why

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

No annotations are present, so the description carries the full burden. It discloses the tool's output behavior (source evidence and full version chain, including superseded and retired versions), and 'Explain' strongly implies a read-only operation. However, it does not explicitly state that no state is modified or describe behavior for unknown/invalid factIds.

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 with no filler. The main purpose is front-loaded, and the usage hint about factId follows naturally. Every sentence earns its place.

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 low-complexity tool with one parameter and no output schema, the description covers what the tool returns and how to get the required input. Minor gaps remain: no explicit no-side-effects statement and no error behavior, but these are relatively small given the simple read/explain nature of the 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 schema only provides type and minLength for factId, with no description. The tool description adds meaningful guidance by saying to pass a factId from facts or recall, giving the parameter a clear source. For a single simple string parameter, this is sufficient, though it lacks format examples or edge-case guidance.

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 ('Explain') and resource ('fact'), and spells out exactly what is returned: source evidence and the full version chain including superseded and retired versions. This clearly distinguishes it from siblings like facts or recall, which focus on retrieval rather than provenance.

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 provides clear context: use this tool when you need to understand why a fact is believed and what evidence/version history supports it. It also tells the agent where to obtain the required factId ('from facts or recall'), but it does not explicitly state when not to use it or name alternatives.

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