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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds behavior those hints cannot express: content is conditionally truncated to roughly a fifth for non-owners, ownership of the book or a purchased chapter unlocks the whole text, and the paid price is surfaced in the preview response. This is exactly the kind of conditional-output disclosure an agent needs before calling.

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?

Four tightly packed sentences, front-loaded with the outcome and followed by the branching rules; nothing is padding. The trailing 'Read-only.' restates what the annotations already say and is the one line that does not earn its place.

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 no output schema, the description carries the return-value burden and does so: full text for free/owned chapters, public lead-in plus standalone price otherwise. Combined with the schema-covered inputs, an agent has everything needed to call this 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?

Schema coverage is 100%, so the three parameters are already documented by the schema; the description adds the sourcing hint that book slugs and chapter ids come from list_books. Minor confusion: 'the call carries its token' implies a token input that does not appear in the schema, leaving the agent to guess how entitlement is transmitted.

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?

Starts with a specific verb+resource ('Return the text of one chapter') and immediately scopes it by entitlement (free = full, paid = lead-in unless owned). It names the related siblings list_books and buy_chapter, so an agent can tell what this tool is and is not without opening the schema.

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?

Gives clear context for when the full text comes back versus a truncated lead-in, and routes the agent to list_books for slugs and chapter ids and to buy_chapter for ownership. It stops short of an explicit 'do not use this for X' exclusion, but the entitlement conditions effectively define the branch points.

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