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

Annotations already cover the safety profile (mutation, open-world, non-destructive), and the description adds genuine behavioral detail beyond them: the presigned PUT flow including the returned Content-Type header, the need to re-call start_caption_job after upload, and the returned job_id. Missing are auth/permission requirements, failure modes for an unreachable source_url, and any indication of processing duration or cost.

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?

Front-loaded with the core action, then a tight bulleted breakdown of the two source modes, then the follow-up chain — efficient for a 19-parameter tool. The 'Optionally choose style, position, colours, font size, animation' sentence largely repeats property names already documented in the schema, so it is not entirely waste-free.

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 19-parameter mutation with no output schema, the description covers what an agent most needs: how to supply input, that job_id is returned, and which sibling to call next. The create_caption_job vs start_caption_job division of labour remains unexplained, which is the one meaningful gap for this complexity level.

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?

With schema coverage at 79%, the schema carries most definitions, but the description adds an important constraint the schema cannot express: the source parameters are mutually exclusive and exactly one must be chosen (the schema marks nothing required). The style/position/colour/font/animation roll-up is mostly a restatement of enum and property descriptions, which caps the value here.

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 ("Start adding animated captions to a video") and outlines the two intake paths, which is far more than a restatement of the name. However, the verb "Start" collides with the sibling tool start_caption_job, and the description never explains how create_caption_job differs from it, leaving a real ambiguity an agent must resolve on its own.

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?

Explicitly constrains input to ONE source and spells out the two mutually exclusive paths (source_url direct link vs filename + file_size presigned upload), plus the required follow-up sequence to wait_for_caption_job and download_captioned_video. It does not say when to prefer this tool over start_caption_job or what happens if no source is supplied despite zero required parameters.

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