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

Annotations already declare readOnly/idempotent/non-destructive, but the description adds real value beyond them: the five-result ceiling and the caveat that results blend 'confirmed coverage' with 'directory coverage' (i.e., some entries are unverified). It also sets expectations for what is deliberately absent (live stock, prices, delivery promises, contact details), which prevents misreading of results.

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 primary capability and result cap come first, followed by the data-quality caveat and the negative scope. No filler, no restatement of the title.

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 shape need not be described, and the description covers scope, result limit, and data-vs-directory reliability. Only minor gaps remain (default fuel behavior, and whether results are empty for uncovered counties), which the enum-only schema doesn't resolve.

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% — county carries its own description and both params are enums, so the schema does most of the work. The description confirms the two dimensions (county and fuel) but adds no semantics for the fuel enum (KEROSENE/GAS_OIL/HVO) or its KEROSENE default, so a 3 baseline is appropriate.

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 and resource ('List ... public oil suppliers'), the result cap ('up to five'), and the exact scoping dimensions (Republic of Ireland county + fuel). The closing clause ('No live stock, prices, delivery promises or contact') implicitly separates it from the price-guide sibling, so an agent can tell what this tool is and is not without reading the 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?

Clear context for use: a county plus a fuel type, with the note that a county only (no address/Eircode) is expected. The 'no prices / no contact' clause steers price or quote requests toward tankful_county_price_guide, but the alternative is never named explicitly, so the routing is inferential rather than stated.

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