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

Annotations already declare readOnly/idempotent/non-destructive/openWorld, so the safety profile is covered. The description goes beyond them by disclosing the cost model (free daily allowance, then per-call x402 payment) and the half-hourly granularity, which materially affects whether an agent should call it repeatedly.

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?

Two tight sentences; the data scope is front-loaded and the pricing note follows. No filler or redundancy.

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?

An output schema exists, so return values need not be explained, and the description covers scope, granularity, and cost. It stops short of resolving overlap with sibling price tools and omits any date-range or timezone conventions.

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% with two fully documented date params, so the baseline is 3. The phrase 'between two dates' merely restates the from/to pair and adds no format or boundary detail (timezone, max range) beyond the schema.

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 (Market Index Data) with granularity and units (half-hourly wholesale price in £/MWh and volume), which is concrete enough to distinguish it from the many other gb-electricity price/forecast siblings. It does not, however, explicitly differentiate itself from close siblings like gb-electricity__system-prices or indicated-forecast.

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 only guidance is commercial: free within the daily allowance, then $0.002 USDC per call via x402. There is no statement of when to use this tool versus system-prices or the various forecast tools, nor any prerequisite or exclusion.

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