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

Annotations only declare readOnly and openWorld; the description adds the material facts an agent needs before invoking: the data source (Nord Pool public portal), the price ($0.002 USDC per successful call), the payment protocol and rails (x402 on Base, Solana or Arc), and that failed calls are not charged. That is exactly the auth/cost context annotations cannot express.

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 sentences, front-loaded with what/where/when, followed by an explicit exclusion and then billing terms. Every clause carries non-redundant information; nothing repeats the schema or annotations.

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 an output schema present the description need not explain return structure, and it covers market scope, units, resolution, source and cost. The remaining gap is small but real: it does not say how far back historical dates are supported or how stale/real-time the published prices are, which matters for a date-parameterized time-series tool.

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% and both parameters already document their formats (area codes, YYYY-MM-DD) and defaults (SE3, tomorrow CET), so the schema carries the parameter burden. The description adds only the constraint of one area plus one date per call, not new syntax or semantics.

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 verb (retrieves day-ahead Elspot prices) with a precisely bounded resource: one Nordic/Baltic bidding area, one delivery date, EUR/MWh, 15-minute resolution, from the Nord Pool public portal. It also names what it is not (intraday, futures), which separates it cleanly from the EPEX and ENTSO-E power siblings.

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 scope limits give clear usage context: Nordic or Baltic bidding areas only, day-ahead market only, one area and one date per call. It never names a specific alternative sibling (e.g. energy_epex_spot_power_europe for continental Europe), so an agent must infer the routing rule rather than being told it explicitly.

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