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

With no annotations provided, the description shoulders the full burden of behavioral disclosure. It explicitly states that this is a write to a sensitive vault, that proof of npub ownership is required, that it merges rather than overwrites, and that it is free. These are valuable behavioral traits beyond the basic operation.

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 compact and front-loaded with the core purpose in the first sentence. Each subsequent sentence adds a distinct piece of information (merge behavior, use cases, cost, auth requirement, sensitivity), with no wasted words or redundancy.

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?

The description covers the essential operational context: what it does (add/update single field), how it behaves (merge), when to use it (examples), prerequisites (proof of ownership), cost (free), and its sensitive nature. Since an output schema exists, return values need not be explained, and no critical gaps remain.

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?

The input schema already provides 100% coverage with detailed descriptions for all four parameters, including the intricate dpop_token requirements. The tool description does not add additional parameter-level semantics beyond reinforcing that the operation merges into existing credentials, which is the baseline expectation given full schema coverage.

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 opens with a specific verb+resource pair: 'Add or update a single patron credential field.' This clearly distinguishes it from sibling tools like brain_delete_patron_credential, brain_get_patron_credential_fields, and brain_receive_patron_credentials, and the 'single field' qualifier sets it apart from batch operations.

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 provides concrete use cases ('setting an account identifier after OAuth, changing a default brain') and explains the merge behavior ('without affecting other fields'), which gives context for when to use it. However, it does not explicitly mention when not to use it or name alternative tools, so it falls short of a 5 by the rubric.

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