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

The annotations indicate readOnlyHint=false and destructiveHint=false, but do not fully convey the financial implications. The description clarifies this is a purchase action, creating a prepaid session that will be funded later. It does not explicitly state the risks of opening a session or that it may incur costs on the buyer's wallet, but the description implies a commercial transaction, which is useful beyond annotations. However, it does not discuss session expiration, refunds, or the exact behavior on session creation, which would add behavioral depth.

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 concise at two sentences, front-loading the core purpose and free preview. It includes concrete details like pricing and session limits, which are useful, but the phrase 'Buy per-query access' is slightly redundant with 'Open a prepaid session'. The structure is effective, but could be tightened by removing the example listing or reducing redundancies. Still well within appropriate length.

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 tool is complex (6 parameters, no output schema, financial implications), and the description provides a high-level workflow and pricing context. However, it lacks details on what happens after opening (how to fund, how to query, session expiration, or the return of a session ID). Since there is no output schema, the description should hint at the return value or post-conditions. Despite sibling tools, this description leaves the agent to infer next steps. Given the complexity, this is a notable gap.

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 covers several parameters with descriptions (listing_id, listing_slug, buyer_address), but only 50% coverage. The description adds value by clarifying the real-world semantics: the listing is a 'devtowatch' example, session limits (max 20 queries/session), and that buyer_address can be omitted to default to the platform wallet. However, it does not explain max_queries default or open_tx_hash/proof_escrow_id, but the schema's descriptions are partly rich. Given the medium coverage, the description compensates somewhat, but not fully, so 3 is appropriate.

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 tool's purpose: opening a prepaid data session for per-query access to live data listings. It specifies the resource (data session), the action (open), and the commercial context (prepaid, per-query, with a free preview via data_preview). The description also provides a concrete example listing with pricing and limits, distinguishing it from related tools like data_preview and data_session_fund.

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 mentions the alternative `data_preview` for the first free taste, implying that data_session_open is for paid access. It also mentions funding and querying after opening, indirectly referencing data_session_fund and data_session_query. This gives clear guidance on the workflow and when to use this tool versus the preview option.

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