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

Goes well beyond the readOnly/idempotent annotations by disclosing the computation method (snapshot diffing), why it must be done that way, and an explicit failure contract: comparable:false with a reason rather than a fabricated empty diff when no suitable snapshot exists. That failure-mode disclosure is exactly what an agent needs to avoid misreading a result.

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, front-loaded with the resource and operation, followed by the mechanism and the edge-case contract. Every clause carries information; nothing is redundant with the schema or annotations.

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 one-parameter read tool with no output schema, the description covers purpose, mechanism, and the key non-success case. It names one output field (comparable) but does not sketch the rest of the return shape, which an agent would otherwise have to discover by calling it.

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% and the single parameter is fully documented, so baseline is 3. The description adds real semantics by tying the date to snapshot availability ('no snapshot exists at or before the requested date'), telling the agent that certain dates will yield comparable:false rather than data.

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: products added to or removed from the Article 57 register since a date. The diff-of-dated-snapshots explanation makes it unmistakable versus siblings like ema_article57_search or ema_article57_product, which return current state rather than deltas.

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?

The 'what changed' framing plus the note that the source has no change-date column gives clear context for when this tool is the right one. It stops short of naming alternatives or stating when not to use it, but the intended use case is unambiguous.

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