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

Annotations already declare readOnlyHint=false, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered structurally. The description adds almost nothing beyond those hints: it doesn't say whether the update is a merge or replace of the payment object, whether buyer fields exist at all, or what happens to an already-completed 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 core sentence is front-loaded and waste-free. The appended example only passes checkout_id and omits the payment payload, so it consumes space without demonstrating the tool's actual value, and it duplicates what the schema already shows.

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 tool with a nested payment object, zero required parameters, and an output schema, the description leaves the essential question unanswered: is checkout_id required? It also advertises a "buyer" capability absent from the schema, which is misleading rather than merely incomplete.

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 both checkout_id and payment are documented structurally, which sets the baseline at 3. The description's mention of "payment rail details" loosely maps to the nested payment object but gives no insight into its substructure, and "buyer details" has no corresponding parameter.

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 uses a specific verb and resource ("Update ... details on a checkout session") and names the two categories being modified, which is clear at a glance. It does not explicitly distinguish itself from update_cart or complete_checkout, and it references "buyer details" that don't appear in the schema.

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?

There is no guidance on when to use this versus update_cart, complete_checkout, or cancel_checkout, and no prerequisites or preconditions are stated. The single example only illustrates the call shape, not the situation that calls for it.

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