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

Annotations only indicate a non-read-only mutation. The description adds useful behavioral context by mentioning the price, the session query cap, and that this is a paid prepaid offering. However, it does not explain what the tool returns, what happens with open_tx_hash/proof_escrow_id, or why the text says 'Open a prepaid session, then fund' when prepaid already implies funding. The schema's max_queries limit (50) also differs from the 'max 20 queries/session' mention, leaving ambiguity.

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 short, front-loaded, and uses the first sentence to state both purpose and the free-preview alternative. The listing/pricing line is a bit of a detour, but it still earns its place by giving concrete cost and quota context. It avoids filler and remains fairly scannable.

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?

Given the tool has six parameters, no output schema, and only weak annotations, the description covers the high-level workflow but still leaves gaps: the agent is not told what a successful open returns, what 'prepaid session' means exactly, how to get the listing_slug/listing_id for the given listing, or why open_tx_hash/proof_escrow_id may be needed. It is enough for a first routing decision but not fully complete for correct invocation.

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?

With schema description coverage at 50%, the description adds some value by clarifying the economic/limit semantics (0.01 USDC/query, max 20 queries/session). It does nothing to explain open_tx_hash, proof_escrow_id, or how max_queries should be used beyond implied session cap; listing_id/listing_slug are already documented in the schema. The compensation is only partial.

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 names the action ('Buy per-query access') and resource ('live data listings') and says the tool 'Open[s] a prepaid session'. It also points to data_preview as the free alternative, so a user can tell them apart. It is slightly less crisp than a distilled one-liner because it mixes a specific listing/pricing example into the purpose statement, but the core action is unambiguous.

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 explicitly recommends 'first taste free via data_preview' and lays out the proper sequence: 'Open a prepaid session, then fund and query.' This is a clear when-to-use signal relative to the free preview and the downstream fund/query tools. It lacks explicit 'do NOT use when...' exclusions, but the workflow guidance is strong enough that an agent can route correctly.

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