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

Annotations already declare readOnlyHint and openWorldHint, so the bar is lower, yet the description still adds real value: the window is defined as today minus N days at 00:00 through now with the current day included but incomplete, and the exact error payload {'error': 'foreign_object'} is disclosed for pixels not bound to the workspace's ad account.

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 purpose and time window before per-parameter notes, and every sentence carries information (including the partial-day caveat). It is dense and uses a compact telegraphic style, but nothing is repetitive or filler.

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?

An output schema exists, so return values need not be explained; combined with full parameter semantics, the time-window edge case, and the documented error case, an agent has everything needed to invoke this read-only stats tool 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 coverage is 0%, so the description must carry the full burden, and it does: days is defined via the calendar-window rule, aggregation lists concrete accepted values (event/PageView/Purchase/Lead, device_type, browser_type, host, url), pixel_id gets a sourcing route and a constraint, and workspace is described as a slug that is optional when only one workspace exists.

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 resource (Meta pixel event statistics), the output shape (totals plus daily progression per value), and the exact time window. An agent can distinguish it from meta_list_pixels (which only supplies the ID) or meta_ad_performance without consulting another 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?

Explicitly routes the agent to meta_list_pixels to obtain pixel_id and states the foreign_object failure condition, which is actionable precondition guidance. It does not, however, contrast this tool with other reporting siblings such as meta_ad_performance or meta_campaign_performance, so it stops short of full alternative guidance.

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