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

Annotations already declare a non-read-only, non-idempotent mutation, and the description adds genuinely useful context beyond them: it advances the session to ready-for-payment, requires a sandbox credential, and clarifies there is no human login. It doesn't describe what happens on repeated calls or failure modes, but the auth and side-effect disclosure is substantive.

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 purpose and state effect in the first sentence, then layers preference, guide, and credential instructions. It is dense and mostly earns its length, though the credential paragraph is long enough that some repetition of the schema's agent_key semantics could be trimmed.

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 3-param mutation tool with an output schema (so return values need no explanation) and full annotation coverage, the definition supplies the prerequisite credential flow, the guide pointer, and the card alternative. Complete enough to invoke correctly; only edge-case behavior (repeat calls, failure handling) is unaddressed.

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 every parameter (cart_id, agent_key, payment_mandate with all sub-fields) is fully documented in the schema. The description reinforces the agent_key credential path but adds no format or constraint detail the schema lacks, so the baseline 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?

States a specific verb and resource ('Preview the complete order summary before payment') and adds the state transition it causes ('Advances the session to ready-for-payment state'), which cleanly separates it from complete_checkout and the ucp_* checkout siblings.

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?

Gives an explicit alternative ('PREFER THE CART CARD'), points at the checkout guide resource, and spells out the prerequisite order: call create_sandbox_key first, then pass agent_key or an Authorization header. It also rules out the wrong path ('Do not ask a person to log in').

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