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

With no annotations, the description carries the full burden and does well: it discloses the JSON result, the optional auto-fix mode, and hard quota/rate limits plus branding on free output. What it omits is what happens on an unparseable or oversized file (hard error vs partial results), and whether autoFix rewrites in place or only returns a repaired copy (the schema implies the latter, the description does not).

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?

Purpose leads, return shape follows, then tier limits and the escalation rule in a clean sequence. The pricing section is a bit long relative to the functional content, but every sentence is actionable (limits, rate, upgrade path) rather than filler.

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 3-parameter tool with no output schema and no annotations, the description covers purpose, cost/limit constraints, and the failure-to-upgrade path, which is what an agent most needs. The one remaining gap is return-value detail — 'Returns JSON with check results' doesn't say how issues are graded or whether a pass/fail summary exists — which matters since no output schema is supplied.

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 `file`, `autoFix` and `deepScan` are already documented in structured form. The description only echoes autoFix ('Can optionally auto-fix common issues') and never mentions `deepScan` or the base64 requirement, so it adds essentially nothing beyond the schema — baseline 3.

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 and resource ('Validate an EPUB file') plus what is checked (errors, compatibility issues, spec compliance), and notes it returns JSON. This clearly separates it from the transformation siblings (merge_epub, split_epub, compress_epub), though it never names an alternative explicitly.

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?

It gives concrete operating context (free vs Pro size/daily/rate limits) and an explicit escalation rule: on a daily_limited or Pro-only refusal, call `upgrade_to_pro` for a checkout link. It does not discuss when validation is unnecessary (e.g., after a successful merge) or how it relates to edit_epub_metadata, but for a single-purpose tool that gap is minor.

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