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

Beyond the annotations (readOnlyHint=false, idempotentHint=true, destructiveHint=false), the description adds valuable context: merchant-of-record fee split (70/30), the fact it's a sale not a donation, that get_sermon will then return the full transcript, and that purchases are idempotent (no double-charge). This goes well beyond what annotations declare, giving clear behavioral expectations without contradicting them.

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 front-loaded with the main purpose and every sentence earns its place: fee structure, post-purchase behavior, idempotency, credit insufficiency handling, and the x402 alternative. It is dense but not bloated, with no redundant repetition of schema or annotations.

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?

Given the single-parameter schema, no output schema, and modest annotations, the description covers all necessary context: what the tool does, how payment works, what happens after purchase (get_sermon returns transcript), failure modes (insufficient credits), and alternative payment routes. It paints a complete mental model for an agent to invoke it correctly.

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?

The input schema already fully describes sermon_id with examples and explanation ('The paid sermon to purchase, e.g. a sermon_id from search_sermons or browse_catalog'). Schema coverage is 100%, so the description adds little parameter-specific meaning beyond the schema. The description mentions the sermon_id in the context of x402, but that's not new semantic detail for the parameter itself.

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 opens with a specific verb+resource: 'Buy permanent access to a paid sermon using your prepaid marketplace credits.' This clearly states what the tool does and uniquely positions it among siblings like get_sermon (which retrieves after purchase) and pay_with_x402 (an alternative payment method).

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 provides explicit when-to-use and when-not-to-use guidance, including alternatives: 'If you have insufficient credits, check get_credit_balance and top up. No key / no credits? Pay per-call in USDC on Base via x402 instead — call pay_with_x402 with this sermon_id.' It also differentiates from donations, clarifying this is a content sale.

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