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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds substantial context beyond them: payment terms ($0.003/call via x402, USDC on Base only), that errors are not charged, data freshness (~5 min), the source (OKX public data), and an explicit disclaimer that the crowding hint is not a forecast. This is exactly the value-add structured fields cannot carry.

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?

Front-loaded with purpose, then output shape, payment, freshness, and disclaimer in a logical order. It is a dense block of distinct facts with little redundancy, though the number of separate clauses makes it slightly heavier than the tightest examples.

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?

With no parameters and no output schema, the description carries the full load, and it does: it enumerates the returned fields (instId, fundingRate, fundingTime, crowding hint with thresholds), specifies the source, pricing, freshness, and disclaimers. An agent has everything needed to call and interpret it.

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?

Zero parameters and 100% schema coverage, so the baseline is 4. The description reinforces 'Call GET with no parameters,' which removes any ambiguity about invocation but adds no per-parameter meaning (there are none to add).

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?

States a specific resource (perpetual futures funding rates) with the exact assets covered (BTC, ETH, SOL) and the data source (OKX USDT swaps). An agent can distinguish this from siblings like money_market_rates, gas, or market_quote without opening a schema.

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?

Gives a clear when-to-use trigger ('Use when you need current perpetual futures funding rates') and a practical when-to-avoid ('Data changes about every 5 min; calling more often returns the same values'). It stops short of naming sibling alternatives for related rate data, so it is not a full 5.

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