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

Annotations already declare this is a non-read-only, non-idempotent, closed-world mutation. The description adds value beyond that by disclosing a failure mode (provider may refuse already-expired requests) and the accepted timestamp format, which is genuine behavioral context an agent needs before calling.

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?

Two tight sentences, front-loaded with the action and followed by the constraint. No filler, no restatement of the title.

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 three-parameter mutation with no output schema, the description covers the action, the format, and one refusal condition. It omits prerequisites (must the request be pending?), whether the change notifies signers, and whether the new expiry must be in the future, leaving a few gaps an agent might need.

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 description coverage is 100% and all three parameters are required and documented in the schema. The description only restates the RFC 3339 requirement for expires_at, adding nothing the schema does not already say, so the baseline of 3 applies.

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 gives a specific verb+resource ('Change when an existing signature request expires') and names the required timestamp format. It is unambiguous as to what is being modified, though it does not differentiate from any sibling (none of which touch expiration) and uses 'signature request' where the name/params say 'assignment'.

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?

Usage is implied: only apply this to an existing, not-yet-expired request. The caveat that 'Assinafy may refuse for requests that already expired' is a useful conditional, but there is no explicit when-to-use vs. when-not-to-use routing or mention of alternatives such as re-issuing the assignment.

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