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

Annotations already declare readOnlyHint=true and destructiveHint=false, and the description adds substantial behavioral context: the pinned EPUBCheck 5.3.0 engine, no-network sandbox, exact-byte SHA-256, truncation metadata, and the pass-meaning caveat. It also discloses the live download and 110-second deadline, which is critical for an agent to set user expectations. No contradiction with annotations.

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 dense but well-organized: it front-loads the core validation action, then the return payload, then usage guidance, then caveats. Every sentence earns its place, though the return-value enumeration is long and could be trimmed slightly without losing essential information.

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 the tool's complexity (3 params, nested file object, output schema, sibling tool), the description is complete: it covers source selection, constraints, return values, limitations, and user communication. The output schema exists, so the description needn't re-explain return structure, but it still summarizes the key fields. Nothing an agent needs to call it 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 description coverage is 100%, so the baseline is 3. The description adds value by clarifying the 'never both' exclusivity rule, the direct-public-URL requirement, the 25 MB attachment limit, and the maxMessages default/range semantics. It doesn't add much beyond the schema for the file object's fields, but the added context on source exclusivity and limits justifies a 4.

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 ('validates') and resource ('one EPUB against the EPUB 2/3 specification'), and explicitly distinguishes itself from the sibling tool by naming the alternative (ebook_compare) and the one-file scope. It also enumerates the exact verdict fields, making the tool's purpose unmistakable.

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 ('Use for one-file readiness and diagnosis') and clear exclusions ('never renders the book, judges accessibility, or guarantees any store will accept it'). It also states the one-source rule (url or file attachment, never both) and the 110-second deadline with a user-facing warning, leaving no ambiguity about invocation context.

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