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

Annotations declare a non-readonly, non-idempotent, non-destructive local mutation, and the description adds real value beyond them: the dynamic price model tied to station reserves (2 to 20 CWM). It doesn't say what happens on insufficient credits or a full tank, but the cost behavior is disclosed.

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 tight sentences: action and location first, then the cost model, then the parameter shortcut. No filler and nothing important buried.

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, an agent might want to know what is returned (fuel added, credits spent) or the failure modes (insufficient credits, tank already full). Those are the only meaningful gaps; the pricing and unit semantics are fully covered.

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 coverage is 100%, so baseline is 3; the description earns above that by explaining that the units parameter has a cost consequence driven by station reserve pricing, which the schema does not convey. The omission-equals-fill-tank rule is restated from the schema, adding little.

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?

A specific verb+resource ('Buy fuel') plus a stated precondition ('at your current docked station') that separates it from the generic buy/sell siblings. An agent can tell immediately this is the refuel-at-dock action rather than a market order.

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?

Usage context is clear — you must be docked, and omitting units fills the tank. There are no explicit exclusions or pointers to alternatives (e.g., that buy/place_orders handle other commodities or larger fuel purchases), so it stops short of full routing guidance.

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