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

Annotations already declare the safety profile (non-read-only, non-destructive, not open-world), so the bar is lower, and the description meaningfully adds the multi-step contract: it returns per-part signed URLs and requires a follow-up finish call. However it omits behavior an agent needs for failure handling – signed URL lifetime, whether the reserved slot expires, and any size limits that change the part count.

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?

Three short sentences, front-loaded with the primary action and return value, followed by the required next step and the disambiguating condition. Nothing is redundant with the name or annotations.

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 correctly supplies the return shape (object key plus signed URLs per part) and the downstream step, which is the critical missing piece an agent would otherwise lack. It would be complete with a note on slot/URL expiry before lunara_finish_upload is called.

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% and all seven parameters are documented in-schema, including defaults for primaryLang/secondaryLang and the omit-to-reuse rule for projectId, so the description carries no burden here. It adds no parameter-level meaning beyond the flow, which is the baseline-3 case.

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 concrete operation (reserve an upload slot), the exact return payload (object key plus a signed URL per part), and scopes it with 'a file you hold locally', which separates it from the URL-based sibling lunara_subtitle_from_url. An agent knows what this does and what it produces 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?

Gives an explicit trigger condition ('file you hold locally') and the mandatory follow-up step ('After uploading the bytes call lunara_finish_upload'), which prevents a half-completed flow. It stops short of naming the alternative tool for remote files or stating what happens if the reserved slot is never finished.

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