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

Annotations already declare readOnlyHint=false, idempotentHint=false, and destructiveHint=false, so the agent knows this is a state-changing, non-idempotent operation. The description adds the commercial context (paid, prepaid, max 20 queries/session) and the workflow (open, fund, query), which is useful. However, it does not disclose side effects like whether opening a session consumes funds immediately, whether sessions expire, or what happens on failure. With annotations covering the basic safety profile, 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?

The description is compact and front-loaded with the core purpose ('Buy per-query access'), then gives the example listing and workflow. The 'Listing: googleblogwatch...' line is specific and useful, though it is a concrete example that may become stale. The final sentence is a concise workflow summary. No wasted words, but the example listing detail could be seen as slightly extraneous.

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 6-parameter tool with no output schema, the description gives the essential workflow (open, fund, query) and the key constraint (exactly one of slug or id, max queries). However, it does not explain what the response contains, how to handle the open_tx_hash or proof_escrow_id parameters, or what happens after opening (e.g., need to fund). The sibling data_session_fund and data_session_query exist, so the workflow hint helps, but the description is not fully complete for an agent to call this correctly without opening the schema.

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 50%, and the schema already documents listing_id, listing_slug, and buyer_address with clear descriptions. The description adds the 'exactly one of listing_slug or listing_id' constraint and the 'max 20 queries/session' context, which helps. However, it does not explain open_tx_hash, proof_escrow_id, or max_queries beyond what the schema provides. The description partially compensates for the coverage gap but does not fully carry the burden.

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'), a resource ('live data listings'), and a concrete example listing with pricing. It also names the sibling data_preview as the free first taste, which distinguishes this paid session-opening tool from the free preview tool.

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 clearly says to use data_preview for the free first taste, implying data_session_open is for paid access. It also gives a workflow hint ('Open a prepaid session, then fund and query'), which orients the agent to the session lifecycle. It does not explicitly enumerate when not to use it or name all alternatives, but the context is clear.

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