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

The description adds significant value beyond annotations: it states data is 'sampled from public schedule data, not live times – treat as planning estimates,' specifies direction-insensitivity, and describes error handling. This provides behavioral context that readOnlyHint and idempotentHint cannot convey.

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 sentences with no unnecessary words. Core information is front-loaded, making it easy to scan. Every sentence earns its place.

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?

Despite no output schema, the description fully lists what the tool returns (fastest/typical duration, direct trains, fewest changes, operators, guide URL, atlas routes) and explains error behavior. For a small tool with 2 parameters, this is complete.

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 coverage is 100% with descriptions for both parameters. The description enhances by noting 'City name only, no station needed; accent-insensitive,' which adds meaning beyond the schema's example values.

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 European cities, including duration, directness, operators, guide URL, and legendary atlas routes. The verb 'journey facts' is specific and the resource is well-defined. It distinguishes from siblings like search_routes (mentioned as fallback) and famous_routes (likely lists notable 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?

The description tells when to use: to get journey facts between two cities. It also explains error behavior (uncovered pair returns an error with a tip to use search_routes) and mentions direction-insensitivity. It does not explicitly list alternatives or when-not-to-use, but the guidance is clear enough.

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