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

With no annotations provided, the description carries the burden and usefully declares that the tool is free and processes no data, indicating no side effects or cost. It stops short of explicitly stating read-only status or output format, but for a zero-parameter capabilities listing this is largely sufficient.

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 tight sentences front-load the return content and then add the cost/behavior note. Every word earns its place with no redundancy.

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?

There is no output schema and no annotations, so the description must convey what an agent can expect. It thoroughly enumerates the categories of returned information (modes, fields, limits) and clarifies that the call is free and data-free, making it complete for a discovery tool.

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 tool takes zero parameters, and the schema is an empty object. The description correctly does not discuss parameters; the baseline for a parameterless tool is 4.

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 uses a specific verb ('Returns') and enumerates exactly what is returned: join types, key matching modes, conflict strategies, fan-out modes, added fields, and row limits. This makes it immediately distinguishable from the sibling 'join_rows', which performs joins rather than describing capabilities.

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 phrase 'Free, processes no data' implies a safe discovery call, but there is no explicit statement of when to use this versus 'join_rows' (e.g., 'call this before joining to learn supported modes'). The usage context is only implied.

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