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

Annotations already cover safety (readOnlyHint, idempotentHint, destructiveHint false), so the bar is lower; the description still adds real context: the per-call price, the PaymentRequired failure mode and retry path via params._meta["x402/payment"], the key requirement, and the important caveat that caps are advisory rather than enforced. These are traits no annotation conveys. The only blemish is internal tension between "no key" for x402 and "Needs a key mts_", which forces the agent to resolve two auth paths itself.

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?

Every sentence carries functional information (price, auth, fallback, plan), so little is pure waste, but the ordering is poor: the description opens with pricing rather than the purpose, and the core action appears in a parenthetical clause. Heavy parenthetical nesting makes it dense to parse for an agent deciding whether to call it.

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 3-parameter, no-required-parameter read tool with a fully described schema, the description covers cost, auth, payment fallback, data scope, and the advisory caveat. With no output schema present, a brief note on what a returned cap or remaining-limit figure looks like would have closed the remaining gap.

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 schema already documents payer, apiKey, and sessionId in detail, making 3 the baseline. The description adds only that the 24h spend figure is supplemented by a session-scoped figure, which ties sessionId to output semantics without deepening any single parameter's meaning.

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?

The description states a specific verb and resource: "read the spend caps of a payer address plus what it spent in the last 24h (and in a session)." It also frames the tool's role as "Advisory (complements your wallet's enforced hard limits)," which distinguishes it from enforcement. However, the purpose sentence is buried mid-paragraph after pricing boilerplate, and it never distinguishes itself from a plausibly overlapping sibling such as spend_check.

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?

It gives clear routing for the authentication branch, explicitly naming an alternative: "No key? call settle_trial_key (free, 50 calls/7 days)." It also explains that paying via x402 removes the need for a key at 0.002 USDC per call. What is missing is any guidance on when to prefer this tool over other spend-related siblings or when reading caps is unnecessary.

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