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

Annotations already declare readOnlyHint and openWorldHint, so safety is covered. The description adds genuinely non-structured context: the cost of $0.005 USDC per call via x402 and the data source (NREL PVWatts), which tells the agent this is geared toward a rooftop-system model rather than generic solar irradiance. No note on rate limits, caching, or latency for a paid external call.

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 sentences, front-loaded with the capability and outputs before the pricing note. The output enumeration is a long run-on list, but each item corresponds to a distinct returned metric and no sentence is filler.

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 correctly enumerates the return values, and it discloses the per-call cost. The main remaining gap for an eight-parameter, zero-required-parameter tool is that it never explains the location inputs (lat/lon vs near) or that now-required choices exist, so an agent could call it with no location at all.

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 50%, so half the parameters (lat, lon, near, tilt, azimuth) carry no inline documentation, and the description does not fill that gap — it never says that coordinates and/or an address supply the location, what tilt/azimuth default to, or that all eight parameters are optional. It only indirectly maps cost_per_watt and price_per_kwh to the savings/cost/payback outputs, so it lands at the baseline.

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 (solar potential) and enumerates the exact outputs — yearly/monthly rooftop output, sun hours, capacity factor, savings, install cost, payback — and names the underlying model (NREL PVWatts). This is enough for an agent to separate it from nearby siblings like energy, energy_mix, or weather without opening a schema.

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?

There is no statement of when to use this versus alternatives, nor which of lat/lon or 'near' to prefer, nor any prerequisite or exclusion. The only extra guidance offered is the price, which is useful but is not usage routing. An agent must infer context entirely.

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