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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so the safety profile is fully covered. The description adds the external-service dependency and what a result contains, but says nothing about rate limits, throttling, error cases when a postcode is invalid, or coverage gaps — modest added value on top of the annotations.

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 action and scope, with filters last. Slightly padded by enumerating return fields (providers, speeds, prices, contracts, features, sign-up link) that the output schema already supplies.

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 and full schema coverage for 12 parameters, the description need not explain return structure, and it correctly covers scope and filters. Its main omission is the multi-call workflow implied by visitor_id/address_key (reuse across calls), which an agent must infer from the schema alone.

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%, so baseline is 3. The description echoes postcode, UPRN/UDPRN and the four filter dimensions but adds no syntax, defaults, or behavior beyond what each schema property already documents (including the nuanced visitor_id/address_key chaining, which only the schema explains).

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 precise verb and resource ('Check which fibre and broadband deals are available'), bounds it geographically (UK postcode), and names both the identifier modes (postcode vs UPRN/UDPRN) and the backing service (fibrecompare.com). This is specific enough that an agent can distinguish it from the generic 'fetch'/'search' siblings without opening 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?

Gives clear context: postcode-level by default, address-level when a UPRN/UDPRN or matching address is supplied, plus the optional filter dimensions. It does not state when NOT to use it or name which sibling to prefer for, e.g., looking up a single known deal, so it stops short of full routing guidance.

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