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

No annotations are provided, so the description carries the burden. It discloses a key behavioral requirement: 'Requires npub for credit billing,' which is non-obvious and useful. However, it does not describe the operation's side effects, error behavior, or expected return (though output schema exists). This is adequate but has gaps.

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, front-loaded with the action, and contains no fluff. 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?

Given the tool's simplicity and the presence of an output schema, the description covers the essential purpose and a critical prerequisite (npub). It does not explain all parameters, but the schema covers those. Minor gap: it doesn't mention how the active brain is used, but that is not critical.

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 83%, so baseline is 3. The description emphasizes npub as required, which aligns with the property description, but adds no new semantic meaning beyond the schema. It does not explain dpop_token or the optional brain_id behavior beyond what schema already provides.

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 action: 'Add a URL attachment to a thought.' This clearly identifies the resource (URL attachment) and target (thought), and distinguishes it from sibling tools like brain_add_file_attachment. The additional npub billing note doesn't obscure the primary purpose.

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 clearly implies usage: to attach a URL to a thought. It also provides an important prerequisite: 'Requires npub for credit billing.' However, it does not explicitly mention alternatives or exclusions (e.g., when to use brain_add_file_attachment instead), so it lacks explicit when-not guidance.

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