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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint false, so the tool is known to be safe and non-mutating. The description adds substantial behavioral detail: free guides return the full body immediately, paid guides return payment requirements (a URL) instead of the body, and a PAYMENT-SIGNATURE header is needed to complete payment. It also specifies the cost and payment networks. This goes far beyond the annotations and is crucial for correct invocation.

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 three sentences with zero wasted words. The core purpose is front-loaded ('Fetch a guide by id'), followed by the free/paid distinction and payment flow. Every sentence adds essential information, and the structure is logical and scannable.

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 there is no output schema, the description fully explains what the tool returns in both cases (free vs paid), including the payment completion step. It covers the cost, payment methods, and the required header, which is all an agent needs to correctly invoke the tool and handle the result. Nothing critical 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?

There is only one parameter, 'id', and the schema already describes it as 'Guide id, from search_guides' with 100% coverage. The description repeats the purpose ('by id') but adds no new format, syntax, or constraints beyond the schema. Baseline of 3 is appropriate since the schema fully documents the parameter.

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 verb ('Fetch') and resource ('a guide by id'), clearly distinguishing it from search_guides which finds guides. It also clarifies the free vs paid distinction, so an agent knows exactly what this tool does and how it differs from siblings.

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 implies usage: use this when you have a guide id (from search_guides, as noted in the schema). It provides clear context about free vs paid behavior and how to complete payment, but it does not explicitly state exclusions or name alternatives like 'use search_guides to find guides first.' Still, the context is clear enough for correct selection.

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