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

Annotations already declare readOnlyHint=false, idempotentHint=true, destructiveHint=false and a closed world. The description adds genuinely non-redundant context: sign-in is required, nothing is validated automatically (validation is on request only), and only day tickets are supported, with season tickets explicitly excluded. It does not describe the response, but the behavioral profile is well covered.

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-loads the core action and scope in the first clause, then adds constraints in compact sentences with no filler. Slightly dense as a single paragraph but every sentence 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 an 8-parameter mutation tool with no output schema and no nesting, the description covers auth, idempotency, ticket-type limits and the check-on-request behavior. The main gap is what the caller gets back on save versus update, which matters more here precisely because there is no output schema.

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 the schema already documents every parameter including commute_id restore semantics and weekday numbering. The description only echoes the field list (stations, departure time, travel days, fare) without adding format or edge-case meaning beyond the schema. Baseline 3 applies.

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 pair (saves or updates) plus the exact resource (one direction of a usual train journey / commute) and enumerates the fields it stores. It clearly separates outward vs. homeward as two distinct routes. It does not explicitly differentiate itself from the sibling add_train_journey, so it stops short of a 5.

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 some routing guidance implicitly: saving the same route/time again updates rather than duplicates, and each direction is its own route. However it never says when to reach for this instead of add_train_journey, check_commute, or list_commutes, so the alternative-selection guidance is left to inference.

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