach_escrow by focusing on the funding-package purchase flow. However, it doesn't explicitly contrast with data_session_fund, which could be a close alternative, so it's clear but not fully differentiated.

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 gives clear context: use this to buy per-query access, and mentions 'first taste free via data_preview' as an alternative for trying before buying. It also says 'Returns fund instructions after data_session_open,' implying the prerequisite of an open session. It doesn't explicitly state when NOT to use it (e.g., when you already have funds or want to attach escrow), but the context is reasonably clear.

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