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

Annotations already declare read-only, idempotent, non-destructive, but the description adds real behavior beyond them: the implicit default 30-day window, inclusive date bounds that cannot exceed today, and the crucial interpretive caveat that attributedComparison is Klaviyo's attribution claim which can explain a drop but never proves causation for a rise. That attribution-semantics warning is exactly the kind of context annotations cannot carry.

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-loads what the tool returns and who should use it, then the date-handling rule; every sentence carries functional content. Some sentences (particularly the attribution caveat and the MUST-send-dates clause) are long and packed, which slightly reduces scannability but not correctness.

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 and zero required parameters, the description supplies the return contents (revenue, sends, open/click rate per flow and campaign), the default time window, and the comparison output's provenance. It does not describe output shape or whether SMS metrics actually appear given the email framing, but it is complete enough to call the 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 100 percent, so the baseline is 3; the description adds the semantic consequence of `compare` — that attributedComparison returns Klaviyo-attributed revenue for flows, campaigns, and total across both periods with the delta — which is a result meaning the schema does not convey. The both-or-neither and not-after-today constraints are repeated from the schema, so the gain is modest but real.

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+resource: reads Klaviyo flows and email campaigns for a period, with named metrics (revenue, sends, open rate, click rate) at per-flow and per-campaign granularity. Naming Klaviyo and email/SMS implicitly separates it from campaign tools in the sibling list, but it never explicitly contrasts itself with get_campaigns, so differentiation is inferential rather than stated.

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 explicit use cases ('asked about email or SMS performance, or which flow earns the most') and a hard routing rule: a named month, week, or two operator-supplied dates MUST be passed here because the no-date default is the last 30 days. It also signals when to prefer yoyWeekday over the default comparison, covering the main alternative-selection decisions an agent faces.

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