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

Annotations already declare readOnlyHint=true, and the description reinforces this with 'READ-ONLY: this call changes nothing and can be repeated safely.' It adds useful behavioral context: the document is server-authored, contains no untrusted citizen text, and is versioned. It does not describe output format, but for a read-only guide with no parameters and no output schema, the key behavioral traits are well covered.

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 a single dense paragraph that front-loads the purpose and then lists contents and usage. It is somewhat long but every clause adds information: contents, audience, when to read, versioning, server-authored nature, and read-only safety. It could be slightly more structured, but it is efficient and not padded.

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 zero-parameter, read-only reference tool, the description is nearly complete. It tells the agent what the document covers, when to consult it, and that it is safe. The only minor gap is not describing the return format or how the document is delivered, but with no output schema and no parameters, the description carries enough context 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?

The tool has zero parameters, so the schema is trivially complete. The description adds context about what the document contains, which is the only meaningful semantic information an agent needs. Baseline 4 for zero-parameter tools is appropriate.

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 clearly identifies the tool as a versioned reference document for the payment rail, covering how-and-why, steps for funders/workers/verifiers, limits, moderation, and signing bytes. It distinguishes itself from sibling tools like offers_guide and rail_security by being the comprehensive rail guide, and explicitly frames itself as a document to read before acting.

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 explicit when-to-use guidance: read before posting, submitting, binding, paying, or verifying, and re-read when rules_version changes. It also states what the tool is not for (it contains no untrusted citizen text) and implies it is a reference rather than an action tool, which helps an agent choose it over siblings.

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