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

With no annotations, the description carries the full burden and does so: it discloses the billing consequence (unit price × quantity from next renewal, unit credits × quantity), the absence of proration, and the 1 CRD cost charged even for rejected calls except 429 RATE_LIMITED. It also states that error results carry a request_id to quote to support — unusually complete behavioral disclosure for a mutation tool.

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 mutation and its billing impact, then identity resolution, then cost, then error handling. Every clause carries information an agent needs (price model, no-op case, credit cost, rate-limit exemption); nothing is redundant or padded.

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 financial, non-reversible-in-effect mutation with 7 params and no output schema, the description covers preconditions, cost, failure modes, and error payload hints well. The remaining gap is the success response shape (what fields come back after a quantity change), which the description leaves unstated.

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 the baseline is 3; the description adds genuine meaning beyond the schema by explaining that user_id/email are alternatives that require exactly one live subscription and that subscription_id is the escape hatch on 409. The x402_payment retry condition and the free-on-429 nuance are also only implicit in the schema text.

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 with scope: "Change a live subscription's quantity (seats/packs, 1–10000)." It is clearly distinguishable from siblings such as cancel_subscription, list_subscriptions, and update_pricing_tier 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?

Gives concrete conditional guidance: identify the customer via user_id/email only when exactly one live subscription exists, otherwise expect a 409 and pass subscription_id. It also flags that the same quantity is a no-op. It stops short of naming sibling tools to call for recovery (e.g., list_subscriptions) or of explicitly stating when to prefer this over other subscription tools.

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