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

With no annotations supplied, the description carries the full burden. "Opaque" usefully signals the value should not be parsed, but nothing explains what happens when a supplied cart_session_id is expired, malformed, or belongs to another session, nor whether validation failure raises an error or silently creates a new session. The create-vs-validate fork is the tool's central behavior and it is left unexplained.

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?

One sentence, front-loaded with the operative verbs and resource, with no padding or repetition. Its brevity is appropriate to a one-parameter tool, though the space saved is not used to disambiguate the create/validate branch.

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-value structure need not be explained, which relieves much of the burden. However, for a session-bootstrapping tool with zero annotation coverage and an undocumented optional parameter, the definition leaves the failure and idempotency behavior unstated, which is material for correct invocation.

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 description coverage is 0% for the single optional cart_session_id parameter, and the description never mentions it. The agent cannot tell from either source whether omitting it creates a fresh session and supplying it validates an existing one, which is the key semantic distinction the description needed to supply.

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 names a specific verb pair and resource: "Create or validate the opaque cart session." That clearly separates it from siblings like add_to_cart, update_cart, and render_cart, which all operate on existing cart contents rather than establishing the session itself. The phrase "used by this app surface" is vague but does not obscure the core purpose.

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 explicit when-to-use guidance, no mention of ordering relative to add_to_cart or begin_checkout, and no statement about whether this is idempotent or only called once per session. The phrase "used by this app surface" hints at context but gives the agent nothing actionable about when to invoke it versus simply calling a cart tool directly.

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