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

With no readOnly or destructive hints in annotations, the description carries the burden. It discloses that each destination triggers a separate lookup, which implies performance costs and influences parameter selection. This is valuable behavioral context beyond the schema. It does not mention return format or error cases, but the disclosed behavior is relevant.

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?

The description is a single, front-loaded sentence that immediately states purpose and then gives a practical caveat. Every word earns its place, with no redundancy or filler.

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?

Given a simple tool with 5 well-documented parameters and no output schema, the description covers the core semantics and usage nuance. The phrase 'put them in order' implies the return structure. It lacks explicit mention of output format, but the tool's purpose is clear enough for correct invocation.

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%, so all parameters already have descriptions. The description adds semantic guidance for the 'to' parameter ('ask about the places you are actually choosing between') and reinforces the meaning of the multi-destination comparison. However, it does not add significant depth beyond the schema, so the baseline of 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?

The description clearly states the tool's function: 'Given a starting point and several destinations, say which is quickest to reach and put them in order.' This uses a specific verb ('say') and resource ('routes'), and the scope of comparing multiple destinations distinguishes it from sibling tools like routes_directions and routes_eta.

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 provides explicit usage context: 'Each destination is a separate routing lookup, so ask about the places you are actually choosing between rather than a long list.' This tells the agent when to use the tool and warns against misuse, though it does not explicitly name alternative tools for point-to-point routing.

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