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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description still adds real value with 'No sign-in needed' (auth requirement) and the presence of a 'last checked' freshness field. It stops short of describing behavior for an unknown operator or matching failures.

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?

Front-loaded with the verb and resource before the payload enumeration, and the two trailing sentences are short and purposeful. The long field list is dense but each clause describes a distinct returned value, so little is wasted.

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 carries the return-value burden well by enumerating delay threshold, tiers, deadline, claim link, automatic scheme, and freshness date. It is complete for a single read-only lookup, missing only error/edge behavior for unrecognized operator names.

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% and the schema already supplies examples ('Avanti', 'GWR', 'Southern', 'LNER') and the two-letter code option. The description's 'as people say it, such as Avanti, GWR or Southern' largely restates that, so the baseline 3 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 (Returns) plus a precise resource (one train operator's Delay Repay rules) and enumerates the exact payload fields. No sibling tool overlaps this resource, so an agent can identify it immediately without opening any 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?

Usage is implied: the enumeration of rules, tiers, and deadlines makes clear this is a pre-claim lookup, and 'No sign-in needed' removes one barrier. However, it never states when to prefer this over check_train_delay or find_money_owed, nor any when-not condition, so guidance stays implicit.

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