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

Annotations already indicate a state-changing (readOnlyHint=false), idempotent, non-destructive operation. The description adds useful behavior context: 'Platform-executes funding' clarifies that the funding is executed on the platform's side, and it presents a concrete per-query cost. It does not disclose failure modes, insufficient funds handling, or what happens on repeat calls, but the idempotence annotation covers one of those gaps.

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 two sentences, front-loaded with the core action ('Buy per-query access to live data listings') and then the free preview alternative. The listing name and price are concise but arguably product-specific; still, they are relevant and not verbose. The structure is efficient with no redundant filler.

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 no output schema, the description covers the purpose and workflow adequately. However, it does not describe the return value, success/failure indicators, or what happens after funding (e.g., whether funds are automatically attached to the session). With no output schema, the description carries more burden for return behavior, and that is missing. The reference to siblings like data_session_funding_package is also absent.

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%: the only parameter, session_id, is fully documented as 'UUID of a data session you opened (from data_session_open).' The tool description adds no extra parameter-level detail, so the baseline score of 3 applies. The description's references to workflow do not clarify the parameter beyond what the schema already provides.

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 uses a specific verb ('Buy'), a clear resource ('per-query access to live data listings'), and a concrete target ('data session'). It names the exact listing and pricing, and differentiates from the free alternative data_preview. It clearly establishes the tool's role in the workflow between opening a session and querying it.

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 data_preview as a free alternative ('first taste free via data_preview') and states the follow-up action ('so you can data_session_query'), implying it should be used after opening a session and before querying. However, it does not distinguish this tool from sibling data_session_funding_package or data_session_attach_escrow, so exclusions are missing.

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