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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds meaningful behavioral context beyond that: SSRF-guarded and 2 MB-capped preflight, alignment by cue index, the specific comparison dimensions (cue-count delta, timing drift, CPS regressions, structural-error deltas), and the explicit non-goals (never translation quality, transcription accuracy, or sync against video). It also discloses the up-to-60-second fetch/parse time. The only minor gap is that it doesn't describe the exact output shape, but an output schema exists.

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 purpose and use cases, then the input modes, then the comparison dimensions, then the sibling routing, then the non-goals and latency caveat. Every sentence earns its place, though the long middle sentence listing all comparison dimensions is a bit heavy. It is appropriately sized for a tool with this complexity.

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 comparison tool with an output schema, 100% schema coverage, and read-only annotations, the description is complete. It covers what the tool does, how to provide inputs, what it checks, what it deliberately does not check, the sibling alternative, and the latency expectation. An agent has everything needed to select and invoke it correctly.

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 schema already documents all three parameters (urls, files, profile) with detailed descriptions and strict rules. The description adds the ordering semantics ('in the order they should appear side by side') and the profile's role ('applied to both tracks'), but it doesn't add much beyond what the schema already provides. Baseline 3 is appropriate when the schema carries the parameter documentation burden.

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 ('Compare'), a precise resource ('exactly two caption or subtitle files'), and enumerates concrete use cases (original vs translated, before/after timing fix, format migration). It also explicitly distinguishes itself from the sibling caption_check by naming the single-file alternative, so an agent can tell them apart without opening the schema.

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 caption_check for a single file') and clear input modes (two attached files or two public URLs, never both). It also states the strict either/or rule and the 60-second latency expectation, so the agent knows the operational context before invoking.

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