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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the agent knows this is a safe, deterministic local lookup; the bar for extra disclosure is low. The description adds 'No input' and 'use this once', which is minor behavioral context, but most of the text re-states what the output schema already contains.

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?

Front-loaded with the core purpose, then usage, then return shape. The sentences are dense and mostly earn their place, though the trailing enumeration of returned fields overlaps with the output schema and adds bulk.

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?

With an output schema present and annotations covering safety, the description is complete enough to call correctly. It redundantly narrates return fields, but nothing an agent needs to invoke this zero-input tool is missing.

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?

This is a zero-parameter tool, so the baseline is 4. The description reinforces 'No input', which is consistent with the empty schema and leaves nothing ambiguous.

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 opening 'What a refused call looks like' names the resource (refusal/error shape) and the tool is clearly a schema-describing lookup. The line 'A tool's output schema covers its answers only' implicitly separates this from describe_schema, though it never names the sibling outright, so an agent must infer the boundary.

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?

Explicit guidance: 'use this once, to branch on refusals or validate them' tells the agent both the purpose (branch/validate) and a usage constraint (once, not repeatedly). It frames the contrast with normal answer-schema inspection but stops short of naming describe_schema as the alternative.

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