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

Goes well beyond the annotations: it discloses that the first response is an isError=true PaymentRequired quote, that a buyer wallet must authorize an exact Solana USDC amount, where the settlement receipt appears (_meta['x402/payment-response']), and that there are 'no keys, automatic charges or simulated sales.' That is exactly the extra 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?

Four dense sentences with the payment flow front-loaded and no filler; the first sentence anchors purpose and the rest walks the transaction lifecycle. Slightly heavy in one block, but every sentence 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 paid API call with no output schema, it describes where the verified data and settlement receipt are returned, which covers the return contract. The gap is product-level semantics — what each product_id actually yields or requires — which an agent must infer from siblings.

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 only 17% across 6 parameters, so the description must compensate, and it does not explain product_id enum meaning, or the username/usernames/question/language fields. It does add genuine value for the payment mechanism itself (params._meta['x402/payment'] and the retry-with-same-arguments rule), which is why this is not lower.

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 ('Execute a real x402-protected InstaVeil Instagram data API via MCP') and frames the tool as the execution endpoint, which implicitly separates it from the discovery/prep siblings (list_paid_products, build_purchase_request, get_purchase_instructions). An agent can tell this is the tool that actually runs the paid call rather than preparing one.

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?

Explicitly prescribes the two-step flow: first call without payment returns a free 402 quote, then retry tools/call with identical arguments plus a signed PaymentPayload. It even names the fallback path ('payment_signature for clients without x402-over-MCP support'), so when-to-use and how-to-retry are fully specified.

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