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

With no annotations provided, the description carries the full burden, and it does so well: it labels the check 'Stateless,' lists the returned fields, warns that 'A named handle is a CLAIM until checked against the published record,' and notes it is 'Public on every lane.' It does not detail error or failure behavior, which keeps this from a 5, but it is clearly transparent about the operation's nature and caveats.

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 dense but every clause earns its place: core cryptographic check, return shape, preferred local alternative, mandatory-parameter rationale, and the handle-claim caveat. It is front-loaded with the main purpose and contains no filler or redundant restatement.

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 security-sensitive verification tool with no output schema, the description supplies the return fields, preconditions, and an alternative path, which is enough for an agent to invoke it correctly. Slight jargon such as 'psid,' 'mode,' and 'lane' and the absence of error-case guidance prevent a perfect score, but no essential invocation detail is missing.

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 schema already documents all three parameters; the description adds meaningful security context beyond the schema by emphasizing that rp must be YOUR domain and challenge must be the one-time challenge YOU issued, and why those constraints prevent replay attacks. This goes beyond the baseline for 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 states a specific verb and resource: a 'Stateless dual-signature check ... of a NULLAUTH1 assertion for YOUR rp domain and the ONE-TIME challenge YOU issued.' The title adds the critical scoping phrase 'for agents that ARE the relying party,' which distinguishes this from any generic login or assertion tool and from the sibling null_* tools.

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 explicitly tells agents when not to call this tool: 'Prefer verifying locally with @null/login-rp (no call at all); this tool is the convenience path.' It also gives a clear security precondition by stating rp and challenge are mandatory and explaining that without them 'cross-site replays would verify.'

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