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

Annotations are mostly false flags (not read-only, not idempotent, not destructive), so the description carries the burden. It adds meaningful behavior: buying per-query access, opening a prepaid session, requiring authorization, and being non-guest-callable. It does not detail whether funds are moved immediately or whether the session is refundable, but the prepaid purchase intent is clear.

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?

Three dense sentences with no filler. The core purpose is front-loaded, followed by auth, the identification rule, and the REST endpoint. Every clause adds operational value.

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 description covers the immediate call well, and the input-schema constraints add useful implementation detail. But there is no output schema and no mention of what the tool returns or how the opened session is used with sibling tools like data_session_fund or data_session_query. An agent can invoke it but not anticipate the result or next step.

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 about 50%, so the description must compensate. It usefully clarifies that listing_slug and listing_id are mutually exclusive and that buyer_address defaults to the caller's platform wallet. However, it leaves max_queries, open_tx_hash, and proof_escrow_id unexplained in both schema and description, which is a real gap for a purchase-oriented tool.

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 (open a prepaid buyer session), a concrete resource (live public data listings), and an exact protocol (POST /api/v1/data-sessions). It also distinguishes itself from data_preview, the free-taste alternative, so an agent can tell this tool apart from related data-session 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?

It clearly says this is the paid path and points to data_preview for a free first taste. It specifies the caller requirements (agent API key, not guest-callable) and the listing identification rule (exactly one of slug or id), giving an agent unambiguous conditions for when to invoke it.

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