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

Annotations already declare readOnlyHint=true, and the description adds useful behavioral detail about the return structure (couple's shared core numbers plus text interpretations for relationship dynamics, finances, conflict points, children, growth areas). It doesn't contradict annotations and goes beyond them by describing output composition.

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 sentences, each earning its place: a clear action sentence, a return-value sentence, and an explicit usage sentence. No filler, no repetition, and the most important information is front-loaded.

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?

Despite lacking an output schema, the description adequately explains what the tool returns (structured reading with specific areas). Combined with comprehensive parameter descriptions in the schema and read-only annotations, the description is complete for an agent to select and invoke the tool correctly.

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 100%, with each parameter already described (birth dates in DD-MM-YYYY format, locale enum). The description doesn't add extra parameter semantics beyond mentioning 'two birth dates', so it aligns with the baseline where the schema does the heavy lifting.

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 calculates relationship compatibility from two birth dates using the Destiny Matrix method. It specifies the output (shared core numbers plus text interpretations for dynamics, finances, conflict points, children, growth areas) and distinguishes from a generic matrix calculation.

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 says 'Use this when a user asks how compatible two people are' and mentions a numerology compatibility reading, providing clear context. It doesn't name the sibling tool as an alternative or state exclusions, so it lacks a direct when-not-to-use, but the usage direction is strong.

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