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

Annotations already declare readOnly/idempotent/non-destructive/open-world, so the bar is lower, yet the description adds genuinely non-obvious behavior: the tool returns a paid x402 request document rather than the data, the agent must call the returned URL with an x402 or MPP client, cost is $0.001 per call, and bad input is refused before any charge. That two-step payment protocol is the single most important thing an agent must know and it is disclosed. It stops short of describing the response envelope or pagination behavior.

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-loaded with the trigger condition, then scope, then the payment/invocation protocol, in two dense sentences with no filler. The phrase 'Returns the exact paid x402 request for your input' is slightly awkward and could be tighter, which keeps it off a 5.

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 three-optional-parameter read tool with no output schema, the description covers the scope, the returned entry fields, the price, the validation-before-charge guarantee, and the otherwise-invisible fact that a second HTTP call is required to obtain data. Little an agent needs to invoke it correctly is missing.

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 100%, so the baseline is 3. The description restates 'up to 100 keys' and 'optionally under prefix', which the schema already conveys via maximum: 100 and the prefix property; it adds nothing about cursor semantics or default limit, so it does not exceed the schema.

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 ('lists up to 100 keys ... optionally under prefix') and scopes it to 'the namespace of the paying wallet with size, stored_at and expires_at'. This is clearly distinguishable from siblings kv_get (single-key read) and kv_put (write) without opening their schemas.

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?

Opens with an explicit trigger: 'Use when an agent needs to see what it has stored', which is the right condition and implicitly excludes the single-key case handled by kv_get. It gives no explicit when-not guidance or named alternative, which keeps it below a 5.

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