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

Annotations already declare a non-read-only, non-destructive, non-idempotent write, so the bar is lower; the description still adds value by noting no payment is involved, that the call fails when the shift is full or cancelled, and that the user should be confirmed before the write. It doesn't address duplicate-signup behavior or what the response contains, which is minor given the output schema.

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?

Three tight sentences with zero filler: identity of the action, parameter provenance, failure conditions, sibling alternative, and the write confirmation warning, all front-loaded.

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 single-parameter mutation with an output schema, the description covers everything an agent needs: auth subject, parameter source, failure modes, the alternative tool, and a confirmation requirement. Return values need not be explained since an output schema exists.

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% for the single parameter, so baseline is 3, but the description adds genuine meaning: shift_id identifies one shift within an event, not the event itself, and must be sourced from get_school_event. That disambiguation goes beyond the schema's own text.

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?

States a specific verb and resource ('Signs the authenticated user up for a specific volunteer shift') and immediately scopes what shift_id refers to versus the event, which separates it from sibling register_for_event and cancel_my_volunteer_signup.

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?

Explicit routing: says to obtain shift_id from the event's shifts via get_school_event, and names register_for_event as the alternative for attending as a guest. It also states the failure condition (shift full or cancelled), leaving little to inference.

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