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

Annotations already cover safety (readOnly, idempotent, openWorld), so the description correctly spends its words on provenance instead: Octopus-published versus Renewably-estimated values, the `estimate` flag, and the fact that other tariffs only carry supplier-published prices. That is real behavioral context an agent cannot get from the structured fields; only rate limits or failure modes are absent.

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 the core answer (what, where, when) before the tariff-provenance detail. No filler, but the final sentence is ungrammatical and 'take no panels' is an oblique way of saying solar inputs are ignored, costing a little clarity.

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?

For a 9-parameter tool with no output schema, the description covers provenance and export semantics but leaves the response shape implicit and defers all solar modelling behaviour to external documentation. An agent has enough to call it, but not a confident picture of what comes back per half-hour slot.

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 each parameter already carries a thorough description, so the baseline is 3. The description adds one genuinely new cross-parameter fact — that an export tariff's prices are what it pays for export and that solar parameters do not apply there — but that point is stated in a garbled clause ('An export tariff's are...'), which blunts its value.

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 opening sentence states the resource (half-hourly prices), the scope (a region), and the time window (from midnight today), which is specific enough to separate it from find_cheapest_window or the plan_* siblings. It does not name an alternative explicitly, so it stops 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?

Usage is implied rather than stated: fetch prices for a region under a given tariff, and the note that export tariffs report what they pay for export helps pick the right mode. There is no guidance on when to use this versus find_cheapest_window (which presumably consumes the same prices) or the plan_* tools, and no mention of prerequisites.

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