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

Annotations already declare readOnly/idempotent/non-destructive/closed-world, so the safety profile is carried elsewhere. The description adds genuinely new context: the ads.read scope requirement and the fact that the tool produces an explanatory best-performer result rather than raw numbers. Minor gap is that it doesn't say how many ads must pre-exist or what happens if a snapshot_id is stale.

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 short sentences, front-loaded with the action and cardinality, then construction instructions, then the prerequisite. No filler and nothing buried.

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 only 3 required parameters, an output schema handling return values, and annotations covering the safety profile, the description supplies everything needed to invoke correctly: cardinality bounds, item shape, metric sourcing, and auth scope.

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 only 33% (only workspace_id is documented), so the description has to compensate and largely does: it explains that each array item is a key/snapshot_id pair and that the comparison spans a single metric. It still doesn't explain what 'key' is used for (display label?) or where snapshot_id comes from in the list_ads payload, so the compensation is partial.

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 ('Compares 2 to 20 ads ... on one metric') plus the output ('explains which performed best and why'), and ties itself to the sibling that produces the inputs (postly_list_ads). An agent can distinguish this from postly_list_ads and postly_get_ads_overview without opening a 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 says the ads must come from postly_list_ads and that ads.read is required, which is real routing and prerequisite guidance. It stops short of saying when not to use it (e.g., use postly_get_ads_overview for a broad account snapshot), so no explicit exclusions or alternatives.

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