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

Annotations already declare readOnly=false, openWorld=true, idempotent=false, destructive=false, so the safety profile is covered. The description adds real value on top: it discloses the return payload (payment_url, session_id, claim_token) and that the purchase is completed by a human out-of-band via Stripe, which is behavior the annotations cannot convey. It does not mention session/token expiry or whether credits are granted automatically on claim.

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 compact sentence carries the action, the return values, the downstream sibling, and both parameter modes. Nothing is padded and the action plus output is 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?

With no output schema, the description correctly shoulders the burden of describing what comes back, and it covers both calling modes for a 2-parameter tool. The only gap is post-payment semantics — whether the purchase must be claimed, and any lifetime on session_id/claim_token — which an agent orchestrating the flow would want.

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 baseline would be 3, but the description adds the meaning of the omitted api_key case (create a new key) that the schema only implies with "Existing key to top up (optional)". That single clause materially changes how an agent will call the tool.

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 ("Start a credit purchase") and immediately describes the outcome (a Stripe payment_url plus session_id and claim_token). It also names the sibling it feeds into (check_purchase), so an agent can distinguish it from check_balance and the other siblings without opening a 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 an explicit branching rule — omit api_key for a new key, pass it to top up — and points at check_purchase as the follow-up call. It stops short of stating when not to use the tool or any preconditions around the human actually paying, so it is clear context rather than full when/when-not guidance.

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