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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds meaningful context beyond that: it explains the tool is rule-based, that spoofing is possible, and critically that billing is 1 credit per successful call with schema-invalid inputs rejected before billing. This is exactly the kind of financial and reliability context agents need.

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 front-loaded with the core purpose, then tacks on the spoofing caveat, return shape, and billing terms in two compact sentences. Every sentence earns its place, and there is no repetitive 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, read-only parsing tool with full schema coverage and annotations covering the safety profile, the description is complete. It explains the return shape even though no output schema exists, discloses billing conditions, and notes the rule-based limitation. An agent has everything needed to call it 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%, so the single required parameter and its constraints (minLength 1, maxLength 2000, description 'The User-Agent header value') are fully documented. The description reinforces that the input is a User-Agent string but adds no syntax or format details beyond what the schema already provides, making the baseline 3 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 states a specific verb ('Identifies') and enumerates exactly what is extracted: browser, engine, OS, device class, crawlers, AI bots, and HTTP clients. It is immediately clear what the tool does, though it does not explicitly distinguish itself from nearest siblings like url-parse or header-audit.

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 no explicit when-to-use guidance, no alternatives, and no exclusions. The caveat that user agents can be spoofed is a behavioral warning, not usage routing. An agent must infer that this is the right tool whenever a UA string needs parsing.

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