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

Annotations only declare readOnlyHint=true, so the description carries the real burden here and delivers: it discloses that data is 'merged across providers', that the window is ~10 days by default, that points=0 suppresses the series, and that positive means longs pay shorts. Provider-merge and suppression semantics are genuine behavioral detail beyond the annotation.

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 first sentence front-loads the purpose well, but the middle is discursive, repeatedly re-explaining why trajectory beats a trailing delta and why a local low/high matters. The same idea ('the trajectory is the point', 'shape a trailing delta flattens away') is argued more than once and could be compressed without losing meaning.

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 output schema, the description must describe the return shape, and it does so thoroughly: summary metrics, trajectory windows, bucket series, and 1y percentile stats with dates. For a 2-param read tool this is nearly complete; only error/empty-data or provider-availability behavior is unaddressed.

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%, so the baseline is 3. The description's points guidance ('omit for the full ~10-day window; 0 to suppress it') essentially restates what the schema already documents, adding no new format or constraint. Symbol gets no mention in the prose at all.

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 names a specific resource (current perpetual funding rate for one coin) with concrete outputs: hourly rate, annualized estimate, 24h/7d trajectory, 8h-bucket series, and 1y percentile line. That unambiguously separates it from derivatives_open_interest, derivatives_liquidations, and derivatives_flow. It never names a sibling to contrast against, which is why it falls 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?

It gives strong guidance on how to interpret the output ('judge the rate against where it has actually been rather than an absolute threshold'), but no guidance on when to select this tool over the derivatives_* siblings, nor any stated prerequisites. Usage is implied by the analytical framing rather than explicit.

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