List available Happy Endpoint APIs

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

No annotations are provided, so the description carries the full burden. It states that the tool lists data sources and subscription locations, implying a non-destructive, informational read operation. However, it does not disclose potential behavioral details such as whether the tool actively checks reachability, requires authentication, or has any side effects. The phrase 'can reach' is somewhat ambiguous and could imply network calls or simply configuration data.

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 two sentences long, with the first sentence front-loading the primary action and resource, and the second sentence providing usage guidance. Every word adds value; there is no redundancy, fluff, or unnecessary detail.

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 simple tool with zero parameters and no output schema, the description adequately explains what it does and when to use it. It states the tool lists data sources and subscription locations, giving a clear idea of the return content. It could be more explicit about the exact format or structure of the output, but for such a low-complexity tool, the coverage is sufficient.

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 has zero parameters, and the schema coverage is 100% (vacuously). The description adds no parameter-specific information, which is acceptable because there are no parameters to explain. The baseline for 0-parameter tools is 4, and no deduction is warranted.

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 specific action ('List') and clearly identifies the resource ('data sources this server can reach') plus the additional detail of where to subscribe. This clearly distinguishes it from sibling tools that retrieve specific data (e.g., search_properties, get_property_details), as this tool is about discovering available endpoints rather than querying them.

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 second sentence provides explicit use cases: when a call fails due to a missing subscription or to see what data is available. This gives clear context for when to use the tool. It does not name alternative tools explicitly, but the context is sufficient to infer that this is for metainformation rather than actual data retrieval.

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