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

With no annotations, the description bears the full transparency burden. It openly discloses that this is a write to a 'sensitive credential vault,' requires proof of ownership, is free, and merges without affecting other fields. This goes beyond a generic 'update' statement. It doesn't mention rate limits or idempotency, but the critical 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 two short paragraphs with the main action in the first sentence. Every sentence adds value: merge behavior, use cases, cost, security requirement, and vault sensitivity. No fluff or 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?

Given the tool's complexity (4 required params, no annotations, output schema present), the description covers essential context: purpose, merge semantics, use cases, cost, and proof requirement. The output schema exists, so return-value details are not needed. Minor gaps like accepted field names are not covered, but the description is sufficiently complete for an agent to select and invoke the tool safely.

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 a thorough explanation of the dpop_token format. The description adds only the concept of a 'single field' and merge semantics, which is relevant but not necessary for understanding individual parameters. Since the schema does the heavy lifting, a baseline of 3 is 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 opens with 'Add or update a single patron credential field,' which clearly states the action (add/update), the resource (patron credential field), and the scope ('single'). This distinguishes it from siblings like 'delete_patron_credential' or 'update_operator_credential.' The merge behavior further clarifies it's a partial update, not a wholesale replacement.

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'), making it clear when this tool is appropriate. It also includes practical constraints: 'Free' and 'Proof of npub ownership is required.' However, it does not explicitly name alternatives or state when not to use it, 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.