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 that this is 'a write to the patron's sensitive credential vault,' requires 'Proof of npub ownership,' merges without affecting other fields, and is free. It does not discuss reversibility or failure modes, but the key behavioral traits are covered.

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 five short sentences, front-loaded with the core action and immediately followed by side effects, use cases, cost, and security context. There is no filler or redundant repetition of schema 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 4-parameter sensitive write with an output schema and full schema coverage, the description covers core behavior, the auth precondition, side effects, and use cases. It does not explicitly route around sibling tools like delete_patron_credential or receive_patron_credentials, but the output schema and detailed dpop_token schema fill most remaining gaps.

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 describes all four parameters with 100% coverage, so the description does not need to re-explain them. The phrase 'single patron credential field' adds a bit of semantic context for the `field` parameter, but 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 opens with 'Add or update a single patron credential field,' naming a specific verb ('Add or update'), resource ('patron credential field'), and scope ('single'). The merge note and examples further distinguish it from bulk credential tools like receive_patron_credentials and from update_operator_credential.

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?

It gives concrete use cases: 'setting an account identifier after OAuth, changing a default brain, etc.' This tells an agent when to use it. However, it does not explicitly state when not to use it or name alternatives such as delete_patron_credential or update_operator_credential, so it stops short of a 5.

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