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

Annotations (readOnlyHint=true, destructiveHint=false) already establish the safety profile. The description adds value by detailing what the tool computes and returns (overall score, per-dimension breakdown, strengths, the single biggest fix), giving the agent expectations beyond the annotation. It does not contradict annotations.

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?

Two sentences that front-load the core purpose and output details, then provide usage guidance. No filler or redundancy. Every phrase earns its place.

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?

The tool is straightforward: one required parameter (url) with two optional ones, all documented in the schema. The description lists the result structure and gives a usage trigger. It does not mention failure modes or edge cases, but for a read-only scoring tool, this is adequate. No output schema exists, so the description's enumeration of outputs fills that gap.

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% — url, kind, and intent each have descriptions. The tool description does not add additional meaning beyond what the schema already provides; it only mirrors the fields. With full coverage, a baseline of 3 is appropriate.

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 opens with a specific verb+resource: 'Virality/performance prediction for a finished ad (image or video URL)' and lists the detailed output dimensions (overall score, per-dimension breakdown, strengths, biggest fix). It clearly distinguishes this from sibling tools like plan_ad or render_ad, which have different purposes.

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 states when to use: 'Use BEFORE spending on distribution, or to rank variants.' This provides clear context for invocation. It does not name an alternative tool or give a when-not-to-use, but the timing guidance is actionable and sufficient for most cases.

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