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

Although readOnlyHint already indicates safety, the description adds the exact return format ({result: {"<email>": {contact: ...}}}) and the HTTP method (POST), which are not in the annotations or schema. This goes beyond the annotation and gives the agent concrete expectations for output.

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: the first states purpose and read-only nature, the second provides return format and endpoint. Every sentence is informative, with no redundant or filler content.

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 single-parameter tool with no output schema, the description covers all essential aspects: what it does, how it behaves, what it returns, and the underlying API call. No critical details are 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?

With 100% schema coverage, the parameter is already well-documented. The description adds a concrete example of how the emails parameter maps to the result key, which reinforces the semantics. Baseline 3 plus this extra context justifies a 4.

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 starts with 'Look up marketing contacts by exact email address', which is a specific verb+object and clearly differentiates from sibling tools like list_marketing_lists or add_or_update_contacts. It also includes the HTTP endpoint, further scoping the action.

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 gives clear context: exact-email lookup, read-only, and the return shape. It doesn't explicitly name alternative tools or say 'when not to use', but the exactness and read-only nature imply that mutation or listing tools are alternatives. A 4 is appropriate for clear context without explicit exclusions.

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