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

No annotations are present, so the description carries the full behavioral disclosure burden. It does this well: it reveals that without a subscription the tool returns a real but delayed sample, that live data requires an active subscription key, and that live data can also be purchased per call in USDC via x402. This is valuable operational context an agent needs to predict what the call will do.

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 short sentences, with the core action front-loaded and the subscription/cost details following. There is no filler, and each sentence adds distinct information: what the endpoint is, what a free call returns, and what a paid/live call involves.

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 two-parameter fetch tool with no output schema, the description is largely sufficient. It explains the data mode, cost, and how to preview the response shape. The main gap is that it mentions an active subscription key without specifying how the key is supplied (header vs parameter), which an agent may need to know for a real live-data call.

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 already documents both parameters at 100% coverage, so the baseline is 3. The description adds the meaningful constraint that the endpoint value must come from search_market_data, but it doesn't explain parameter syntax or formats 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 opens with a specific verb-resource pair: 'Fetch data from an endpoint returned by search_market_data.' This clearly distinguishes it from the search sibling and makes the tool's role as a follow-up fetch operation immediately obvious. While it doesn't compare against every sibling, the scope 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 states when to use the tool: after search_market_data has returned an endpoint, and it explains the subscription-based branches (sample vs live data). It doesn't explicitly say 'do not use this to search for endpoints,' but the precondition 'endpoint returned by search_market_data' makes the intended workflow clear.

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