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

Annotations already cover safety (readOnly, idempotent, openWorld), so the bar is lower, yet the description adds real operational context: citation provenance (source URL + verified_at), null-means-not-stated semantics, response size (~23 KB for the all-protocols array), invalid-input rejection for out-of-enum ids, and that access is free with no auth. These are facts an agent cannot infer from the annotations or schema alone.

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 purpose is front-loaded and every sentence carries information; the field enumeration is long but functional, telling the agent what to expect in the payload. The trailing note about out-of-enum ids is mildly redundant with the schema's enum, which keeps it short of a perfect score.

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?

With an output schema present the description needn't document return values, and it nonetheless supplies the response shape, size, provenance semantics, and error behavior. For a single-optional-parameter read tool, nothing an agent needs to select or invoke it correctly is missing.

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% and the schema already explains omit-vs-set behavior, so the baseline is 3. The description still adds value beyond the schema by quantifying the response-size difference (~23 KB array), clarifying the null-means-unspecified convention, and stating that out-of-enum ids are rejected as invalid input.

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?

It names a specific verb and resource ('Return source-cited facts for agent payment protocols') and enumerates the exact fact categories returned (spec URL, HTTP status codes, payment rail, networks, facilitators, pay flow, example), so an agent knows precisely what it gets. It also explicitly separates itself from siblings by naming search_sources and get_support_info as the tools for related-but-different needs.

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 stated in both directions: 'Use this for how a protocol works' plus explicit alternatives for adjacent needs ('Use search_sources for papers or docs about it, and get_support_info for this site's own paid L402 API'). The call-shape guidance (omit id for all nine, pass id for one) is tied to the concrete scenario of wanting breadth vs. depth.

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