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

With no annotations, the description carries the burden and does reasonably well: it discloses reuse semantics ('or reuse an open one for the same plan'), a named failure mode (billing_unavailable when no Stripe key), and a precondition. It stops short of describing auth requirements and what success returns beyond the URL.

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?

Two dense sentences with zero filler, and the most decision-relevant facts (create/reuse, default plan, failure mode, prerequisite) are front-loaded.

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 two-optional-param tool with no output schema, the description covers purpose, default, idempotent reuse, error surface, and prerequisite. Only the auth-header nuance lives solely in the schema, which is acceptable.

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 parameters are already documented, including the pro default and the tenant_key/auth-header precedence rule. The description's 'defaulting to the pro plan' merely restates the schema, adding no new meaning — baseline 3.

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 — creating (or reusing) a Stripe Checkout URL — plus the scope (upgrade this tenant, default pro plan). This is clearly distinguishable from billing.plans and billing.status without opening any 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?

Explicitly tells the agent to call billing.plans first as a precondition check, which is real routing guidance. It does not, however, contrast this tool with billing.status or describe when checkout is *not* the right call.

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