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

With no annotations, the description carries the full burden, and it does disclose the most important trait — the call is free and 'without spending anything,' which is non-obvious for a payment-status tool. However, it says nothing about authentication expectations, error behavior for an invalid credit id, or what happens when neither parameter is supplied (both are optional).

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?

A single front-loaded sentence with a colon introducing the two branches; the cost note ('Free') is appended without waste. Nothing is repeated and every clause carries information.

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-parameter read tool this covers the selection logic and cost, which is most of what an agent needs. But there is no output schema, and 'see its status' / 'see its run-credit balance' leaves the return shape — possible status values, balance units — unspecified.

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 already 100%, so the baseline is 3; the description earns above that by tying each parameter to a distinct outcome — 'credit' yields status, 'api_key' yields run-credit balance. It also clarifies that 'credit' is a web run-credit id from Stripe checkout, adding context beyond the schema's 'Stripe checkout session id.'

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 — 'check payment state' — and immediately explains the two modes of operation (credit status vs api_key balance). It is clearly distinct from the sibling sanity_check_* tools, but it never names them or explicitly contrasts itself, so it stops short of full sibling differentiation.

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?

The description tells the agent exactly which parameter selects which behavior: 'pass credit ... to see its status, or api_key to see its run-credit balance.' That is clear routing between the two use cases. It stops short of 5 because it never says when not to use this tool or points to a related alternative for other payment questions.

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