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

Goes well beyond the readOnly/idempotent annotations by disclosing the full x402 payment lifecycle: price (0.04 USDC on Base), who pays (the calling agent's own wallet), the two-call handshake, and the explicit guarantee that the server never holds a wallet or pays on the agent's behalf. This is exactly the behavioral context annotations cannot carry.

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 payment tag and the core purpose, and every sentence is actionable. Minor redundancy: the price and the 'your own wallet pays, never this server's' guarantee are each stated twice.

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 paid, no-output-schema tool the description covers the payment handshake fully, which is the hardest part for an agent to get right. It is thin on what a 'Signal brief' actually contains and on failure modes (e.g., date outside the archive), so an agent still lacks some result-shape context.

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 both date and payment_header in detail. The description's 'Body { date: YYYY-MM-DD }' merely restates the schema without adding format edge cases or fallback behavior for unavailable dates. Baseline 3 is appropriate.

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 — point-in-time purchase of ONE archive date's full Signal brief — and immediately distinguishes the scope from 'buying full history,' which maps to signal_history. An agent can tell this apart from sibling tools without opening the 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 a clear use case (backtesting or auditing a single curated day) and the key procedural rule: call first with no payment_header to get the challenge, then re-call with the same arguments plus the signed header. It does not, however, route the agent to signal_day_preview for a free look or name signal_history / signal_latest explicitly as alternatives.

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