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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so safety is covered. The description adds genuinely non-obvious context: a free tier with a per-caller daily quota, and that payment credentials must travel in MCP request headers (X-Payment / X-Payment-Tx / Bearer) and not as parameters. That is real operational value beyond the annotations.

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 useful content is front-loaded in the first paragraph, but the description then repeats the same facts twice more in [zh] and [en] blocks, restating the vendor asset name, free quota, and argument list. The bilingual duplication roughly triples length for no new information, and the payment-header note is buried at the end.

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 simple two-optional-parameter read tool with a full set of annotations and no output schema, the description covers what comes back, cost/quota, and auth placement, which is most of what an agent needs. The remaining gap is the unexplained parameters, which matters because the schema gives them no semantics at all.

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 0% and neither parameter has a description in the schema. The text only names them ('参数 currency / top 可选') and notes they are optional; it never explains what 'top' selects (count of contracts? results?) or what values 'currency' accepts (BTC/ETH?). With zero schema coverage the description had to carry this and does not.

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 opening sentence states a concrete question the tool answers and enumerates the returned fields: spot index price, perp premium, curve shape (contango/backwardation), and annualized basis for nearest/farthest expiry. That is a specific verb-plus-resource picture, not a restatement of the name. It does not differentiate from the sibling derivatives-basis.basis_curve, so it stops short of a 5.

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 phrase 'right now' implies this is the snapshot tool versus some historical alternative, and the caller is told the result is free with a daily per-caller quota. But there is no explicit when-to-use/when-not statement or named alternative. Usage is implied rather than guided.

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