Explain a fact

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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety behavior is covered. The description adds that omitting fact produces a list of explainable facts and that source.counted has a source-id requirement, but it does not add much beyond that about rate limits, permissions, or other runtime behavior.

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 short and front-loads the core purpose before listing what an explanation contains. It is efficient, though the opening rhetorical question is slightly indirect and the return-value list partly duplicates the output 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?

Given the output schema and annotations, the description provides enough to call the tool correctly: required slug, optional fact for listing, and source usage with source.counted. It does not fully explain every enum value, but that is largely carried by the schema.

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 coverage is 67%, with slug and source already described in the schema. The description clarifies the optional list behavior when fact is omitted and reinforces the source.counted/source relationship, but it does not explain the meaning of the individual fact enum values beyond the schema's enumeration.

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 makes clear that the tool explains why a derived project fact or figure has a particular value, and it lists the returned explanation components such as rule, version, source, lineage, and evidence ids. The rhetorical question format is less direct than a verb-first statement, and it does not explicitly distinguish this from siblings like ask_hey or get_evidence.

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 useful usage detail by saying 'Omit fact to list what can be explained' and that source.counted takes a source id. However, it does not say when to use this tool versus ask_hey, get_evidence, or other explanation/research siblings, so the guidance is implied rather than explicit.

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