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

No annotations are provided, so the description carries the full burden. It discloses session-scoped behavior, key format, and the optional serverUrl override with production default. It doesn't mention error handling or side effects but adequately explains the tool's function for a simple setter.

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?

Three concise sentences, front-loaded with purpose and prerequisite, each sentence adds value. No redundancy or fluff.

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 simple 2-param setter with full schema coverage, the description provides all necessary context: purpose, prerequisite, optional use case, and default behavior. No gaps.

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 covers both parameters with detailed descriptions (100% coverage). The description adds the session requirement but doesn't add significant parameter-level meaning beyond what the schema already provides. Baseline 3 is appropriate.

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 clearly states the tool's function: 'Set your FaceSign API key for this session' with a specific verb and resource. It also distinguishes itself from sibling tools by noting it must be called before any other FaceSign tools, establishing it as a setup prerequisite.

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?

Explicitly states when to call this tool ('must be called before any other FaceSign tools') and provides guidance on when to set serverUrl (only if explicitly targeting a non-default server). This clearly differentiates usage from alternatives and gives actionable instructions.

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