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

Annotations already establish that the call is read-only and idempotent. The description adds useful behavioral detail beyond that: results are ordered from nascente to foz, include per-municipality km stretches, and include fishing locations from the collection.

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 compact sentences: the first front-loads what the tool returns, the second states exactly when to invoke it. No filler or repetition of the schema.

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 single-parameter, read-only lookup with no output schema, the description covers the essential return content and the triggering user intents. An agent has enough context to select and call the tool 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?

The only parameter, name, is fully described in the schema with an example ('rio taquari'). The description adds no additional parameter semantics, so the baseline of 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?

The description clearly identifies the resource (a mapped river) and the specific output: municipalities in source-to-mouth order, km segments, and fishing spots. It does not explicitly contrast with sibling tools like get_location or search_locations, but the river-route focus is distinctive enough to set it apart.

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 gives explicit trigger conditions: 'Use quando o usuário perguntar por onde um rio passa, que cidades ficam nele, ou onde pescar ao longo de um rio.' It does not mention when not to use it or name an alternative tool, so it falls just short of a 5.

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