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

Annotations already establish it as read-only, idempotent, non-destructive and closed-world, so the safety profile is covered. Beyond that, the description adds real behavioral signal: it is 'fail-closed,' enumerates the checks it enforces (trusted key, algorithm, nonce, freshness, replay status, signature coverage), and discloses a cost of '0.003 USDC.' This is meaningful context the annotations do not provide.

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?

A single dense sentence that front-loads the core concept ('Fail-closed RFC 9421-style webhook preflight') before enumerating requirements and cost. Every clause carries information, though the packing of so many checks makes it slightly hard to scan.

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 tool with two nested objects, 0% schema descriptions and no output schema, the description supplies a fair amount (the enforcement model, the checked properties, the cost). But it omits any explanation of the `event` payload shape and does not state what the caller receives on pass vs. fail, which matters for a fail-closed gate.

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 0% for two nested objects, so the description must carry parameter meaning. It usefully enumerates the verification requirements (trusted key, algorithm, nonce, freshness, replay, coverage of method/target/content), which maps onto the `verification` sub-fields. However, it says nothing about the `event` object (delivery_id, timestamp, payload) or how freshness ties to the timestamp, leaving half the parameter surface unexplained.

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 names a specific verb and resource: a 'webhook preflight' that verifies signatures 'RFC 9421-style.' It is clearly a verification/validation gate for webhooks, distinguishing it functionally from the many payment and policy preflights among its siblings. It stops short of explicitly naming which sibling to prefer, but the purpose itself is unambiguous.

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?

Usage is only implied by the word 'preflight' – the reader can infer it runs before accepting a webhook. There is no statement of when to use it versus the adjacent preflight/verify siblings or any precondition (e.g., call before processing the payload). No exclusions or alternatives are given.

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