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

Annotations only declare the safety profile (readOnly/idempotent/openWorld); the description goes well beyond by disclosing the x402 paid flow, the 402 challenge on the first call, auto-payment via the fiatdock-mcp package, and the crucial billing guarantee that failures are NOT charged. That is exactly the behavioral context an agent needs before invoking a paid 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 price and purpose, then packs in return shape, search behavior, and payment mechanics efficiently. There is mild redundancy between 'a query that matches no open market answers 404 and is not charged' and 'any answer the tool cannot produce is >= 400 and is NOT charged,' but both reinforce a billing-critical rule.

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?

An output schema exists, so return values need no prose. Everything an agent must know to call this correctly — cost, payment handshake, error semantics, billing exclusions, and query behavior — is present.

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, but the description adds meaning beyond the schema — it clarifies that price is 'the price of one share, 0-1 USDC, read as the market's implied probability' and that the npm package handles payment automatically, which the schema does not state. It does not restate limit/query semantics beyond what the schema already carries.

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 — 'Live prediction markets from Polymarket' — and then enumerates exactly what each record contains (outcome, price as implied probability, 24h/total volume, liquidity, closing date, link). That level of return-field specificity makes it trivially distinguishable from siblings like perp_market or token_price.

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 clear operational context: 'Search by topic, or omit the query for the markets with the most 24h volume,' plus an explicit negative case (no match → 404, not charged). It does not name a competing sibling tool to route against, so it stops short of a 5.

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