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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds value by disclosing exactly what kind of information is returned (principal, scopes, rate limit, brands), which is meaningful behavioral context beyond the annotations. It doesn't cover error behavior or response format, but for a non-destructive identity lookup this is sufficient.

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?

Two concise sentences with no filler. The first sentence front-loads the core purpose and return fields; the second provides the practical usage trigger. Every word 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 zero-parameter, read-only introspection tool with strong annotations, the description fully covers what the agent needs: what the tool returns, when to call it, and why it is safe. No output schema exists, but the description's enumeration of returned fields is sufficient for the agent to decide and invoke the tool successfully.

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?

There are zero parameters, and schema description coverage is 100%, so there is nothing for the description to add about parameters. The baseline of 4 applies here; the description appropriately focuses on the tool's behavior and context rather than inventing parameter details.

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 states a specific verb and resource: 'Identify the calling key' followed by the exact data returned (principal, scopes, rate limit, brands it can reach). This clearly distinguishes it from the sibling tools, which are all action-oriented (create, get, list, configure) rather than identity introspection.

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 explicitly tells the agent when to invoke this tool: 'Call this first when debugging a permission problem.' It also explains why (it answers 'what am I allowed to do' without guessing), giving clear situational guidance and a natural first-step role among the sibling tools.

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