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

Annotations only say readOnlyHint=false and destructiveHint=false, so the description carries real weight and does: no authentication required, an email is sent on new-email registration, an existing LockTrip account silently switches to a magic-link branch with no token, and the guest session's permissions are enumerated (cannot list, view or cancel bookings, no `get_payment_url`, no `confirm_booking`). The only missing piece is token lifetime or expiry behavior for the session.

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 opening sentence front-loads purpose, inputs, and outputs, then branches into the existing-account case and the permission envelope. It is dense and slightly long-winded in the session-limitation sentence, but nearly every clause carries operational value.

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 auth entry point with no output schema, the description covers the success shape, the alternate magic-link outcome, downstream tool wiring, and the capability boundary of the resulting session. An agent has everything needed to call it and interpret either response.

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% and there is one parameter, so the schema already documents `email`; the description still adds that it is the email the user provided for the booking and that no password accompanies it, which disambiguates it from a normal credential login. It does not add format validation guidance beyond that.

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 ('Starts a guest checkout from the user's `email` only') and names the exact return payload (token, userId, email, isNewUser). It is clearly separable from siblings like `confirm_booking` or `list_bookings`, which it explicitly says a guest session cannot use.

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 gives an explicit trigger ('Use it when the user wants to book, with the email the user gave for the booking'), names the downstream alternatives that consume the token, and adds a when-NOT-to: do not call again if `magicLinkSent` is true for that email. Alternatives and exclusions are both spelled out.

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