Nearest River Mile Marker to a Location

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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so safety is clear. The description adds substantial behavioral detail beyond annotations: it explains the search logic (nearest within radius, great-circle distance calculation), the return structure including bearing and plain-language finding, and error response format. This is valuable context that complements the annotations.

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 concise and well-structured. It leads with the core purpose in the first sentence, then provides a clear example of the JSON return and error format. Every sentence serves a purpose, and the structure allows quick scanning for an agent. No wasted text.

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 the low parameter count (3), full schema coverage, presence of annotations, and output schema, the description is complete. It covers the core logic, return structure, error handling, and provenance information. An agent has all necessary context to invoke and correctly interpret results.

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 baseline is 3. The description does not add new parameter-level detail beyond what the schema provides; it only reiterates the max_radius_meters search behavior. The lat/lon parameters are not elaborated beyond the schema. Thus, the description adds marginal value to parameter understanding.

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 starts with a clear verb and resource: 'Return the single closest river mile marker to a WGS84 coordinate.' It pinpoints the exact operation (find nearest) and resource (river mile marker), and implicitly distinguishes from siblings by focusing on 'single closest' rather than listing or querying multiple markers.

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 explicitly frames the use case: 'Answers "what river mile am I at?" / "nearest marker to this point"', providing clear context for when to use this tool. It also mentions the max_radius_meters search boundary. However, it does not explicitly indicate when NOT to use (e.g., if multiple markers are needed) or reference sibling tools as alternatives, leaving a slight gap in exclusion guidance.

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