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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), so the description's job is to add context — which it does by explaining the 'addable' flag and that books Google refuses as a source include a reason. It also exposes the underlying endpoint (GET /sources/books). No pagination or rate-limit detail, keeping it short of a 5.

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?

Three tightly packed sentences, front-loaded with purpose and followed by field list, usage routing, and endpoint. Every sentence earns its place, though the field enumeration is slightly dense.

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 fully: it names the fields returned and explains the conditional refusal reason. Nothing an agent needs to call and consume this tool 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 coverage is 100% with a single, fully documented optional email parameter. The description adds nothing about the parameter, so the baseline 3 applies — the schema does all the work here.

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?

States a specific verb (List) and resource (Google Play Books library / ebooks the account owns), and enumerates the returned fields (book id, title, authors, addable, refusal reason). This is clearly distinguishable from sibling list tools like notebook_list_notebooks or notebook_list_sources.

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?

Explicitly routes the agent to the right follow-up: 'Pass the ids with addable: true to notebook_add_sources books,' implying the tool is used to discover sources before adding them. No explicit when-not guidance or comparison against alternatives, but the downstream condition is concrete.

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