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

Annotations cover only the safety profile (readOnly, idempotent, non-destructive); the description adds substantial context beyond that: offline execution, no DRM removal or script execution, no repair or publisher certification, no logging of input/results, temp files deleted, a 45-second runtime ceiling, and a prompt-injection warning about returned text. That is exactly the extra behavioral layer the annotations cannot express.

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?

Dense but nearly every clause carries distinct information (tool identity, engine version, exclusions, privacy, price, runtime, safety warning), and the core purpose is front-loaded. Minor waste from the stray '..' and the mildly redundant 'Offline; 10 MB' / 'Unsafe/encrypted input refused' pairing.

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?

An output schema exists so return values need not be described, and the description covers the operational envelope an agent needs: input limits, refusal behavior, pricing, latency, privacy handling, and an untrusted-output warning. Only the lack of explicit alternative-routing keeps it from a 5.

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 a single parameter with 100% schema description coverage — the schema already documents base64 encoding, the 10 MB decoded cap, and the encrypted-file exclusion. The description restates the size limit and refusal rule but adds no new syntax or format detail, so the baseline 3 for high schema coverage applies.

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 and resource — validate/check EPUB 2/3 files using pinned EPUBCheck 5.4.0 — which clearly separates it from the PDF/office siblings in the list. The phrasing is slightly garbled ('Returns checks one EPUB 2/3 with pinned EPUBCheck'), which keeps it just short of a 5, but the intent is unambiguous.

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 operational constraints (10 MB cap, encrypted/unsafe input refused, no repair, up to 45 s, paid) that implicitly define when the tool applies, but it never names an alternative or says explicitly 'use this when you need spec conformance, not repair' beyond the negative list. Usage is implied rather than routed.

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