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

With no annotations, the description carries the full behavioral burden, and it does so thoroughly. It discloses that the tool is payment-gated, that unpaid calls return an MCP error with structuredContent containing PaymentRequired terms, that the identical call must be retried with PaymentPayload in params._meta, and that optional listing fields are written with the deed.

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?

The description is dense but every sentence adds necessary operational detail, and the core purpose is front-loaded. The payment flow and px explanation are valuable, though the phrasing is slightly packed and the 'optional listing fields' reference is a bit indirect.

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 payment-gated mutation tool with no output schema and no annotations, the description covers the critical workflow: payment failure, payment retry, coord assignment, and pixel encoding. It stops short of describing the success response or any post-purchase constraints, which would make it fully complete, but the provided information is sufficient for correct invocation.

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 83%, and the description adds genuine meaning beyond the schema: it explains that px encodes 100 pixels as 200 hex characters with one palette index each, and clarifies the 'next' coord behavior. Only the 'framework' parameter remains undocumented, preventing a 5.

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 action ('Claim one 10x10 block'), the resource, and the payment condition ($1.00 USDC via x402). This clearly distinguishes buy_block from read-only siblings like get_block/list_unclaimed and from set_pixels, which modifies pixels rather than purchasing a block.

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?

The description gives clear context: use this tool to claim a block by paying $1.00 USDC, and explains the x402 payment flow including the retry mechanism. It does not explicitly name alternative tools or when not to use them, but the purpose is unambiguous enough for an agent to route correctly.

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