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

Annotations already declare destructiveHint=true, idempotentHint=false, and readOnlyHint=false, so the bar is lower; the description still adds real behavior — credit spend, refund-on-failure, async job semantics, and the polling cadence. It does not say what the underlying source video's fate is beyond implying preservation via 'a new video beside it'.

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 purpose is front-loaded, which is good, but the prose is tangled with comma splices and dense clauses ('its take carried on with new lines written from the product page, into a new video beside it'). Roughly four sentences, and each covers a distinct point, so length is defensible even if readability suffers.

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?

With no output schema, the description carries the burden well: it explains that the call returns an id immediately and how to poll it to completion, plus the credit and eligibility constraints. A mutation of this weight arguably warrants a note on failure modes or what happens to the original video, which is the only real gap.

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 id, part, credits, and idempotency_key; per the rubric that sets a baseline of 3. The description's note about the price get_video lists in after_steps mostly restates what the credits parameter description already says, adding little beyond it.

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?

Names a specific verb+resource ('make one finished video longer') and distinguishes the output ('into a new video beside it'), plus references the app behavior it emulates. It's clear what the tool does, though the run-on phrasing takes work to parse and it doesn't explicitly contrast with siblings like change_video or other_sizes.

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?

Gives an explicit precondition ('Only for a video whose after_steps offers it'), states the cost and refund policy, and prescribes the follow-up loop ('call get_video with it about once a minute until its status is ready or failed'). It doesn't name an alternative tool for when this isn't appropriate, but the eligibility gate is unusually precise.

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