Start a new download

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

Annotations already declare readOnlyHint=false and destructiveHint=true, and the description is consistent with that by framing the tool as a write. It adds a real operational detail (parent directory must exist), but leaves key behaviors unexplained: whether an existing file at the target is overwritten, what the tool returns so the download can be tracked or later cancelled, and whether authentication or permissions are involved.

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 opening sentence and the path precondition both earn their place, but the middle sentence ('MEASURED: Downloads.createDownload exists on the module from resource://gre/modules/Downloads.sys.mjs ...') is internal implementation trivia that helps neither selection nor invocation and consumes roughly a third of the text.

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 should tell the agent what comes back (a download handle/id) so a subsequent manage, pause, or cancel call is possible; it does not. It also omits failure modes such as invalid URLs or existing targets, leaving a mutation tool only partially specified despite the sound path precondition.

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?

Both parameters are already fully documented in the schema at 100% coverage, so the baseline applies. The description does add one non-obvious constraint not in the schema — the parent directory must pre-exist — but offers no format details, size limits, or destination-conflict behavior beyond that.

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 ('Begin a download of a URL to a local file') and explicitly distinguishes itself from the sibling browser_download_manage by calling itself 'the write half that browser_download_manage does not cover.' An agent can separate it from browser_list_downloads, browser_list_download_events, and browser_set_download_dir without opening any 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?

Names the complementary alternative (browser_download_manage) and gives a concrete precondition: 'Pass an absolute target path; the parent directory must exist.' It stops short of stating the inverse condition (use browser_download_manage to list, pause, or cancel a download), so usage is strongly implied but not exhaustively framed.

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