build_index

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

With no annotations, the description carries the full burden of behavioral disclosure, and it delivers: it explains that the tool converts book text via calibre's converter, stores data in a local SQLite FTS5 index outside the library, only processes unindexed books, and may take several minutes initially. This gives the agent a strong understanding of side effects, persistence, and cost.

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 well-structured with a concise summary up front followed by an Args section. Every sentence adds value: the operation, the caching behavior, the storage location, the idempotency, the time cost, and parameter semantics. There is no filler or repetition of the schema.

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 tool with only two optional parameters and an output schema, the description covers the important operational details: what gets built, where it is stored, which books are eligible, and the potential long runtime. The agent can confidently decide when to call it and what to expect, even without annotations.

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 input schema provides only titles and defaults with 0% coverage, so the description fully compensates by explaining both parameters: book_ids defaults to all unindexed books, and limit caps how many books are processed in a single call. This is exactly the semantic information an agent needs to invoke the tool correctly.

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 opens with 'Index book contents for full-text search', which is a specific verb and resource that clearly distinguishes build_index from sibling search tools like search_books and search_in_book. The parenthetical 'one-time, cached' further clarifies its role as a maintenance/indexing operation rather than a query operation.

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 clearly establishes when this tool is relevant: it is a one-time cached indexing step that processes only unindexed books, and warns that an initial full-library run can take minutes. It does not explicitly name alternatives or state when not to use it, but the context is clear enough for an agent to recognize this as the setup step for full-text search.

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