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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered; the description adds real value on top with the reporting-window rule (complete days only, today excluded) and the definition of Conversions as primary/bid-relevant conversions. It says nothing about latency, rate limits, or empty-result behavior, so it is not fully transparent.

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 compact sentences, front-loaded with the resource and metrics before the time-window and workspace caveats. The nested parentheticals make it dense but nothing is wasted.

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 read-only, two-parameter report with an output schema, the description covers what an agent needs: which dimension is broken out, which metrics come back, how the date window behaves, and how to resolve the workspace. Return-value structure is correctly left to the output schema.

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 load and largely does: it explains the `days` window semantics (last N complete days, today excluded) and defines `workspace` as a customer-workspace slug that is optional when only one workspace exists. The default value of 30 for `days` is never mentioned, leaving a small gap.

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?

The description names a specific report and its breakdown dimension ("Performance nach Gerätetyp (Mobile, Desktop, Tablet)") plus the exact metrics returned, which implicitly separates it from ads_campaign_performance, ads_demographic_performance and ads_geo_performance. It never names a sibling or states which one to pick instead, so it stops short of the top score.

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?

Usage context is implied by the window definition ("letzten N Tage, vollständige Tage bis gestern, heute nicht enthalten") and the note that workspace is optional for single-workspace accounts. There is no explicit when-to-use/when-not guidance and no routing to alternative performance tools for other breakdowns.

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