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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false and destructiveHint=false, so the safety profile is covered. The description adds two things beyond them: the sign-in requirement (auth prerequisite) and the explicit absence of external National Rail calls, which clarifies data freshness rather than merely restating openWorldHint.

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 short sentences, zero filler, with the core purpose front-loaded and constraints following. Every clause carries information.

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 simple read-only lister with no output schema and rich annotations, the description covers the essentials: what is returned, the auth requirement, and the absence of live lookups. The only omission is any mention of the include_stopped behavior, which the schema already supplies.

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 single parameter include_stopped is fully documented there, so the baseline is 3. The description adds nothing about when to pass include_stopped (e.g., to surface routes stopped via stop_commute), so it does not exceed the schema.

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 ('Lists the person's saved train routes') and enumerates the returned fields (ids, usual times, days, fares), which cleanly separates it from siblings like list_my_claims or list_savings. An agent can identify it 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?

'Makes no National Rail lookups' and 'Requires sign-in to Untap' tell the agent when this is the right tool (static saved data, authenticated context) rather than a live-lookup sibling such as check_commute. It stops short of naming an explicit alternative, so it is clear context but not full when/when-not guidance.

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