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

Annotations only state readOnly=false, idempotent=false, destructive=false; the description goes well beyond that by disclosing the wallet-charging formula (quantity × ticket price), the failure modes (insufficient balance, sold out, registration closed), and the fact that the error payload contains a regional Paxaver top-up page. The financial-impact warning and the 'no per-student split' caveat are exactly the kind of context annotations cannot carry.

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?

Front-loads the core action, then semantics, then failure modes, then the sibling alternative, then the financial warning — a sensible priority order with no filler sentences. It is dense, with a long parenthetical that packs quantity semantics and the ticket-split clarification together, which costs a little readability.

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?

An output schema exists, so return values need not be described. For a non-idempotent, financially-impactful mutation, the description covers actor (authenticated user), cost mechanics, failure conditions, prerequisites (registration must be open, checked via the closed flag referenced in the schema) and the alternative tool — nothing material to correct invocation 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 event_id, quantity bounds, and school_slug; baseline would be 3. The description still adds genuine semantic value by explaining what quantity means ('tickets bought for the caller, 1 to 20 - there is no per-student split') and how it maps to cost, which resolves an ambiguity the bare 'Number of tickets' schema text leaves open.

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+resource ('Registers the authenticated user for a school event') plus the side effect ('issues tickets to the caller'). It distinguishes itself from near-neighbours by naming sign_up_for_volunteer_shift as the alternate path for volunteering, so an agent can separate it from register_for_program and the volunteer tool without opening schemas.

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?

Explicitly names the alternative tool and the condition that selects it ('To volunteer at an event rather than attend, use sign_up_for_volunteer_shift'), and adds an operational prerequisite ('confirm before registering' for paid events). It also enumerates the three failure conditions, giving the agent clear expectations before invoking.

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