City-to-city by train

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

Disclosures beyond annotations: figures are sampled from public schedule data (not live), treat as planning estimates, direction-insensitive. Annotations already indicate read-only and idempotent, and description adds important context without contradiction.

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 concise sentences, front-loaded with purpose. Every sentence adds value: core function, output summary, data source caveat, and error handling. No wasted words.

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 two-parameter tool with no output schema, the description covers all needed aspects: purpose, data type, limitations, and error behavior. Completely sufficient for an agent to use correctly.

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 already has 100% coverage with descriptions. Description adds that both parameters are city names only (no station needed), accent-insensitive, and that the tool is direction-insensitive, significantly aiding correct parameter usage.

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?

The description clearly states it provides journey facts between two cities, listing specific details (fastest/typical duration, direct trains, operators, atlas routes) and European coverage. It distinguishes from siblings like search_routes and famous_routes.

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?

Explicitly states when to use (for journey facts between two cities) and what happens for uncovered pairs (error with search_routes tip). No explicit when-not-to-use, but alternative is clearly implied.

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