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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds behavioral value by clarifying that the response is reference metadata (source= values, regional lenses, filtering guidance) rather than actual news records, and it cross-references 'dedicated tools' for regional lenses — useful routing information beyond 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?

One compact sentence that front-loads what is returned and ends with the call condition. Every clause earns its place and nothing is repeated from the annotations or 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 zero-parameter, read-only introspection tool with an output schema and full annotations, the description tells the agent what it will learn and when to use it. Only minor gaps remain, such as whether the content is static or cheap to call repeatedly.

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?

With zero parameters, the baseline is 4. The description goes further by documenting the vocabulary that other tools' parameters accept (source= and region= filter values), which is meaningful semantic content even though there are no parameters of its own.

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 names a specific verb ('Describe') and a concrete resource (Horizon's ingestion sources, source= filter values, regional lenses, and filtering guidance). It implies a distinction from the query siblings via 'before querying', but never names an alternative, so differentiation is inferred rather than explicit.

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?

It gives a clear call condition: 'Call this to understand the corpus before querying.' This is more than implied usage, though it stops short of stating when not to call it (e.g., if the schema of source values is already known).

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